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J.P. Magué & B. Ménard

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[1]  arXiv:1804.06844 [pdf, other]
Title: Modelling the structure of star clusters with fractional Brownian motion
Comments: 11 Pages, submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

The degree of fractal substructure in molecular clouds can be quantified by comparing them with Fractional Brownian Motion (FBM) surfaces or volumes. These fields are self-similar over all length scales and characterised by a drift exponent $H$, which describes the structural roughness. Given that the structure of molecular clouds and the initial structure of star clusters are almost certainly linked, it would be advantageous to also apply this analysis to clusters. Currently, the structure of star clusters is often quantified by applying $\mathcal{Q}$ analysis. $\mathcal{Q}$ values from observed targets are interpreted by comparing them with those from artificial clusters. These are typically generated using a Box-Fractal (BF) or Radial Density Profile (RDP) model. We present a single cluster model, based on FBM, as an alternative to these models. Here, the structure is parameterised by $H$, and the standard deviation of the log-surface/volume density $\sigma$. The FBM model is able to reproduce both centrally concentrated and substructured clusters, and is able to provide a much better match to observations than the BF model. We show that $\mathcal{Q}$ analysis is unable to estimate FBM parameters. Therefore, we develop and train a machine learning algorithm which can estimate values of $H$ and $\sigma$, with uncertainties. This provides us with a powerful method for quantifying the structure of star clusters in terms which relate to the structure of molecular clouds. We use the algorithm to estimate the $H$ and $\sigma$ for several young star clusters, some of which have no measurable BF or RDP analogue.

[2]  arXiv:1804.06845 [pdf, other]
Title: A Low Lyman Continuum Escape Fraction of $<10\%$ for Extreme [OIII] Emitters in an Overdensity at z$\sim3.5$
Comments: 10 pages, 8 figures (key result in Figures 7, 8). Accepted for publication in MNRAS. Comments welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Recent work has suggested extreme [OIII] emitting star-forming galaxies are important to reionization. Relatedly, [OIII]/[OII] has been put forward as an indirect estimator of the Lyman Continuum (LyC) escape fraction ($f_{esc}$) at $z\gtrsim4.5$ when the opaque IGM renders LyC photons unobservable. Using deep archival U-band (VLT/VIMOS) imaging of a recently confirmed overdensity at $z\sim3.5$ we calculate tight constraints on $f_{esc}$ for a sample (N=73) dominated by extreme [OIII] emitters. We find no Lyman Continuum signal ($f_{esc}^{rel} < 6.3^{+0.7}_{-0.7} \%$ at $1\sigma$) in a deep U-band stack of our sample (31.98 mag at 1$\sigma$). This constraint is in agreement with recent studies of star-forming galaxies spanning $z\sim1-4$ that have found very low average $f_{esc}$. Despite the galaxies in our study having an estimated average rest-frame EW([OIII]$\lambda5007$)$\sim400\AA$ and [OIII]/[OII]$\sim 4$ from composite SED-fitting, we find no LyC detection, which brings into question the potential of [OIII]/[OII] as an effective probe of the LyC--a majority of LyC emitters have [OIII]/[OII]$>3$, but we establish here that [OIII]/[OII]$>3$ does not guarantee significant LyC leakage for a population. Since even extreme star-forming galaxies are unable to produce the $f_{esc}\sim10-15\%$ required by most theoretical calculations for star-forming galaxies to drive reionization, there must either be a rapid evolution of $f_{esc}$ between $z\sim3.5$ and the Epoch of Reionization, or hitherto observationally unstudied sources (e.g. ultra-faint low-mass galaxies with $\log(M/M_\odot)\sim7-8.5$) must make an outsized contribution to reionization.

[3]  arXiv:1804.06846 [pdf, other]
Title: An Unexpected Dip in the Solar Gamma-Ray Spectrum
Comments: 12 pages, 5 figures. Supplemental Material includes an additional 12 pages, 14 figures. Comments are welcome!
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR); High Energy Physics - Phenomenology (hep-ph)

The solar disk is a bright source of multi-GeV gamma rays, due to the interactions of hadronic cosmic rays with the solar atmosphere. However, the underlying production mechanism is not understood, except that its efficiency must be greatly enhanced by magnetic fields that redirect some cosmic rays from ingoing to outgoing before they interact. To elucidate the nature of this emission, we perform a new analysis of solar atmospheric gamma rays with 9 years of Fermi-LAT data, which spans nearly the full 11-year solar cycle. We detect significant gamma-ray emission from the solar disk from 1 GeV up to $\gtrsim200$ GeV. The overall gamma-ray spectrum is much harder ($\sim E_{\gamma}^{-2.2}$) than the cosmic-ray spectrum ($\sim E_{\rm CR}^{-2.7}$). We find a clear anticorrelation between the solar cycle phase and the gamma-ray flux between 1-10 GeV. Surprisingly, we observe a spectral dip between $\sim$30-50 GeV in an otherwise power-law spectrum. This was not predicted, is not understood, and may provide crucial clues to the gamma-ray emission mechanism. The flux above 100 GeV, which is brightest during the solar minimum, poses exciting opportunities for HAWC, LHAASO, IceCube, and KM3NeT.

[4]  arXiv:1804.06849 [pdf, other]
Title: Tree-Level Bispectrum in the Effective Field Theory of Large-Scale Structure extended to Massive Neutrinos
Comments: 34 pages, 12 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We compute the tree-level bispectrum of dark matter in the presence of massive neutrinos in the mildly non-linear regime in the context of the effective field theory of large-scale structure (EFTofLSS). For neutrinos, whose typical free streaming wavenumber ($k_{\rm fs}$) is longer than the non-linear scale ($k_{\mathrm{NL}}$), we solve a Boltzmann equation coupled to the effective fluid equation for dark matter. We solve perturbatively the coupled system by expanding in powers of the neutrino density fraction ($f_{\nu}$) and the ratio of the wavenumber of interest over the non-linear scale ($k/k_{\mathrm{NL}}$) and add suitable counterterms to remove the dependence from short distance physics. For equilateral configurations, we find that the total-matter tree-level bispectrum is approximately $16f_{\nu}$ times the dark matter one on short scales ($k > k_{\rm fs}$). The largest contribution stems from the back-reaction of massive neutrinos on the dark matter growth factor. On large scales ($k < k_{\rm fs}$) the contribution of neutrinos to the bispectrum is smaller by up to two orders of magnitude.

[5]  arXiv:1804.06851 [pdf, other]
Title: A Distant Sample of Halo Wide Binaries from SDSS
Comments: 11 pages, 15 figures. Submitted to MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)

Samples of reliably identified halo wide binaries are scarce. If reasonably free from selection effects and with a small degree of contamination by chance alignments, these wide binaries become a powerful dynamical tool, having provided one of the very few experiments capable of constraining the nature of dark matter in the Milky Way halo. Currently, however, the best available sample of halo wide binaries is confined to the solar neighborhood, and is plagued by small number statistics at the widest separations. We present the results of a program aimed to probe the wide binary population of the Galactic halo at significantly larger distances, and which informs future searches that could improve the statistics by orders of magnitude. Halo stars were taken from the Sloan Digital Sky Survey after analyzing the Galactic orbits of stars in the reduced proper motion diagram. We then select candidate binaries by searching for pairs with small differences in proper motion and small projected separation on the sky. Using medium-resolution spectroscopy, a subsample of candidates is validated via radial velocities, finding that about 68% of candidate pairs up to 20$^{\prime\prime}$ separation can be considered genuine halo wide binaries, to the limits of the available data. Photometric distance estimates show that the components of our validated candidates lie at the same distances, independently confirming the robustness of the selection method. These results should prove valuable to guide the optimal assembly of larger catalogs of halo wide binaries from upcoming large databases such as Gaia and LSST.

[6]  arXiv:1804.06852 [pdf, other]
Title: Resolving the ISM at the peak of cosmic star formation with ALMA - The distribution of CO and dust continuum in z~2.5 sub-millimetre galaxies
Comments: Submitted to ApJ, 16 pages, 6 figures
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We use ALMA observations of four sub-millimetre galaxies (SMGs) at $z\sim2-3$ to investigate the spatially resolved properties of the inter-stellar medium (ISM) at scales of 1--5 kpc (0.1--0.6$''$). The velocity fields of our sources, traced by the $^{12}$CO($J$=3-2) emission, are consistent with disk rotation to first order, implying average dynamical masses of $\sim$3$\times10^{11}$M$_{\odot}$ within two half-light radii. Through a Bayesian approach we investigate the uncertainties inherent to dynamically constraining total gas masses. We explore the covariance between the stellar mass-to-light ratio and CO-to-H$_{2}$ conversion factor, $\alpha_{\rm CO}$, finding values of $\alpha_{\rm CO}=1.1^{+0.8}_{-0.7}$ for dark matter fractions of 15 \%. We show that the resolved spatial distribution of the gas and dust continuum can be uncorrelated to the stellar emission, challenging energy balance assumptions in global SED fitting. Through a stacking analysis of the resolved radial profiles of the CO(3-2), stellar and dust continuum emission in SMG samples, we find that the cool molecular gas emission in these sources (radii $\sim$5--14 kpc) is clearly more extended than the rest-frame $\sim$250 $\mu$m dust continuum by a factor $>2$. We propose that assuming a constant dust-to-gas ratio, this apparent difference in sizes can be explained by temperature and optical-depth gradients alone. Our results suggest that caution must be exercised when extrapolating morphological properties of dust continuum observations to conclusions about the molecular gas phase of the ISM.

