31 articles on Sunday, February 17

arXiv:1902.05553v1 [pdf, other]
First Results from the TNG50 Simulation: The evolution of stellar and gaseous disks across cosmic time
Comments: Submitted to MNRAS. Highlights: Figures 9, 14, 15. See companion paper by Nelson et al. submitted today. Visuals at http://www.tng-project.org. Comments welcome

We present a new cosmological, magnetohydrodynamical simulation for galaxy formation: TNG50, the third and final installment of the IllustrisTNG project. TNG50 evolves 2x2160^3 dark-matter particles and gas cells in a volume 50 comoving Mpc across. It hence reaches a numerical resolution typical of zoom-in simulations, with a baryonic element mass of 8.5x10^4 Msun and an average cell size of 70-140 parsecs in the star-forming regions of galaxies. Simultaneously, TNG50 samples ~700 (6,500) galaxies with stellar masses above 10^10 (10^8) Msun at z=1. Here we investigate the structural and kinematical evolution of star-forming galaxies across cosmic time (0.5 < z < 6). We quantify their sizes, disk heights, 3D shapes, and degree of rotational vs. dispersion-supported motions as traced by rest-frame V-band light (i.e. roughly stellar mass) and by Halpha light (i.e. star-forming and dense gas). The unprecedented resolution of TNG50 enables us to model galaxies with sub-kpc half-light radii and with <300-pc disk heights. Coupled with the large-volume statistics, we characterize a diverse, redshift- and mass-dependent structural and kinematical morphological mix of galaxies all the way to early epochs. Our model predicts that for star-forming galaxies the fraction of disk-like morphologies, based on 3D stellar shapes, increases with both cosmic time and galaxy stellar mass. Gas kinematics reveal that the vast majority of 10^9-11.5 Msun star-forming galaxies are rotationally-supported disks for most cosmic epochs (Vmax/sigma>2-3, z<5), being dynamically hotter at earlier epochs (z>1.5). Despite large velocity dispersion at high redshift, cold and dense gas in galaxies predominantly arranges in disky or elongated shapes at all times and masses; these gaseous components exhibit rotationally-dominated motions far exceeding the collisionless stellar bodies.

arXiv:1902.05554v1 [pdf, other]
First Results from the TNG50 Simulation: Galactic outflows driven by supernovae and black hole feedback
Comments: MNRAS submitted, see also companion paper by Pillepich et al. (today). Visualizations, movies, and an image gallery of paper figures available on the TNG50 website: www.tng-project.org

We present the new TNG50 cosmological, magnetohydrodynamical simulation -- the third and final volume of the IllustrisTNG project. This simulation occupies a unique combination of large volume and high resolution, with a 50 Mpc box sampled by 2160^3 gas cells (baryon mass of 8x10^4 solar masses). The median spatial resolution of star-forming ISM gas is ~100-140 parsecs (from z=0 to z=6). This resolution approaches or exceeds that of modern 'zoom' simulations of individual massive galaxies, while the volume contains ~20,000 resolved galaxies with M* > 10^7 solar masses. Herein we show first results from TNG50, focusing on galactic outflows driven by supernovae as well as supermassive black hole feedback. We find that the outflow mass loading is a non-monotonic function of galaxy stellar mass, turning over and rising rapidly above 10^10.5 solar masses due to the action of the central black hole (BH). Outflow velocity increases with stellar mass, and at fixed stellar mass, outflows are faster at higher redshift. The phase structure of galactic winds is complex, and we demonstrate that the TNG model can produce high velocity, multi-phase outflows which include cool, dense components. These outflows reach speeds in excess of 3000 km/s with an ejective, BH-driven origin. Critically, we show how the relative simplicity of model inputs (and scalings) at the injection scale produces complex behavior at galactic and halo scales. For example, despite isotropic wind launching, outflows exhibit natural collimation and an emergent bipolarity. We present a correlation between outflow velocity and offset from the star-forming main sequence -- galaxies above the SFMS drive faster outflows, although this correlation inverts at high mass with the onset of quenching, whereby low luminosity, slowly accreting, massive black holes drive the strongest outflows.