[7]  arXiv:1804.06854 [pdf, other]
Title: The information content in cold stellar streams
Comments: submitted to AAS journals, comments welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Cold stellar streams---produced by tidal disruptions of clusters---are long-lived, coherent dynamical features in the halo of the Milky Way. Due to their different ages and different positions in phase space, different streams tell us different things about the Galaxy. Here we employ a Cramer--Rao (CRLB) or Fisher-matrix approach to understand the quantitative information content in eleven known streams (ATLAS, GD-1, Hermus, Kwando, Orinoco, PS1A, PS1C, PS1D, PS1E, Sangarius and Triangulum). This approach depends on a generative model, which we have developed previously, and which permits calculation of derivatives of predicted stream properties with respect to Galaxy and stream parameters. We find that in simple analytic models of the Milky Way, streams on eccentric orbits contain the most information about the halo shape. For each stream, there are near-degeneracies between dark-matter-halo properties and parameters of the bulge, the disk, and the stream progenitor, but simultaneous fitting of multiple streams will constrain all parameters at the percent level. At this precision, simulated dark matter halos deviate from simple analytic parametrizations, so we add an expansion of basis functions to give the gravitational potential more freedom. As freedom increases, the information about the halo reduces overall, and it becomes more localized to the current position of the stream. In the limit of high model freedom, a stellar stream appears to measure the local acceleration at its current position; this motivates thinking about future non-parametric approaches. The CRLB formalism also permits us to assess the value of future measurements of stellar velocities, distances, and proper motions. We show that kinematic measurements of stream stars are essential for producing competitive constraints on the distribution of dark matter, which bodes well for stream studies in the age of Gaia.

[8]  arXiv:1804.06855 [pdf, other]
Title: Dust Evolution in Galaxy Cluster Simulations
Comments: 19 pages, 15 figures, submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We implement a state-of-the-art treatment of the processes affecting the production and Interstellar Medium (ISM) evolution of carbonaceous and silicate dust grains within SPH simulations. We trace the dust grain size distribution by means of a two-size approximation. We test our method on zoom-in simulations of four massive ($M_{200} \geq 3 \times 10^{14} M_{\odot}$) galaxy clusters. We predict that during the early stages of assembly of the cluster at $z \gtrsim 3$, where the star formation activity is at its maximum in our simulations, the proto-cluster regions are rich of dusty gas. Compared to the case in which only dust production in stellar ejecta is active, if we include processes occurring in the cold ISM,the dust content is enhanced by a factor $2-3$. However, the dust properties in this stage turn out to be significantly different than those observationally derived for the {\it average} Milky Way dust, and commonly adopted in calculations of dust reprocessing. We show that these differences may have a strong impact on the predicted spectral energy distributions. At low redshift in star forming regions our model reproduces reasonably well the trend of dust abundances over metallicity as observed in local galaxies. However we under-produce by a factor of 2 to 3 the total dust content of clusters estimated observationally at low redshift, $z \lesssim 0.5$ using IRAS, Planck and Herschel satellites data. This discrepancy can be solved by decreasing the efficiency of sputtering which erodes dust grains in the hot Intracluster Medium (ICM).

[9]  arXiv:1804.06856 [pdf, other]
Title: Estimating stellar birth radii and the time evolution of the Milky Way's ISM metallicity gradient
Comments: 14 p., 9 fig., submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present a semi-empirical, largely model-independent approach for estimating Galactic birth radii, r_birth, for Milky Way disk stars. The technique relies on the justifiable assumption that a negative radial metallicity gradient in the interstellar medium (ISM) existed for most of the disk lifetime. Stars are projected back to their birth positions according to the observationally derived age and [Fe/H] with no kinematical information required. Applying our approach to the AMBRE:HARPS and HARPS-GTO local samples, we show that we can constrain the ISM metallicity evolution with Galactic radius and cosmic time, [Fe/H]_ISM(r, t), by requiring a physically meaningful r_birth distribution. We find that the data are consistent with an ISM radial metallicity gradient that flattens with time from ~-0.15 dex/kpc at the beginning of disk formation, to its measured present-day value (-0.07 dex/kpc). We present several chemo-kinematical relations in terms of mono-r_birth populations. One remarkable result is that the kinematically hottest stars would have been born locally or in the outer disk, consistent with thick disk formation from the nested flares of mono-age populations and predictions from cosmological simulations. This phenomenon can be also seen in the observed age-velocity dispersion relation, in that its upper boundary is dominated by stars born at larger radii. We also find that the flatness of the local age-metallicity relation (AMR) is the result of the superposition of the AMRs of mono-r_birth populations, each with a well-defined negative slope. The solar birth radius is estimated to be 7.3+-0.6 kpc, for a current Galactocentric radius of 8 kpc.

[10]  arXiv:1804.06859 [pdf, ps, other]
Title: Shepherding in a Self-Gravitating Disk of Trans-Neptunian Objects
Comments: Submitted; 21 pages, 15 figures. Comments are welcome
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

A relatively massive and moderately eccentric disk of trans-Neptunian objects (TNOs) can effectively counteract apse precession induced by the outer planets, and in the process shepherd highly eccentric members of its population into nearly-stationary configurations which are anti-aligned with the disk itself. We were sufficiently intrigued by this remarkable feature to embark on an extensive exploration of the full spatial dynamics sustained by the combined action of giant planets and a massive trans-Neptunian debris disk. In the process, we identified ranges of disk mass, eccentricity and precession rate which allow apse-clustered populations that faithfully reproduce key orbital properties of the much discussed TNO population. The shepherding disk hypothesis is to be sure complementary to any potential ninth member of the Solar System pantheon, and could obviate the need for it altogether. We discuss its essential ingredients in the context of Solar System formation and evolution, and argue for their naturalness in view of the growing body of observational and theoretical knowledge about self-gravitating disks around massive bodies, extra-solar debris disks included.

[11]  arXiv:1804.06875 [pdf, other]
Title: Universe opacity and CMB
Authors: Vaclav Vavrycuk
Comments: 21 pages, 14 figures
Journal-ref: Monthly Notices of the Royal Astronomical Society, 2018
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

A cosmological model, in which the cosmic microwave background (CMB) is a thermal radiation of intergalactic dust instead of a relic radiation of the Big Bang, is revived and revisited. The model suggests that a virtually transparent local Universe becomes considerably opaque at redshifts $z > 2-3$. Such opacity is hardly to be detected in the Type Ia supernova data, but confirmed using quasar data. The opacity steeply increases with redshift because of a high proper density of intergalactic dust in the previous epochs. The temperature of intergalactic dust increases as $(1+z)$ and exactly compensates the change of wavelengths due to redshift, so that the dust radiation looks apparently like the radiation of the blackbody with a single temperature. The predicted dust temperature is $T^{D} = 2.776 \, \mathrm{K}$, which differs from the CMB temperature by 1.9\% only, and the predicted ratio between the total CMB and EBL intensities is 13.4 which is close to 12.5 obtained from observations. The CMB temperature fluctuations are caused by EBL fluctuations produced by galaxy clusters and voids in the Universe. The polarization anomalies of the CMB correlated with temperature anisotropies are caused by the polarized thermal emission of needle-shaped conducting dust grains aligned by large-scale magnetic fields around clusters and voids. A strong decline of the luminosity density for $z > 4$ is interpreted as the result of high opacity of the Universe rather than of a decline of the global stellar mass density at high redshifts.

[12]  arXiv:1804.06878 [pdf, other]
Title: Delayed Shock-induced Dust Formation in the Dense Circumstellar Shell Surrounding the Type IIn Supernova SN 2010jl
Comments: Accepted for publication in The Astrophysical Journal, 26 Pages
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)

The light curves of Type IIn supernovae are dominated by the radiative energy released through the interaction of the supernova shockwaves with their dense circumstellar medium (CSM). The ultraluminous Type IIn supernova SN 2010jl exhibits an infrared emission component that is in excess of the extrapolated UV-optical spectrum as early as a few weeks post-explosion. This emission has been attributed by some as evidence for rapid formation of dust in the cooling postshock CSM. We investigate the physical processes that may inhibit or facilitate the formation of dust in the CSM. When only radiative cooling is considered, the temperature of the dense shocked gas rapidly drops below the dust condensation temperature. However, by accounting for the heating of the postshock gas by the downstream radiation from the shock, we show that dust formation is inhibited until the radiation from the shock weakens, as the shock propagates into the less dense outer regions of the CSM. In SN 2010jl dust formation can therefore only commence after day 380. Only the IR emission since that epoch can be attributed to the newly formed CSM dust. Observations on day 460 and later show that the IR luminosity exceeds the UV-optical luminosity. The post-shock dust cannot extinct the radiation emitted by the expanding SN shock. Therefore, its IR emission must be powered by an interior source, which we identify with the reverse shock propagating through the SN ejecta. IR emission before day 380 must therefore be an IR echo from preexisting CSM dust.

[13]  arXiv:1804.06880 [pdf, ps, other]
Title: The Masses and Accretion Rates of White Dwarfs in Classical and Recurrent Novae
Comments: 24 pages, 6 figures, Astrophysical Journal in press
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Models have long predicted that the frequency-averaged masses of white dwarfs in Galactic classical novae are twice as large as those of field white dwarfs. Only a handful of dynamically well-determined nova white dwarf masses have been published, leaving the theoretical predictions poorly tested. The recurrence time distributions and mass accretion rate distributions of novae are even more poorly known. To address these deficiencies, we have combined our extensive simulations of nova eruptions with the Strope et al (2010) and Schaefer et al (2010) databases of outburst characteristics of Galactic classical and recurrent novae to determine the masses of 92 white dwarfs in novae. We find that the mean mass (frequency averaged mean mass) of 82 Galactic classical novae is 1.06 (1.13) Msun, while the mean mass of 10 recurrent novae is 1.31 Msun. These masses, and the observed nova outburst amplitude and decline time distributions allow us to determine the long-term mass accretion rate distribution of classical novae. Remarkably, that value is just 1.3 x 10^{-10} Msun/yr, which is an order of magnitude smaller than that of cataclysmic binaries in the decades before and after classical nova eruptions. This predicts that old novae become low mass transfer rate systems, and hence dwarf novae, for most of the time between nova eruptions. We determine the mass accretion rates of each of the 10 known Galactic RN, finding them to be in the range 10^{-7} - 10^{-8} $ Msun/yr. We are able to predict the recurrence time distribution of novae and compare it with the predictions of population synthesis models.