arXiv:1902.05557v1 [pdf, other]
Visual Orbits of Spectroscopic Binaries with the CHARA Array. I. HD 224355
Comments: Accepted for publication in AJ. 10 pages, 6 figures

We present the visual orbit of the double-lined spectroscopic binary HD 224355 from interferometric observations with the CHARA Array, as well as an updated spectroscopic analysis using echelle spectra from the Apache Point Observatory 3.5m telescope. By combining the visual and spectroscopic orbital solutions, we find the binary components to have masses of M1 = 1.626 +/- 0.005 Msun and M2 = 1.608 +/- 0.005 Msun, and a distance of d = 63.98 +/- 0.26 pc. Using the distance and the component angular diameters found by fitting spectrophotometry from the literature to spectral energy distribution models, we estimate the stellar radii to be R1 = 2.65 +/- 0.21 Rsun and R2 = 2.47 +/- 0.23 Rsun. We then compare these observed fundamental parameters to the predictions of stellar evolution models, finding that both components are evolved towards the end of the main sequence with an estimated age of 1.9 Gyr.

arXiv:1902.05558v1 [pdf, other]
A panchromatic view of the bulge globular cluster NGC 6569
Comments: 25 pages, 16 Figures, 1 Table. Accepted for publication in ApJ

We used high-resolution optical HST/WFC3 and multi-conjugate adaptive optics assisted GEMINI GeMS/GSAOI observations in the near-infrared to investigate the physical properties of the globular cluster NGC 6569 in the Galactic bulge. We have obtained the deepest purely NIR color-magnitude diagram published so far for this cluster using ground-based observations, reaching $K_{s}$ $\approx$ 21.0 mag (two magnitudes below the main-sequence turn-off point). By combining the two datasets secured at two different epochs, we determined relative proper motions for a large sample of individual stars in the center of NGC 6569, allowing a robust selection of cluster member stars. Our proper motion analysis solidly demonstrates that, despite its relatively high metal content, NGC 6569 hosts some blue horizontal branch stars. A differential reddening map has been derived in the direction of the system, revealing a maximum color excess variation of about $\delta E(B-V)$ $\sim$ 0.12 mag in the available field of view. The absolute age of NGC 6569 has been determined for the first time. In agreement with the other few bulge globular clusters with available age estimates, NGC 6569 turns out to be old, with an age of about 12.8 Gyr, and a typical uncertainty of 0.8-1.0 Gyr.

arXiv:1902.05561v1 [pdf, other]
Asteroseismology of main-sequence F stars with \textit{Kepler}: overcoming short mode lifetimes
Comments: 11 pages, 11 Figures, Published in MNRAS

Asteroseismology is a powerful way of determining stellar parameters and properties of stars like the Sun. However, main-sequence F-type stars exhibit short mode lifetimes relative to their oscillation frequency, resulting in overlapping radial and quadrupole modes. The goal of this paper is to use the blended modes for asteroseismology in place of the individual separable modes. We used a peak-bagging method to measure the centroids of radial-quadrupole pairs for 66 stars from the \textit{Kepler} LEGACY sample, as well as {\theta} Cyg, HD 49933, HD 181420, and Procyon. We used the relative quadrupole-mode visibility to estimate a theoretical centroid frequency from a grid of stellar oscillation models. The observed centroids were matched to the modelled centroids with empirical surface correction to calculate stellar parameters. We find that the stellar parameters returned using this approach agree with the results using individual mode frequencies for stars, where those are available. We conclude that the unresolved centroid frequencies can be used to perform asteroseismology with an accuracy similar to that based on individual mode frequencies.

arXiv:1902.05569v1 [pdf, other]
The Wide Field Infrared Survey Telescope: 100 Hubbles for the 2020s
Comments: 14 pages, 3 figures, 2 tables