[14]  arXiv:1804.06888 [pdf, other]
Title: The Shocking Power Sources of LINERs
Authors: M. Molina (1), M. Eracleous (1), A. J. Barth (2), D. Maoz (3), J. C. Runnoe (1,4), L. C. Ho (5), J. C. Shields (6), J. L. Walsh (7) (1 Penn State, 2 UC Irvine, 3 Tel Aviv U, 4 U Michigan, 5 KIAA Peking, 6 Ohio U, 7 TAMU)
Comments: Submitted to ApJ on 4/17/18
Subjects: Astrophysics of Galaxies (astro-ph.GA)

The majority of low-ionization nuclear emission-line regions (LINERs) harbor supermassive black holes with very low accretion rates. However, the accretion flows do not produce enough ionizing photons to power the emission lines emitted on scales of ~100 pc, therefore additional sources of power are required. We present and analyze Hubble Space Telescope spectra of three nearby luminous LINERs that are spatially resolved on scales of ~9 pc. The targets have multiple indicators of an accreting black hole, as well as a deficient ionizing photon budget. We measure diagnostic emission line ratios as a function of distance from the nucleus and compare them to models for different excitation mechanisms: shocks, photoionization by the accreting black hole, and photoionization by young or old hot stars. The physical model that best describes our targets comprises a low-luminosity, accretion-powered active nucleus that photoionizes the gas within ~20 pc of the galaxy center and shocks that excite the gas at larger distances. We conclude that, in LINERs with low-luminosity active nuclei, shocks by jets or other outflows are crucial in exciting the gas in and around the nucleus, as suggested by other authors.

[15]  arXiv:1804.06908 [pdf, ps, other]
Title: The Connection Between Different Tracers Of The Diffuse Interstellar Medium: Kinematics
Comments: To be published in ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

Using visible, radio, microwave, and sub-mm data, we study several lines of sight toward stars generally closer than 1 kpc on a component-by-component basis. We derive the component structure seen in absorption at visible wavelengths from Ca II, Ca I, K I, CH, CH$^{+}\!,$ and CN and compare it to emission from H I, CO and its isotopologues, and C$^{+}$ from the GOT C+ survey. The correspondence between components in emission and absorption help create a more unified picture of diffuse atomic and molecular gas in the interstellar medium. We also discuss how these tracers are related to the CO-dark H$_{2}$ gas probed by C$^{+}$ emission and discuss the kinematic connections among the species observed.

[16]  arXiv:1804.06915 [pdf, other]
Title: A search for Galactic runaway stars using Gaia Data Release 1 and Hipparcos proper motions
Authors: J. Maíz Apellániz (1), M. Pantaleoni González (1,2), R. H. Barbá (3), S. Simón-Díaz (4,5), I. Negueruela (6), D. J. Lennon (7), A. Sota (8), E. Trigueros Páez (1,6) ((1) CAB, CSIC-INTA, (2) UCM, (3) ULS, (4) IAC, (5) ULL, (6) UA, (7) ESA, (8) IAA)
Comments: Submitted to A&A
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Astrophysics of Galaxies (astro-ph.GA)

CONTEXT.The first Gaia Data Release (DR1) significantly improved the previously available proper motions for the majority of the Tycho-2 stars. AIMS. We want to detect runaway stars using Gaia DR1 proper motions and compare our results with previous searches. METHODS. Runaway O stars and BA supergiants are detected using a 2-D proper-motion method. The sample is selected using Simbad, spectra from our GOSSS project, literature spectral types, and photometry processed using CHORIZOS. RESULTS. We detect 76 runaway stars, 17 (possibly 19) of them with no prior identification as such, with an estimated detection rate of approximately one half of the real runaway fraction. An age effect appears to be present, with objects of spectral subtype B1 and later having travelled for longer distances than runaways of earlier subtypes. We also tentatively propose that the fraction of runaways is lower among BA supergiants that among O stars but further studies using future Gaia data releases are needed to confirm this. The frequency of fast rotators is high among runaway O stars, which indicates that a significant fraction of them (and possibly a majority) is produced in supernova explosions.

[17]  arXiv:1804.06917 [pdf, other]
Title: Spitzer Light Curves of the Young, Planetary-Mass TW Hya Members 2MASS J11193254-1137466AB and WISEA J114724.10-204021.3
Comments: Accepted for publication in the Astronomical Journal
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

We present Spitzer Space Telescope time-series photometry at 3.6 and 4.5 $\mu$m of 2MASS J11193254$-$1137466AB and WISEA J114724.10$-$204021.3, two planetary-mass, late-type ($\sim$L7) brown dwarf members of the $\sim$10 Myr old TW Hya Association. These observations were taken in order to investigate whether or not a tentative trend of increasing variability amplitude with decreasing surface gravity seen for L3-L5.5 dwarfs extends to later-L spectral types and to explore the angular momentum evolution of low-mass objects. We examine each light curve for variability and find a rotation period of 19.39$^{+0.33}_{-0.28}$ hours and semi-amplitudes of 0.798$^{+0.081}_{-0.083}$% at 3.6 $\mu$m and 1.108$^{+0.093}_{-0.094}$% at 4.5 $\mu$m for WISEA J114724.10$-$204021.3. For 2MASS J11193254$-$1137466AB, we find a single period of 3.02$^{+0.04}_{-0.03}$ hours with semi-amplitudes of 0.230$^{+0.036}_{-0.035}$% at 3.6 $\mu$m and 0.453 $\pm$ 0.037% at 4.5 $\mu$m, which we find is possibly due to the rotation of one component of the binary. Combining our results with 12 other late-type L dwarfs observed with Spitzer from the literature, we find no significant differences between the 3.6 $\mu$m amplitudes of low surface gravity and field gravity late-type L brown dwarfs at Spitzer wavelengths, and find tentative evidence (75% confidence) of higher amplitude variability at 4.5 $\mu$m for young, late-type Ls. We also find a median rotation period of young brown dwarfs (10-300 Myr) of $\sim$10 hr, more than twice the value of the median rotation period of field age brown dwarfs ($\sim$4 hr), a clear signature of brown dwarf rotational evolution.

[18]  arXiv:1804.06920 [pdf, ps, other]
Title: Errors, chaos and the collisionless limit
Comments: 16 pages, 9 figures
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Astrophysics of Galaxies (astro-ph.GA)

We simultaneously study the growth of errors and the question of the faithfulness of simulations of $N$-body systems. The errors are quantified through the numerical reversibility of trajectories of small-$N$ spherical systems integrated to high accuracy. Initially, the errors add randomly, before exponential divergence sets in. Though the exponentiation rate is virtually independent of $N$, the instability saturates at scales $1/\sqrt{N}$. This is interpreted by adopting a model due to Goodman, Heggie \& Hut (1993). In the third phase, the (diminished) growth is initially driven by multiplicative enhancement of errors as in the exponential stage. It is then qualitatively different for the errors in the phase space variables and the mean field conserved quantities (energy and momentum); the former grow systematically through phase mixing while the latter grow diffusively. For energy, the $N$-variation of the `relaxation time' of error growth follows expectations of two-body relaxation theory. This is not the case for angular momentum, at least up to the particle numbers and timescales considered, and even less so for the velocities. Due to increasingly smaller saturation scales, the information loss associated with the exponential instability decreases with $N$, especially when viewed in terms of the mean-field conserved quantities. Indeed, the dynamical entropy vanishes at any finite resolution as $N \rightarrow \infty$. In this sense there is convergence to the collisionless limit and confidence that numerical simulations may faithfully represent it, despite the exponential instability and loss of information on phase space trajectories. Nevertheless, the rapid initial growth of errors. and the relatively slow $N$-variation in its saturation, point to the slowness of the convergence.

[19]  arXiv:1804.06930 [pdf]
Title: How big is the Sun: Solar diameter changes over time
Comments: 9 pages, 2 figures
Journal-ref: Sun and Geosphere, 2018, Vol. 13, No 1, pp 63 - 68; ISSN 2367-8852
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The measurement of the Sun's diameter has been first tackled by the Greek astronomers from a geometric point of view. Their estimation of ~1800", although incorrect, was not truly called into question for several centuries. The first pioneer works for measuring the Sun's diameter with an astrometric precision were made around the year 1660 by Gabriel Mouton, then by Picard and La Hire. A canonical value of the solar radius of 959".63 was adopted by Auwers in 1891. Despite considerable efforts during the second half of the XXth century, involving dedicated space instruments, no consensus was reached on this issue. However, with the advent of high sensitivity instruments on board satellites, such as the Michelson Doppler Imager (MDI) on Solar and Heliospheric Observatory (SoHO) and the Helioseismic and Magnetic Imager (HMI) aboard NASA's Solar Dynamics Observatory (SDO), it was possible to extract with an unprecedented accuracy the surface gravity oscillation f modes, over nearly two solar cycles, from 1996 to 2017. Their analysis in the range of angular degree l=140-300 shows that the so-called "seismic radius" exhibits a temporal variability in anti-phase with the solar activity. Even if the link between the two radii (photospheric and seismic) can be made only through modeling, such measurements provide an interesting alternative which led to a revision of the standard solar radius by the International Astronomical Union in 2015. This new look on such modern measurements of the Sun's global changes from 1996 to 2017 gives a new way for peering into the solar interior, mainly to better understand the subsurface fields which play an important role in the implementation of the solar cycles.

[20]  arXiv:1804.06938 [pdf, ps, other]
Title: Mapping the accretion disc of the short period eclipsing binary SDSS J0926+3624
Comments: 7 pages, 6 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We report the analysis of time-series of optical photometry of SDSS J0926+3624 collected with the Liverpool Robotic Telescope between 2012 February and March while the object was in quiescence. We combined our median eclipse timing with those in the literature to revise the ephemeris and confirm that the binary period is increasing at a rate $\dot{P}=(3.2 \pm 0.4)\times 10^{-13} \, s/s$. The light curves show no evidence of either the orbital hump produced by a bright spot at disc rim or of superhumps; the average out-of-eclipse brightness level is consistently lower than previously reported. The eclipse map from the average light curve shows a hot white dwarf surrounded by a faint, cool accretion disc plus enhanced emission along the gas stream trajectory beyond the impact point at the outer disc rim, suggesting the occurrence of gas stream overflow/penetration at that epoch. We estimate a disc mass input rate of $\dot{M}=(9 \pm 1)\times 10^{-12}\,M_\odot \,yr^{-1}$, more than an order of magnitude lower than that expected from binary evolution with conservative mass transfer.

[21]  arXiv:1804.06942 [pdf, other]
Title: Polycrystalline Crusts in Accreting Neutron Stars
Comments: 8 pages, 4 figures, Submitted to ApJ, this article supersedes arXiv:1709.09260
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The crust of accreting neutron stars plays a central role in many different observational phenomena. In these stars, heavy elements produced by H-He burning in the rapid proton capture (rp-) process continually freeze to form new crust. In this paper, we explore the expected composition of the solid phase. We first demonstrate using molecular dynamics that two distinct types of chemical separation occur, depending on the composition of the rp-process ashes. We then calculate phase diagrams for three-component mixtures and use them to determine the allowed crust compositions. We show that, for the large range of atomic numbers produced in the rp-process ($Z\sim 10$--$50$), the solid that forms has only a small number of available compositions. We conclude that accreting neutron star crusts should be polycrystalline, with domains of distinct composition. Our results motivate further work on the size of the compositional domains, and have implications for crust physics and accreting neutron star phenomenology.

[22]  arXiv:1804.06956 [pdf, other]
Title: Deciphering the local Interstellar spectra of primary cosmic ray species with HelMod
Comments: 13 pages, 13 figures, 6 tables, ApJ in press
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Local interstellar spectra (LIS) of primary cosmic ray (CR) nuclei, such as helium, oxygen, and mostly primary carbon are derived for the rigidity range from 10 MV to ~200 TV using the most recent experimental results combined with the state-of-the-art models for CR propagation in the Galaxy and in the heliosphere. Two propagation packages, GALPROP and HelMod, are combined into a single framework that is used to reproduce direct measurements of CR species at different modulation levels, and at both polarities of the solar magnetic field. The developed iterative maximum-likelihood method uses GALPROP-predicted LIS as input to HelMod, which provides the modulated spectra for specific time periods of the selected experiments for model-data comparison. The interstellar and heliospheric propagation parameters derived in this study are consistent with our prior analyses using the same methodology for propagation of CR protons, helium, antiprotons, and electrons. The resulting LIS accommodate a variety of measurements made in the local interstellar space (Voyager 1) and deep inside the heliosphere at low (ACE/CRIS, HEAO-3) and high energies (PAMELA, AMS-02).

[23]  arXiv:1804.06961 [pdf, other]
Title: Primordial Mass and Density Segregation in a Young Molecular Cloud
Authors: Emilo J. Alfaro (1), Carlos Román-Zúñiga (2) ((1) Instituto de Astrofísica de Andalucía, Spain, (2) Instituto de Astronomía, UNAM, Mexico)
Comments: 6 pages, 5 figures. Submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We analyse the geometry of the Pipe Nebula, drawn by the distribution ($Q$-spatial parameter) and hierarchy ($\Lambda$ spatial segregation) of column density peaks previously detected and catalogued. By analysing the mass and volume density of the cores, we determine that both variables shown to be spatially segregated and with a high degree of substructure. Given the early evolutionary state of the Pipe Nebula, our results suggest that both, mass and volume-density segregations, may be primordial, in the sense of appearing in the early phases of the chain of physical mechanisms which conform the cluster-formation process. We also propose that volume density, and not mass, is the parameter that most clearly determines the initial spatial distribution of pre-stellar cores.

[24]  arXiv:1804.06982 [pdf, other]
Title: The Photometric Selection of M-dwarfs using Gaia, WISE and 2MASS photometry
Comments: Abstract cut down to fit character limit
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR)

We present criteria for the photometric selection of M-dwarfs using all-sky photometry, with a view to identifying M-dwarf candidates for inclusion in the input catalogues of upcoming all-sky surveys, including TESS and FunnelWeb. The criteria are based on Gaia, WISE and 2MASS all-sky photometry, and deliberately do not rely on astrometric information. In the lead-up to the availability of truly distance-limited samples following the release of Gaia DR2, this approach has the significant benefit of delivering a sample unbiased with regard to space velocity. Our criteria were developed by using Galaxia synthetic galaxy model predictions to evaluate both M-dwarf completeness and false-positive detections (i.e. non-M-dwarf contamination rates). In addition to the previously known sensitivity of J-H colour for giant-dwarf discrimination at cool temperatures, we find the WISE W1-W2 colour is also effective at discriminating M-dwarfs from cool giants. We have derived two sets of Gaia G > 14.5 criteria - a "high-completeness" set that contains 78,340 stars, of which 30.7-44.4% are expected to be M-dwarfs and contains 99.3% of the total number of expected M-dwarfs; and a "low-contamination" set that prioritises the stars most likely to be M-dwarfs at a cost of a reduction in completeness. This subset contains 40,505 stars and is expected to be comprised of 58.7-64.1% M-dwarfs, with a completeness of 98%. Comparison of the high-completeness set with the TESS Input Catalogue has identified 234 stars not currently in that catalogue, which preliminary analysis suggests could be useful M-dwarf targets for TESS. We also compared the criteria to selection via absolute magnitude and a combination of both methods. We found that colour selection in combination with an absolute magnitude limit provides the most effective way of selecting M-dwarfs en masse.

[25]  arXiv:1804.06986 [pdf, other]
Title: Astrometric Limits on the Stochastic Gravitational Wave Background
Comments: 30 pages, 5 figures, submitted to ApJ
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

The canonical methods for gravitational wave detection are ground- and space-based laser interferometry, pulsar timing, and polarization of the cosmic microwave background. But as has been suggested by numerous investigators, astrometry offers an additional path to gravitational wave detection. Gravitational waves deflect light rays of extragalactic objects, creating apparent proper motions in a quadrupolar (and higher order modes) pattern. Astrometry of extragalactic radio sources is sensitive to gravitational waves with frequencies between roughly $10^{-18}$ and $10^{-8}$ Hz ($H_0$ and 1/3 yr$^{-1}$), overlapping and bridging the pulsar timing and CMB polarization regimes. We present methodology for astrometric gravitational wave detection in the presence of large intrinsic uncorrelated proper motions (i.e., radio jets). We obtain 95% confidence limits on the stochastic gravitational wave background using 711 radio sources, $\Omega_{GW} < 0.0064$, and using 508 radio sources combined with the first Gaia data release: $\Omega_{GW} < 0.011$. These limits probe gravitational wave frequencies $6\times10^{-18}$ Hz $\lesssim f \lesssim 1\times10^{-9}$ Hz. Using a WISE-Gaia catalog of 567,721 AGN, we predict a limit expected from Gaia alone of $\Omega_{GW} < 0.0006$, which is significantly higher than originally forecast. We incidentally detect and report on 22 new examples of optical superluminal motion with redshifts 0.13-3.89.

[26]  arXiv:1804.06997 [pdf, other]
Title: An extreme ultraviolet wave generating upward secondary waves in a streamer-like solar structure
Comments: 5 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Extreme ultraviolet (EUV) waves, spectacular horizontally propagating disturbances in the low solar corona, always trigger horizontal secondary waves (SWs) when they encounter ambient coronal structure. We present a first example of upward SWs in a streamer-like structure after the passing of an EUV wave. The event occurred on 2017 June 1. The EUV wave happened during a typical solar eruption including a filament eruption, a CME, a C6.6 flare. The EUV wave was associated with quasi-periodic fast propagating (QFP) wave trains and a type II radio burst that represented the existence of a coronal shock. The EUV wave had a fast initial velocity of $\sim$1000 km s$^{-1}$, comparable to high speeds of the shock and the QFP wave trains. Intriguingly, upward SWs rose slowly ($\sim$80 km s$^{-1}$) in the streamer-like structure after the sweeping of the EUV wave. The upward SWs seemed to originate from limb brightenings that were caused by the EUV wave. All the results show the EUV wave is a fast-mode magnetohydrodynamic shock wave, likely triggered by the flare impulses. We suggest that part of the EUV wave was probably trapped in the closed magnetic fields of streamer-like structure, and upward SWs possibly resulted from the release of trapped waves in the form of slow-mode. It is believed that an interplay of the strong compression of the coronal shock and the configuration of the streamer-like structure is crucial for the formation of upward SWs.

[27]  arXiv:1804.06999 [pdf, other]
Title: New detections of (sub)millimeter hydrogen radio recombination lines towards high-mass star-forming clumps
Comments: 20 pages, 13 figures, 8 tables. Accepted for publication in A&A
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

Previous radio recombination line (RRL) observations of dust clumps identified in the APEX Telescope Large Area Survey of the Galaxy (ATLASGAL) have led to the detection of a large number of RRLs in the 3mm range. Here, we aim to study their excitation with shorter wavelength (sub)millimeter radio recombination line (submm-RRL) observations. We made observations of submm-RRLs with low principal quantum numbers ($n$ $\leq$ 30) using the APEX 12 m telescope, toward 104 HII regions associated with massive dust clumps from ATLASGAL. The observations covered the H25$\alpha$, H28$\alpha$, and H35$\beta$ transitions. Toward a small subsample the H26$\alpha$, H27$\alpha$, H29$\alpha$, and H30$\alpha$ lines were observed to avoid contamination by molecular lines at adjacent frequencies. We have detected submm-RRLs (signal-to-noise $\geq$ 3 $\sigma$) from compact HII regions embedded within 93 clumps. The submm-RRLs are approximately a factor of two brighter than the mm-RRLs and consistent with optically thin emission in local thermodynamic equilibrium (LTE). The average ratio (0.31) of the measured H35$\beta$/H28$\alpha$ fluxes is close to the LTE value of 0.28. No indication of RRL maser emission has been found. The Lyman photon flux, bolometric, and submm-RRL luminosities toward the submm-RRL detected sources present significant correlations. The trends of dust temperature and the ratio of bolometric luminosity to clump mass, $L_{\rm bol}/M_{\rm clump}$, indicate that the HII regions are related to the most massive and luminous clumps. By estimating the production rate of ionizing photons, $Q$, from the submm-RRL flux, we find that the $Q$(H28$\alpha$) measurements provide estimates of the Lyman continuum photon flux consistent with those determined from 5 GHz radio continuum emission.