The Wide Field Infrared Survey Telescope (WFIRST) is a 2.4m space telescope with a 0.281 deg^2 field of view for near-IR imaging and slitless spectroscopy and a coronagraph designed for > 10^8 starlight suppresion. As background information for Astro2020 white papers, this article summarizes the current design and anticipated performance of WFIRST. While WFIRST does not have the UV imaging/spectroscopic capabilities of the Hubble Space Telescope, for wide field near-IR surveys WFIRST is hundreds of times more efficient. Some of the most ambitious multi-cycle HST Treasury programs could be executed as routine General Observer (GO) programs on WFIRST. The large area and time-domain surveys planned for the cosmology and exoplanet microlensing programs will produce extraordinarily rich data sets that enable an enormous range of Archival Research (AR) investigations. Requirements for the coronagraph are defined based on its status as a technology demonstration, but its expected performance will enable unprecedented observations of nearby giant exoplanets and circumstellar disks. WFIRST is currently in the Preliminary Design and Technology Completion phase (Phase B), on schedule for launch in 2025, with several of its critical components already in production.

arXiv:1902.05571v1 [pdf, other]
The High Time Resolution Universe Pulsar Survey -- XV: completion of the intermediate latitude survey with the discovery and timing of 25 further pulsars
Comments: Accepted for publication in MNRAS; 12 pages, 9 figures, 7 tables

We report on the latest six pulsars discovered through our standard pipeline in the intermediate-latitude region (|b| < 15 deg) of the Parkes High Time Resolution Universe Survey (HTRU). We also present timing solutions for the new discoveries and for 19 further pulsars for which only discovery parameters were previously published. Highlights of the presented sample include the isolated millisecond pulsar J1826-2415, the long-period binary pulsar J1837-0822 in a mildly eccentric 98-day orbit with a > 0.27 M_sun companion, and the nulling pulsar J1638-4233, detected only 10% of the time. Other interesting objects are PSR J1757-1500, exhibiting sporadic mode changes, and PSR J1635-2616 showing one glitch over 6 years. The new discoveries bring the total count of HTRU intermediate-latitude pulsars to 113, 25% of which are recycled pulsars. This is the higest ratio of recycled over ordinary pulsars discoveries of all recent pulsar surveys in this region of the sky. Among HTRU recycled pulsars, four are isolated objects. Comparing the characteristics of Galactic fully-recycled isolated MSPs with those of eclipsing binaries ('spiders'), from which the former are believed to have formed, we highlight a discrepancy in their spatial distribution. This may reflect a difference in the natal kick, hence, possibly, a different formation path. On the other hand, however, isolated fully-recycled MSPs spin periods are, on average, longer than those of spiders, in line with what one would expect, from simple magnetic-dipole spin-down, if the former were indeed evolved from the latter.

arXiv:1902.05574v1 [pdf, other]
A Consistent Reduced Network for HCN Chemistry in Early Earth and Titan Atmospheres: Quantum Calculations of Reaction Rate Coefficients
Comments: 34 pages, 8 figures, 14 tables, accepted for publication in J Phys Chem A

HCN is a key ingredient for synthesizing biomolecules such as nucleobases and amino acids. We calculate 42 reaction rate coefficients directly involved with or in competition with the production of HCN in the early Earth or Titan atmospheres. These reactions are driven by methane and nitrogen radicals produced via UV photodissociation or lightning. For every reaction in this network, we calculate rate coefficients at 298 K using canonical variational transition state theory (CVT) paired with computational quantum chemistry simulations at the BHandHLYP/augcc-pVDZ level of theory. We also calculate the temperature dependence of the rate coefficients for the reactions that have barriers from 50 to 400 K. We present 15 new reaction rate coefficients with no previously known value; 93% of our calculated coefficients are within an order of magnitude of the nearest experimental or recommended values. Above 320 K, the rate coefficient for the new reaction H2CN -> HCN + H dominates. Contrary to experiments, we find the HCN reaction pathway, N + CH3 -> HCN + H2, to be inefficient and suggest that the experimental rate coefficient actually corresponds to an indirect pathway, through the H2CN intermediate. We present CVT using energies computed with density functional theory as a feasible and accurate method for calculating a large network of rate coefficients of small-molecule reactions.