[28]  arXiv:1804.07037 [pdf]
Title: An HI study of the collisional ring galaxy NGC 922
Comments: 11 pages, 8 figures, published in MNRAS
Journal-ref: 2018MNRAS.476.5681E
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present new atomic hydrogen (HI) observations of the collisional ring galaxy NGC 922 obtained using the Australia Telescope Compact Array. Our observations reveal for the first time the vast extent of the HI disc of this galaxy. The HI morphology and kinematics of NGC 922 show that this galaxy is not the product of a simple drop-through interaction, but has a more complex interaction history. The integrated HI flux density of NGC 922 from our observations is 24.7 Jy km s$^{-1}$, which is within the error of the flux value obtained using the $64$-m Parkes radio telescope. This flux density translates to a total HI mass of $1.1*10^{10}$ M$_{\circ}$ and corresponds to an HI to total mass fraction (M$_{HI}$/M$_{tot}$) of approximately $0.11$. The gaseous structures of NGC 922 are more extended to the north and include an HI tail that has a projected physical length of $8$ kpc. Gas warps are also evident in the velocity field of NGC 922 and are more prominent on the approaching and the western side of the disc. In comparison with a large sample of star-forming galaxies in the local Universe, NGC 922 possesses a high gas fraction relative to galaxies with a similar stellar mass of ~$10^{10.4}$ M$_{\circ}$, and exhibits a high specific star formation rate.

[29]  arXiv:1804.07039 [pdf, other]
Title: The GAPS Programme with HARPS-N at TNG XVII. Line profile indicators and kernel regression as diagnostics of radial-velocity variations due to stellar activity in solar-like stars
Comments: 15 pages, 6 figures, 5 tables, one Appendix, accepted to Astronomy and Astrophysics
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Solar and Stellar Astrophysics (astro-ph.SR)

Stellar activity is the ultimate source of radial-velocity (RV) noise in the search for Earth-mass planets orbiting late-type main-sequence stars. We analyse the performance of four different indicators and the chromospheric index $\log R'_{\rm HK}$ in detecting RV variations induced by stellar activity in 15 slowly rotating ($v\sin i \leq 5$ km/s), weakly active ($\log R'_{\rm HK} \leq -4.95$) solar-like stars observed with the high-resolution spectrograph HARPS-N. We consider indicators of the asymmetry of the cross-correlation function (CCF) between the stellar spectrum and the binary weighted line mask used to compute the RV, that is the bisector inverse span (BIS), $\Delta V$, and a new indicator $V_{\rm asy(mod)}$ together with the full width at half maximum (FWHM) of the CCF. We present methods to evaluate the uncertainties of the CCF indicators and apply a kernel regression (KR) between the RV, the time, and each of the indicators to study their capability of reproducing the RV variations induced by stellar activity. The considered indicators together with the KR prove to be useful to detect activity-induced RV variations in $47 \pm 18$ percent of the stars over a two-year time span when a significance (two-sided p-value) threshold of one percent is adopted. In those cases, KR reduces the standard deviation of the RV time series by a factor of approximately two. The BIS, the FWHM, and the newly introduced $V_{\rm asy(mod)}$ are the best indicators, being useful in $27 \pm 13$, $13 \pm 9$, and $13 \pm 9$ percent of the cases, respectively. The relatively limited performances of the activity indicators are related to the very low activity level and $v\sin i$ of the considered stars. For the application of our approach to sun-like stars, a spectral resolution of at least $10^5$ and highly stabilized spectrographs are recommended.

[30]  arXiv:1804.07049 [pdf, other]
Title: A Predicted Astrometric Microlensing Event by a Nearby White Dwarf
Authors: P. McGill (1), L.C. Smith (1), N.W. Evans (1), V. Belokurov (1), R.L. Smart (2) ((1) IoA, Cambridge, (2) Turin)
Comments: 5 pages, MNRAS (Letters), accepted
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

We used the Tycho-Gaia Astrometric Solution catalogue, part of the Gaia Data Release 1, to search for candidate astrometric microlensing events expected to occur within the remaining lifetime of the Gaia satellite. Our search yielded one promising candidate. We predict that the nearby DQ type white dwarf LAWD 37 (WD 1142-645) will lens a background star and will reach closest approach on November 11th 2019 ($\pm$ 4 days) with impact parameter $380\pm10$ mas. This will produce an apparent maximum deviation of the source position of $2.8\pm0.1$ mas. In the most propitious circumstance, Gaia will be able to determine the mass of LAWD 37 to $\sim3\%$. This mass determination will provide an independent check on atmospheric models of white dwarfs with helium rich atmospheres, as well as tests of white dwarf mass radius relationships and evolutionary theory.

[31]  arXiv:1804.07050 [pdf, other]
Title: The Shards of $ω$ Centauri
Authors: G.C. Myeong (1), N.W. Evans (1), V. Belokurov (1), J.L. Sanders (1), S.E. Koposov (1,2) ((1) IoA, Cambridge, (2) CMU, Pittsburgh)
Comments: MNRAS, 6 pages, submitted
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We use the SDSS-Gaia catalogue to search for material stripped off the atypical globular cluster $\omega$ Centauri. As actions are conserved under slow changes of the potential, this allows identification of groups of stars with a common accretion history. We devise a method to assess the significance of halo substructures based on their clustering in action space, using metallicity as a secondary check. This is validated against smooth models and numerically constructed stellar halos. We identify a number of new substructures in the SDSS-Gaia catalogue, including 7 high significance, high energy and retrograde ones. Using a simple model of the accretion of the progenitor of the $\omega$ Centauri, we argue for the possible association of up to 4 of our new substructures (labelled Rg1, Rg3, Rg4 and Rg6) with this event. This sets a minimum mass of $5 \times 10^8 M_\odot$ for the progenitor, so as to bring $\omega$ Centauri to its current location in action -- energy space. Our proposal can be tested by high resolution spectroscopy of the candidates to look for the unusual abundance patterns possessed by $\omega$ Centauri stars.

[32]  arXiv:1804.07060 [pdf, other]
Title: Limits on radio emission from meteors using the MWA
Comments: Accepted for Publication in MNRAS on 11 April 2018, 11 pages, 11 figures, 5 tables
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Instrumentation and Methods for Astrophysics (astro-ph.IM)

Recently, low frequency, broadband radio emission has been observed accompanying bright meteors by the Long Wavelength Array (LWA). The broadband spectra between 20 and 60 MHz were captured for several events, while the spectral index (dependence of flux density on frequency, with $S_\nu \propto \nu^\alpha$) was estimated to be $-4\pm1$ during the peak of meteor afterglows. Here we present a survey of meteor emission and other transient events using the Murchison Widefield Array (MWA) at 72-103 MHz. In our 322-hour survey, down to a $5\sigma$ detection threshold of 3.5 Jy/beam, no transient candidates were identified as intrinsic emission from meteors. We derived an upper limit of -3.7 (95% confidence limit) on the spectral index in our frequency range. We also report detections of other transient events, like reflected FM broadcast signals from small satellites, conclusively demonstrating the ability of the MWA to detect and track space debris on scales as small as 0.1 m in low Earth orbits.

[33]  arXiv:1804.07063 [pdf, ps, other]
Title: MultiDark-Clusters: Galaxy Cluster Mock Light-Cones, eROSITA and the Cluster Power Spectrum
Comments: 21 pages, 13 figures, 2 tables. Comments are welcome
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Cosmological simulations are fundamental tools to study structure formation and the astrophysics of evolving structures, in particular clusters of galaxies. While hydrodynamical simulations cannot sample efficiently large volumes and explore different cosmologies at the same time, N-body simulations lack the baryonic physics that is crucial to determine the observed properties of clusters. One solution is to use (semi-)analytical models to implement the needed baryonic physics. In this way, we can generate the many mock universes that will be required to fully exploit future large sky surveys, such as that from the upcoming eROSITA X-ray telescope. We developed a phenomenological model based on observations of clusters to implement gas density and temperature information on the dark-matter-only halos of the MultiDark simulations. We generate several full-sky mock light-cones of clusters for the WMAP and Planck cosmologies, adopting different parameters in our phenomenological model of the intra-cluster medium. For one of these simulations and models, we also generate 100 light-cones corresponding to 100 random observers and explore the variance among them in several quantities. In this first paper on MultiDark mock galaxy cluster light-cones, we focus on presenting our methodology and discuss predictions for eROSITA, in particular, exploring the potential of angular power spectrum analyses of its detected (and undetected) cluster population to study X-ray scaling relations, the intra-cluster medium, and the composition of the cosmic X-ray background. We make publicly available on-line more than 400 GB of light-cones, which include the expected eROSITA count rate, on Skies & Universes (this http URL).

[34]  arXiv:1804.07072 [pdf, ps, other]
Title: Star formation in the outskirts of DDO 154: A top-light IMF in a nearly dormant disc
Comments: 14 pages, 15 figures, accepted for publication in MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present optical photometry of Hubble Space Telescope (HST) ACS/WFC data of the resolved stellar populations in the outer disc of the dwarf irregular galaxy DDO 154. The photometry reveals that young main sequence stars are almost absent from the outermost HI disc. Instead, most are clustered near the main stellar component of the galaxy. We constrain the stellar initial mass function (IMF) by comparing the luminosity function of the main sequence stars to simulated stellar populations assuming a constant star formation rate over the dynamical timescale. The best-fitting IMF is deficient in high mass stars compared to a canonical Kroupa IMF, with a best-fit slope $\alpha = -2.45$ and upper mass limit $M_U = 16\ M_{\odot}$. This top-light IMF is consistent with predictions of the Integrated Galaxy-wide IMF theory. Combining the HST images with HI data from The HI Nearby Galaxy Survey Treasury (THINGS) we determine the star formation law (SFL) in the outer disc. The fit has a power law exponent $N = 2.92 \pm0.22$ and zero point $A=4.47 \pm 0.65 \times 10^{-7} \ M_{\odot} \ \text{yr}^{-1} \ \text{kpc}^{-2}$. This is depressed compared to the Kennicutt-Schmidt Star Formation Law, but consistent with weak star formation observed in diffuse HI environments. Extrapolating the SFL over the outer disc implies that there could be significant star formation occurring that is not detectable in H$\alpha$. Last, we determine the Toomre stability parameter $Q$ of the outer disc of DDO 154 using the THINGS HI rotation curve and velocity dispersion map. 72% of the HI in our field has $Q\leq 4$ and this incorporates 96% of the observed MS stars. Hence 28% of the HI in the field is largely dormant.