arXiv:1902.05585v1 [pdf, other]
Nitrogen-containing Anions and Tholin Growth in Titan's Ionosphere: Implications for Cassini CAPS-ELS Observations
Comments: 8 pages, 2 figures, accepted for publication in the Astrophysical Journal Letters

The Cassini Plasma Spectrometer (CAPS) Electron Spectrometer (ELS) instrument onboard Cassini revealed an unexpected abundance of negative ions above 950 km in Titan's ionosphere. \textit{In situ} measurements indicated the presence of negatively charged particles with mass-over-charge ratios up to 13,800 \textit{u/q}. At present, only a handful of anions have been characterized by photochemical models, consisting mainly of C$_n$H$^-$ carbon chain and C$_{n-1}$N$^-$ cyano compounds ($n=2-6$); their formation occurring essentially through proton abstraction from their parent neutral molecules. However, numerous other species have yet to be detected and identified. Considering the efficient anion growth leading to compounds of thousands of \textit{u/q}, it is necessary to better characterize the first light species. Here, we present new negative ion measurements with masses up to 200 \textit{u/q} obtained in an \ce{N2}:\ce{CH4} dusty plasma discharge reproducing analogous conditions to Titan's ionosphere. We perform a comparison with high altitude CAPS-ELS measurements near the top of Titan's ionosphere from the T18 encounter. The main observed peaks are in agreement with the observations. However, a number of other species (\textit{e.g.} \ce{CNN-}, \ce{CHNN-}) previously not considered suggests an abundance of N-bearing compounds, containing two or three nitrogen atoms, consistent with certain adjacent doubly-bonded nitrogen atoms found in tholins. These results suggest that an N-rich incorporation into tholins may follow mechanisms including anion chemistry, further highlighting the important role of negative ions in Titan's aerosol growth.

arXiv:1902.05589v1 [pdf, other]
Photoionization Emission Models for the Cyg X-3 X-ray Spectrum

We present model fits to the X-ray line spectrum of the well known High Mass X-ray binary Cyg X-3. The primary observational dataset is a spectrum taken with the $Chandra$ X-ray Observatory High Energy Transmission Grating (HETG) in 2006, though we compare it to all the other observations of this source taken so far by this instrument. We show that the density must be $\geq 10^{12}$ cm$^{-3}$ in the region responsible for most of the emission. We discuss the influence of the dust scattering halo on the broad band spectrum and we argue that dust scattering and extinction is not the most likely origin for the narrow featureseen near the Si K edge. We identify the features of a wind in the profiles of the strong resonance lines and we show that the wind is more apparent in the lines from the lighter elements. We argue that this wind is most likely associated with the companion star. We show that the intensities of most lines can be fitted, crudely, by a single component photoionized model. However, the iron K lines do not fit with this model. We show that the iron K line variability as a function of orbital phase is different from the lower energy lines, which indicates that the lines arise in physically distinct regions. We discuss the interpretation of these results in the context of what is known about the system and similar sys

arXiv:1902.05615v1 [pdf, other]
Limits on Mode Coherence Due to a Non-static Convection Zone
Comments: 6 pages, 9 figures, In 21st European Workshop on White Dwarfs, ed. Castanheira, Vanderbosch, & Montgomery

The standard theory of pulsations deals with the frequencies and growth rates of infinitesimal perturbations in a stellar model. Modes which are calculated to be linearly driven should increase their amplitudes exponentially with time; the fact that nearly constant amplitudes are usually observed is evidence that nonlinear mechanisms inhibit the growth of finite amplitude pulsations. Models predict that the mass of DAV convection zones is very sensitive to temperature (i.e., $M_{\text{CZ}} \propto T_{\text{eff}}^{-90}$) leading to the possibility that even "small amplitude" pulsators may experience significant nonlinear effects. In particular, the outer turning point of finite-amplitude g-mode pulsations can vary with the local surface temperature, producing a reflected wave that is slightly out of phase with that required for a standing wave. This can lead to a lack of coherence of the mode and a reduction in its global amplitude. We compute the size of this effect for specific examples and discuss the results in the context of Kepler and K2 observations.