[35]  arXiv:1804.07081 [pdf, ps, other]
Title: A new method to quantify differentiate collapse models of star formation
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Continuum emissions from dust grains are used as a general probe to constrain the initial physical conditions of molecular dense cores where new stars may born. To get as much information as possible from dust emissions, we have developed a tool, named as $COREGA$, which is capable of identifying positions of dense cores, optimizing a three-dimensional model for the dense cores with well characterized uncertainties. $COREGA$ can also estimate the physical properties of dense cores, such as density, temperature, and dust emissivity, through analyzing multi-wavelength dust continuum data sets. In the numerical tests on $COREGA$, the results of fitting simulated data are consistent with initial built-in parameters. We also demonstrate $COREGA$ by adding random gaussian noises with Monte Carlo methods and show that the results are stable against varying observational noise intensities within certain levels. A beam size $<$ 3 arcsec and rms $<$ 0.2mJy/pixel (1 pixel = 0.1") is needed for ALMA to distinguish different collapse models, such as power law and Bonner-Ebert sphere, during continuum observations of massive dense cores in Orion molecular cloud. Based on its advanced algorithm, $COREGA$ is capable of giving a quick and deep analysis on dust cores.

[36]  arXiv:1804.07083 [pdf, other]
Title: Revisiting the stellar mass -- angular momentum -- morphology relation: extension to higher bulge fraction, and the effect of bulge type
Comments: Accepted to ApJ. 10 pages, 3 figures, 2 tables
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We present the relation between stellar specific angular momentum $j_*$, stellar mass $M_*$, and bulge-to-total light ratio $\beta$ for THINGS, CALIFA and Romanowsky \& Fall datasets, exploring the existence of a fundamental plane between these parameters as first suggested by Obreschkow \& Glazebrook. Our best-fit $M_*-j_*$ relation yields a slope of $\alpha = 1.03 \pm 0.11$ with a trivariate fit including $\beta$. When ignoring the effect of $\beta$, the exponent $\alpha = 0.56 \pm 0.06$ is consistent with $\alpha = 2/3$ predicted for dark matter halos. There is a linear $\beta - j_*/M_*$ relation for $\beta \lesssim 0.4$, exhibiting a general trend of increasing $\beta$ with decreasing $j_*/M_*$. Galaxies with $\beta \gtrsim 0.4$ have higher $j_*$ than predicted by the relation. Pseudobulge galaxies have preferentially lower $\beta$ for a given $j_*/M_*$ than galaxies that contain classical bulges. Pseudobulge galaxies follow a well-defined track in $\beta - j_*/M_*$ space, consistent with Obreschkow \& Glazebrook, while galaxies with classical bulges do not. These results are consistent with the hypothesis that while growth in either bulge type is linked to a decrease in $j_*/M_*$, the mechanisms that build pseudobulges seem to be less efficient at increasing bulge mass per decrease in specific angular momentum than those that build classical bulges.

[37]  arXiv:1804.07086 [pdf, ps, other]
Title: Globular clusters: absolute proper motions and Galactic orbits
Comments: 14 pages, 14 figures, 3 tables, accepted to Astrophysical Bulletin
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We cross-match objects from several different astronomical catalogs to determine the absolute proper proper motions of stars within the 30-arcmin radius fields of 115 Milky-Way globular clusters with the accuracy of 1--2~mas/yr. The proper motions are based on positional data recovered from the USNO-B1, 2MASS, URAT1, ALLWISE, UCAC5, and GAIA DR1 surveys with up to 10 positions spanning an epoch difference of up to $\sim$~65~years, and reduced to GAIA DR1 TGAS frame using UCAC5 as the reference catalog. Cluster members are photometrically identified by selecting horizontal- and red-giant branch stars on color-magnitude diagrams, and the mean absolute proper motions of the clusters with a typical formal error of $\sim$~0.4~mas/yr are computed by averaging the proper motions of selected members. The inferred absolute proper motions of clusters are combined with available radial-velocity data and heliocentric distance estimates to compute the cluster orbits in terms of the Galactic potential models based on Miyamoto and Nagai disk, Hernquist spheroid, and modified isothermal dark-matter halo (axisymmetric model without a bar) and the same model + rotating Ferre's bar (non-axisymmetric). Five distant clusters have higher-than-escape velocities, most likely due to large errors of computed transversal velocities, whereas the computed orbits of all other clusters remain bound to the Galaxy. Unlike previously published results, we find the bar to affect substantially the orbits of most of the clusters, even those at large Galactocentric distances, bringing appreciable chaotization, especially in the portions of the orbits close to the Galactic center, and stretching out the orbits of some of the thick-disk clusters.

[38]  arXiv:1804.07089 [pdf, ps, other]
Title: Two new magnetic cataclysmic variables discovered in the 3XMM catalogue
Comments: 8 pages, 11 figures, A&A, in press
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

X-ray catalogues provide a wealth of information on many source types, ranging from compact objects to galaxies, clusters of galaxies, stars, and even planets. Thanks to the huge volume of X-ray sources provided in the 3XMM catalogue, along with many source specific products, many new examples from rare classes of sources can be identified. Through visualising spectra and lightcurves from about 80 observations included in the incremental part of the 3XMM catalogue, 3XMM-DR5, as part of the quality control of the catalogue, we identified two new X-ray sources, 3XMM J183333.1+225136 and 3XMM J184916.1+652943, that were highly variable. This work aims to investigate their nature. Through simple model fitting of the X-ray spectra and analysis of the X-ray lightcurves of 3XMM J183333.1+225136 and 3XMM J184916.1+652943, along with complementary photometry from the XMM-Newton Optical Monitor, Pan-Starrs and the Stella/WiFSIP and Large Binocular Telescope (LBT) spectra, we suggest that the two sources might be magnetic cataclysmic variables (CVs) of the polar type and we determine some of their properties. Both CVs have very hard spectra, showing no soft excess. They are both situated in the local neighbourhood, located within $\sim$1 kpc. 3XMM J183333.1+225136 has an orbital period of 2.15 hours. It shows features in the lightcurve that may be a total eclipse of the white dwarf. 3XMM J184916.1+652943 has an orbital period of 1.6 hours. Given that only a small sky area was searched to identify these CVs, future sensitive all sky surveys such as the eROSITA project should be very successful at uncovering large numbers of such sources.

[39]  arXiv:1804.07092 [pdf, other]
Title: Non-conservative mass transfer in stellar evolution and the case of V404 Cyg/GS 2023+338
Comments: submitted to MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR)

We consider donor evolution and mass transfer in the microquasar V404 Cyg/GS 2023+338. Based on X-ray observations of its two outbursts, its average mass accretion rate is substantially lower than the theoretical mass-loss rate from its low-mass giant donor. A likely solution to this discrepancy is that a large fraction of the mass flowing from the donor leaves the binary in the form of accretion-associated outflows. We derive an analytical formula describing changes of the donor Roche lobe radius due to its mass change for non-conservative mass transfer, which we parameterize by the fractions of the mass and angular momentum leaving the system. We perform evolutionary calculations for V 404 Cyg. Given our estimated average accretion rate, $>$70 per cent of the mass lost from the donor has to leave the binary. The allowed solution for the actual donor mass loss rate is parameterized by our two outflow parameters. Our results are in agreement with the observed outflows from the outer disc as well as with the variable near-Eddington accretion observed during the outbursts, compatible with outflows from the vicinity of the black hole. In the latter case, the outflowing matter has a negligible angular momentum.

[40]  arXiv:1804.07120 [pdf, ps, other]
Title: Multicolor photometry and spectroscopy of the yellow supergiant with dust envelope HD 179821=V1427 Aquilae
Comments: 30 pages, 12 figures, Accepted for publication in Astronomy Letters
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We present the results of multicolor (UBVJHKLM) photometry (2009-2017) and low-resolution spectroscopy (2016-2017) of the semi-regular variable V1427 Aql=HD 179821, a yellow supergiant with gas-dust envelope. The star displays low-amplitude ({\Delta}V<0.2 mag) semi-periodic brightness variation superimposed on a long-term trend. The light curve shape and timescale change from cycle to cycle. There are temperature variations characteristic for pulsations, and brightness oscillations with no significant change of color are also observed. The UBV data for the 2009-2011 interval are well reproduced by a superposition of two periodic components with P=170 d and P=141 d (or P=217 d - the one year alias of P=141 d). The variation became less regular after 2011, the timescale increased and exceeded 250 days. In 2009-2016 the annual average brightness monotonically increased in V, J, K, whereas it decreased in U and B. The annual average U-B, B-V, and J-K colors grew, the star was getting redder. The cooling and expanding of the star photosphere along with the increasing of luminosity may explain the long-term trend in brightness and colors. Based on our photometric data we suppose that the photosphere temperature decreased by ~400 K in the 2008-2016 interval, the radius increased by ~24 %, and the luminosity grew by ~19 %. Low-resolution spectra in the {\lambda}4000-9000 {\AA} wavelength range obtained in 2016-2017 indicate significant changes in the spectrum of V1427 Aql as compared with the 1994-2008 interval, i.e. the BaII and near-infrared CaII triplet absorptions have turned stronger while the OI {\lambda}7771-4 triplet blend has weakened that points out the decrease of temperature in the region where the absorptions are formed. The evolutionary stage of the star is discussed. We also compare V1427 Aql with post-AGB stars and yellow hypergiants.

[41]  arXiv:1804.07122 [pdf, ps, other]
Title: Statistical Study of Solar White-light Flares and Comparison with Superflares on Solar-type Stars
Comments: 4 pages, 1 figures, paper of the Proceedings of the IAUS340, accepted
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Earth and Planetary Astrophysics (astro-ph.EP)

Recently, many superflares on solar-type stars were discovered as white-light flares (WLFs). A correlation between the energies (E) and durations (t) of superflares is derived as $t\propto E^{0.39}$, and this can be theoretically explained by magnetic reconnection ($t\propto E^{1/3}$). In this study, we carried out a statistical research on 50 solar WLFs with SDO/HMI to examine the t-E relation. As a result, the t-E relation on solar WLFs ($t\propto E^{0.38}$) is quite similar stellar superflares, but the durations of stellar superflares are much shorter than those extrapolated from solar WLFs. We present the following two interpretations; (1) in solar flares, the cooling timescale of WL emission may be longer than the reconnection one, and the decay time can be determined by the cooling timescale; (2) the distribution can be understood by applying a scaling law $t\propto E^{1/3}B^{-5/3}$ derived from the magnetic reconnection theory.

[42]  arXiv:1804.07124 [pdf, other]
Title: Primordial Black Holes from Inflation and Quantum Diffusion
Comments: 20+9 pages, 12 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph)

Primordial black holes as dark matter may be generated in single-field models of inflation thanks to the enhancement at small scales of the comoving curvature perturbation. This mechanism requires leaving the slow-roll phase to enter a non-attractor phase during which the inflaton travels across a plateau and its velocity drops down exponentially. We argue that quantum diffusion has a significant impact on the primordial black hole mass fraction making the classical standard prediction not trustable.

[43]  arXiv:1804.07126 [pdf, other]
Title: Prototype of a laser guide star wavefront sensor for the Extremely Large Telescope
Journal-ref: Monthly Notices of the Royal Astronomical Society, Volume 477, Issue 1, 11 June 2018, Pages 539-547
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

The new class of large telescopes, as the future ELT, are designed to work with Laser Guide Star (LGS) tuned to a resonance of atmosphere sodium atoms. This wavefront sensing technique presents complex issues for an application to big telescopes due to many reasons mainly linked to the finite distance of the LGS, the launching angle, Tip-tilt indetermination and focus anisoplanatism. The implementation of a laboratory Prototype for LGS wavefront sensor (WFS) at the beginning of the phase study of MAORY, the Multi-conjugate Adaptive Optics RelaY for the ELT first light, has been indispensable to investigate specific mitigation strategies to the LGS WFS issues. This paper shows the test results of LGS WFS Prototype under different working conditions. The accuracy within which the LGS images are generated on the Shack-Hartmann (SH) WFS has been cross-checked with the MAORY simulation code. The experiments show the effect of noise on the centroiding precision, the impact of LGS image truncation on the wavefront sensing accuracy as well as the temporal evolution of sodium density profile and LGS image under-sampling.

[44]  arXiv:1804.07137 [pdf, other]
Title: The Peculiar Atmospheric Chemistry of KELT-9b
Comments: 7 pages, 5 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Atmospheric and Oceanic Physics (

The atmospheric temperatures of the very hot Jupiter KELT-9b straddle the transition between gas giants and stars, and therefore between two traditionally distinct regimes of atmospheric chemistry. Despite the high ultraviolet flux from KELT-9, we show using photochemical kinetics calculations that the observable atmosphere of KELT-9b is predicted to be close to chemical equilibrium, which greatly simplifies any theoretical interpretation of its spectra. It also makes the atmosphere of KELT-9b, which is expected to be cloudfree, a tightly constrained chemical system that lends itself to a clean set of theoretical predictions. Due to the lower pressures probed in transmission spectroscopy, we predict the abundance of water to vary by several orders of magnitude across the atmospheric limb depending on temperature, which makes water a sensitive thermometer. Carbon monoxide is predicted to be the dominant molecule under a wide range of scenarios, rendering it a robust diagnostic of the metallicity when analyzed in tandem with water. All of the other usual suspects (acetylene, ammonia, carbon dioxide, hydrogen cyanide, methane) are predicted to be subdominant at solar metallicity, while atomic oxygen, iron and magnesium are predicted to have relative abundances as high as 1 part in 10,000. Neutral atomic iron is predicted to be seen through a forest of optical and near-infrared lines, which makes KELT-9b suitable for high-resolution ground-based spectroscopy with HARPS-N or CARMENES. We summarize the future observational prospects of characterizing the atmosphere of KELT-9b with the Hubble Space Telescope, James Webb Space Telescope and CHEOPS.

[45]  arXiv:1804.07138 [pdf, other]
Title: A 15.7-Minute AM CVn Binary Discovered in K2
Comments: 13 pages, 11 figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We present the discovery of SDSS J135154.46-064309.0, a short-period variable observed using 30-minute cadence photometry in K2 Campaign 6. Follow-up spectroscopy and high-speed photometry support a classification as a new member of the rare class of ultracompact accreting binaries known as AM CVn stars. The spectroscopic orbital period of $15.65 \pm 0.12$\,minutes makes this system the fourth-shortest period AM CVn known, and the second system of this type to be discovered by the Kepler spacecraft. The K2 data show photometric periods at $15.7306 \pm 0.0003$\,minutes, $16.1121 \pm 0.0004$\,minutes and $664.82 \pm 0.06$\,minutes, which we identify as the orbital period, superhump period, and disc precession period, respectively. From the superhump and orbital periods we estimate the binary mass ratio $q = M_2/M_1 = 0.111 \pm 0.005$, though this method of mass ratio determination may not be well calibrated for helium-dominated binaries. This system is likely to be a bright foreground source of gravitational waves in the frequency range detectable by LISA, and may be of use as a calibration source if future studies are able to constrain the masses of its stellar components.

[46]  arXiv:1804.07140 [pdf, ps, other]
Title: A Light Curve Analysis of Gamma-ray Nova V959 Mon -- Distance and White Dwarf Mass
Comments: 18 pages including 9 figures, to appear in the Astrophysical Journal
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); High Energy Astrophysical Phenomena (astro-ph.HE)

V959 Mon is one of the gamma-ray detected novae. It was optically discovered about 50 days after the gamma-ray detection due to proximity to the Sun. The nova speed class is unknown because of lack of the earliest half of optical light curve and short supersoft X-ray phase due to eclipse by the disk rim. Using the universal decline law and time-stretching method, we analyzed the data of V959 Mon and obtained nova parameters. We estimated the distance modulus in the V band to be (m-M)_V=13.15\pm0.3 for the reddening of E(B-V)=0.38\pm0.01 by directly comparing with the similar type of novae, LV Vul, V1668 Cyg, IV Cep, and V1065 Cen. The distance to V959 Mon is 2.5\pm0.5 kpc. If we assume that the early phase light curve of V959 Mon is the same as that of time-stretched light curves of LV Vul, our model light curve fitting suggests that the white dwarf (WD) mass is 0.9-1.15 M_sun, being consistent with a neon nova identification. At the time of gamma-ray detection the photosphere of nova envelope extends to 5-8 R_sun (about two or three times the binary separation) and the wind mass-loss rate is (3-4)\times 10^{-5} M_sun yr^{-1}. The period of hard X-ray emission is consistent with the time of appearance of the companion star from the nova envelope. The short supersoft X-ray turnoff time is consistent with the epoch when the WD photosphere shrank to behind the elevating disk rim, that occurs 500 days before nuclear burning turned off.

[47]  arXiv:1804.07154 [pdf, other]
Title: Testing Dark energy models in the light of $σ_8$ tension
Comments: 17 pages, 15 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

It has been pointed out that there exists a tension in $\sigma_8-\Omega_m$ measurement between CMB and LSS observation. In this paper we show that $\sigma_8-\Omega_m$ observations can be used to test the dark energy theories. We study two models, (1) Hu-Sawicki(HS) Model of $f(R)$ gravity and (2) Chavallier-Polarski-Linder(CPL) parametrization of dynamical dark energy (DDE), both of which satisfy the constraints from supernovae. We compute $\sigma_8$ consistent with the parameters of these models. We find that the well known tension in $\sigma_8$ between Planck CMB and large scale structure (LSS) observations is (1) exacerbated in the HS model and (2) somewhat alleviated in the DDE model. We illustrate the importance of the $\sigma_8$ measurements for testing modified gravity models. Modified gravity models change the matter power spectrum at cluster scale which also depends upon the neutrino mass. We present the bound on neutrino mass in the HS and DDE model.

[48]  arXiv:1804.07200 [pdf, other]
Title: Silence measurements and measures for ET: characterisation of long term seismic noise in the Mátra Mountains
Comments: 30 pages, 18 figures
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); High Energy Physics - Experiment (hep-ex); Geophysics (physics.geo-ph)

The analysis of long term seismological data collected underground in the M\'atra Mountains, Hungary, using the facilities of the M\'atra Gravitational and Geophysical Laboratory (MGGL) is reported. The laboratory is situated inside the Gy\"ongy\"osoroszi mine, Hungary, 88m below the surface. This study focuses on the requirements of the Einstein Telescope (ET), one of the planned third generation gravitational wave observatories, which is designed for underground operation. After a short introduction of the geophysical environment the evaluation of the collected long term data follows including the comparison of a two-week measurement campaign deeper in the mine. Based on our analysis and considering the specialities of long term data collection, refinements of the performance and evaluation criteria are suggested as well as performance estimation of a possible M\'atra site.

[49]  arXiv:1804.07216 [pdf]
Title: The Protons in Primary Cosmic Rays in the Energy Range 10^15-10^17 eV According to Data from the PAMIR Experiment
Journal-ref: 32nd International Cosmic Ray Conference, Beijing 2011
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); High Energy Physics - Experiment (hep-ex); Instrumentation and Detectors (physics.ins-det); Space Physics (

Adjusted data on the fraction of protons in the mass composition of primary cosmic rays (PCRs) in the energy range of 10^15-10^17 eV are presented. Adjustments are made according to detailed calculations of the response of the X-ray emulsion chamber in the PAMIR experiment. It is demonstrated that the fraction of protons in a PCR is 16-18% for E0 = 10^15-10^16 eV and does not change within the error for E0 = 10^16-10^17 eV.

[50]  arXiv:1804.07217 [pdf, other]
Title: The GALAH Survey: Velocity fluctuations in the Milky Way using red clump giants
Comments: 21 pages, 16 figures, submitted to MNRAS
Subjects: Astrophysics of Galaxies (astro-ph.GA)

If the Galaxy is axisymmetric and in dynamical equilibrium, we expect negligible fluctuations in the residual line-of-sight velocity field. However, non-axisymmetric structures like a bar, spiral arms and merger events can generate velocity fluctuations. Recent results using the APOGEE survey find significant fluctuations in velocity for stars in the midplane (|z|< 0.25 kpc) and out to 5 kpc, which suggests that the dynamical influence of the Milky Way's bar extends out to the Solar neighborhood. Their measured power spectrum has a characteristic amplitude of 11 km/s on a scale of ~ 2.5 kpc. The existence of large streaming motions on these scales has important implications for determining the Sun's motion about the Galactic Centre. Using red clump stars from the GALAH and APOGEE surveys, we map the line-of-sight velocity field around the Sun out to distances of 5 kpc and up to 1.25 kpc from the Galactic Plane. By subtracting a smooth axisymmetric model for the velocity field, we study the residual velocity fluctuations and compare our findings with mock survey generated by Galaxia based on an axisymmetric, steady state model. We find negligible large-scale fluctuations away from the plane. In the mid-plane, we reproduce the earlier APOGEE power spectrum results but with 20\% smaller amplitude (9.5 km/s) after taking a few systematic effects into account (e.g. volume completeness). The amplitude power is further reduced to 6.7 km/s if a flexible axisymmetric model is used. Additionally, our mock simulations show that, in the plane, the distances are underestimated for high mass red clump stars and this can lead to spurious power with amplitude of about 5.5 km/s. Taking this into account, we estimate the amplitude of real fluctuations to be less than 4.2 km/s, about a factor of three less than the previous result from APOGEE.

[51]  arXiv:1804.07222 [pdf, other]
Title: Eclipsing damped Ly$α$ systems in the Sloan Digital Sky Survey Data Release 12
Comments: Accepted for publication in MNRAS, 16 pages, 20 figures, 3 tables. Full version of Tables 1 and 2 can be accessed via this http URL
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We present the results of our automatic search for proximate damped Ly$\alpha$ absorption (PDLA) systems in the quasar spectra from the Sloan Digital Sky Survey Data Release 12. We constrain our search to those PDLAs lying within 1500 km s$^{-1}$ from the quasar to make sure that the broad DLA absorption trough masks most of the strong Ly$\alpha$ emission from the broad line region (BLR) of the quasar. When the Ly$\alpha$ emission from the BLR is blocked by these so-called eclipsing DLAs, narrow Ly$\alpha$ emission from the host galaxy could be revealed as a narrow emission line (NEL) in the DLA trough. We define a statistical sample of 399 eclipsing DLAs with log$N$(HI)$\ge$21.10. We divide our statistical sample into three subsamples based on the strength of the NEL detected in the DLA trough. By studying the stacked spectra of these subsamples, we found that absorption from high ionization species are stronger in DLAs with stronger NEL in their absorption core. Moreover, absorption from the excited states of species like SiII are also stronger in DLAs with stronger NEL. We also found no correlation between the luminosity of the Ly$\alpha$ NEL and the quasar luminosity. These observations are consistent with a scenario in which the DLAs with stronger NEL are denser and physically closer to the quasar. We propose that these eclipsing DLAs could be the product of the interaction between infalling and outflowing gas. High resolution spectroscopic observation would be needed to shed some light on the nature of these eclipsing DLAs.

[52]  arXiv:1804.07229 [pdf, other]
Title: Identification of meteorite source regions in the Solar System
Comments: 24 pages, 10 figures, accepted for publication in Icarus
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Over the past decade there has been a large increase in the number of automated camera networks that monitor the sky for fireballs. One of the goals of these networks is to provide the necessary information for linking meteorites to their pre-impact, heliocentric orbits and ultimately to their source regions in the solar system. We re-compute heliocentric orbits for the 25 meteorite falls published to date from original data sources. Using these orbits, we constrain their most likely escape routes from the main asteroid belt and the cometary region by utilizing a state-of-the-art orbit model of the near-Earth-object population, which includes a size-dependence in delivery efficiency. While we find that our general results for escape routes are comparable to previous work, the role of trajectory measurement uncertainty in escape-route identification is explored for the first time. Moreover, our improved size-dependent delivery model substantially changes likely escape routes for several meteorite falls, most notably Tagish Lake which seems unlikely to have originated in the outer main belt as previously suggested. We find that reducing the uncertainty of fireball velocity measurements below $\sim0.1$ km/s does not lead to reduced uncertainties in the identification of their escape routes from the asteroid belt and, further, their ultimate source regions. This analysis suggests that camera networks should be optimized for the largest possible number of meteorite recoveries with measured speed precisions of order 0.1 km/s.

[53]  arXiv:1804.07245 [pdf, ps, other]
Title: Meteor showers from active asteroids and dormant comets in near-Earth space: a review
Authors: Quan-Zhi Ye
Comments: PSS in press
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Small bodies in the solar system are conventionally classified into asteroids and comets. However, it is recently found that a small number of objects can exhibit properties of both asteroids and comets. Some are more consistent with asteroids despite episodic ejections and are labeled as "active asteroids", while some might be aging comets with depleting volatiles. Ejecta produced by active asteroids and/or dormant comets are potentially detectable as meteor showers at the Earth if they are in Earth-crossing orbits, allowing us to retrieve information about the historic activities of these objects. Meteor showers from small bodies with low and/or intermittent activities are usually weak, making shower confirmation and parent association challenging. We show that statistical tests are useful for identifying likely parent-shower pairs. Comprehensive analyses of physical and dynamical properties of meteor showers can lead to deepen understanding on the history of their parents. Meteor outbursts can trace to recent episodic ejections from the parents, and "orphan" showers may point to historic disintegration events. The flourish of NEO and meteor surveys during the past decade has produced a number of high-confidence parent-shower associations, most have not been studied in detail. More work is needed to understand the formation and evolution of these parent-shower pairs.

[54]  arXiv:1804.07260 [pdf, ps, other]
Title: Magnetic and pulsational variability of Przybylski's star (HD101065)
Comments: 11 pages, 4 tables, 7 figures, accepted for publication in MNRAS
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Since its discovery more than half a century ago Przybylski's star (HD101065) continues to excite the astronomical community by the unusual nature of its spectrum, exhibiting exotic element abundances. This star was also the first magnetic chemically peculiar A-type star for which the presence of rapid oscillations was established. Our analysis of newly acquired and historic longitudinal magnetic field measurements indicates that Przybylski's star is also unusual with respect to its extremely slow rotation. Adopting a dipolar structure for the magnetic field and using a sine wave fit to all reported longitudinal magnetic field values over the last 43yr, we find a probable rotation period P_rot ~ 188yr, which however has to be considered tentative as it does not represent a unique solution and has to be verified by future observations. Additionally, based on our own spectropolarimetric material obtained with HARPSpol, we discuss the impact of the anomalous structure of its atmosphere, in particular of the non-uniform horizontal and vertical distributions of chemical elements on the magnetic field measurements and the pulsational variability. Anomalies related to the vertical abundance stratification of Pr and Nd are for the first time used to establish the presence of a radial magnetic field gradient.

[55]  arXiv:1804.07261 [pdf, other]
Title: MontePython 3: boosted MCMC sampler and other features
Comments: 17 pages, 2 figures, code available at this https URL brinckmann/montepython_public
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Instrumentation and Methods for Astrophysics (astro-ph.IM)

MontePython is a parameter inference package for cosmology. We present the latest development of the code over the past couple of years. We explain, in particular, two new ingredients both contributing to improve the performance of Metropolis-Hastings sampling: an adaptation algorithm for the jumping factor, and a calculation of the inverse Fisher matrix, which can be used as a proposal density. We present several examples to show that these features speed up convergence and can save many hundreds of CPU-hours in the case of difficult runs, with a poor prior knowledge of the covariance matrix. We also summarise all the functionalities of MontePython in the current release, including new likelihoods and plotting options.

[56]  arXiv:1804.07291 [pdf, ps, other]
Title: Measuring the circum- and inter-galactic baryon contents with fast radio bursts
Authors: Vikram Ravi
Comments: 8 pages, 4 figures, to be submitted to AAS Journals, comments welcome
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Astrophysics of Galaxies (astro-ph.GA)

Over 80% of the cosmic baryon density is likely to be distributed in the diffuse, $\gtrsim10^{4}$ K circum- and inter-galactic medium (CGM and IGM respectively). We demonstrate that the dispersion measures (DMs) of samples of localized Fast Radio Bursts (FRBs) can be used to measure the distribution of baryons between the CGM and IGM. We propose to separate the CGM and IGM contributions to FRB DMs by including redshift and mass measurements of intervening galaxies. Using simulated samples of FRB sightlines through intervening galaxy halos and an illustrative model for the CGM, and including realistic observational uncertainties, we show that small samples ($O(10^{1})-O(10^{2})$) of localized FRBs are sensitive to the presence of CGM gas. The fractions of baryons in the CGM and IGM can be accurately estimated with 100 FRBs at $z<1$, and the characteristic radial density profiles of CGM halos may also be possible to constrain. The required samples of localized FRBs are expected to be assembled in the coming few years by instruments such as the Australian Square Kilometre Array Pathfinder, the Very Large Array, and the Deep Synoptic Array.

[57]  arXiv:1804.07292 [pdf, ps, other]
Title: Comet C/2017 K2 (PANSTARRS): dynamically old or new?
Comments: 3 pages, 1 figure. Accepted for publication in Research Notes of the AAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

At discovery time, C/2017 K2 (PANSTARRS) was the second most distant inbound active comet ever observed. It has been argued that this object is in the process of crossing the inner Solar System for the first time, but other authors have concluded that it is dynamically old. We have performed full N-body simulations for 3 Myr into the past using the latest public orbit determination for this object and most of them, 67%, are consistent with a bound and dynamically old Oort cloud comet, but about 29% of the studied orbits are compatible with an interstellar origin. Our independent calculations strongly suggest that C/2017 K2 is not a dynamically new Oort cloud comet.

[58]  arXiv:1804.07295 [pdf, other]
Title: Imprints of Oscillatory Bispectra on Galaxy Clustering
Comments: 27+12 pages, 6 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

Long-short mode coupling during inflation, encoded in the squeezed bispectrum of curvature perturbations, induces a dependence of the local, small-scale power spectrum on long-wavelength perturbations, leading to a scale-dependent halo bias. While this scale dependence is absent in the large-scale limit for single-field inflation models that satisfy the consistency relation, certain models such as resonant non-Gaussianity show a peculiar behavior on intermediate scales. We reconsider the predictions for the halo bias in this model by working in Conformal Fermi Coordinates, which isolate the physical effects of long-wavelength perturbations on short-scale physics. We find that the bias oscillates with scale with an envelope similar to that of equilateral non-Gaussianity. Moreover, the bias shows a peculiar modulation with the halo mass. Unfortunately, we find that upcoming surveys will be unable to detect the signal because of its very small amplitude. We also discuss non-Gaussianity due to interactions between the inflaton and massive fields: our results for the bias agree with those in the literature.