arXiv:1902.05637v1 [pdf, other]
Mapping the Interstellar Reddening and Extinction towards Baade's Window Using Minimum Light Colors of ab-type RR Lyrae Stars. Revelations from the De-reddened Color-Magnitude Diagrams
Comments: accepted for publication in AAS journals; most likely The Astrophysical Journal

We have obtained repeated images of 6 fields towards the Galactic bulge in 5 passbands (u, g, r, i, z) with the DECam imager on the Blanco 4m telescope at CTIO. From over 1.6 billion individual photometric measurements in the field centered on Baade's window, we have detected 4877 putative variable stars. 474 of these have been confirmed as fundamental mode RR Lyrae stars, whose colors at minimum light yield line-of-sight reddening determinations as well as a reddening law towards the Galactic Bulge which differs significantly from the standard R_V = 3.1 formulation. Assuming that the stellar mix is invariant over the 3 square-degree field, we are able to derive a line-of-sight reddening map with sub-arcminute resolution, enabling us to obtain de-reddened and extinction corrected color-magnitude diagrams (CMD's) of this bulge field using up to 2.5 million well-measured stars. The corrected CMD's show unprecedented detail and expose sparsely populated sequences: e.g., delineation of the very wide red giant branch, structure within the red giant clump, the full extent of the horizontal branch, and a surprising bright feature which is likely due to stars with ages younger than 1 Gyr. We use the RR Lyrae stars to trace the spatial structure of the ancient stars, and find an exponential decline in density with Galactocentric distance. We discuss ways in which our data products can be used to explore the age and metallicity properties of the bulge, and how our larger list of all variables is useful for learning to interpret future LSST alerts.

arXiv:1902.05652v1 [pdf, other]
The SAMI Galaxy Survey: Satellite galaxies undergo little structural change during their quenching phase
Comments: 11 pages, 7 figures. Accepted for publication in MNRAS

At fixed stellar mass, satellite galaxies show higher passive fractions than centrals, suggesting that environment is directly quenching their star formation. Here, we investigate whether satellite quenching is accompanied by changes in stellar spin (quantified by the ratio of the rotational to dispersion velocity V/$\sigma$) for a sample of massive ($M_{*}>$10$^{10}$ M$_{\odot}$) satellite galaxies extracted from the SAMI Galaxy Survey. These systems are carefully matched to a control sample of main sequence, high $V/\sigma$ central galaxies. As expected, at fixed stellar mass and ellipticity, satellites have lower star formation rate (SFR) and spin than the control centrals. However, most of the difference is in SFR, whereas the spin decreases significantly only for satellites that have already reached the red sequence. We perform a similar analysis for galaxies in the EAGLE hydro-dynamical simulation and recover differences in both SFR and spin similar to those observed in SAMI. However, when EAGLE satellites are matched to their 'true' central progenitors, the change in spin is further reduced and galaxies mainly show a decrease in SFR during their satellite phase. The difference in spin observed between satellites and centrals at $z\sim$0 is primarily due to the fact that satellites do not grow their angular momentum as fast as centrals after accreting into bigger halos, not to a reduction of $V/\sigma$ due to environmental effects. Our findings highlight the effect of progenitor bias in our understanding of galaxy transformation and they suggest that satellites undergo little structural change before and during their quenching phase.

arXiv:1902.05728v1 [pdf, other]
Magnetic field vector maps of nearby spiral galaxies

We present a method for determining directions of magnetic field vectors in a spiral galaxy using two synchrotron polarization maps, an optical image, and a velocity field. The orientation of the transverse magnetic field is determined with a synchrotron polarization map of higher frequency band and the $180^\circ$-ambiguity is solved by using sign of the Rotation Measure (RM) after determining geometrical orientation of a disk based on a assumption of trailing spiral arms. The advantage of this method is that direction of magnetic vector for each line of sight through the galaxy can be inexpensively determined with easily available data and with simple assumptions. We applied this method to three nearby spiral galaxies using archival data obtained with the Very Large Array (VLA) to demonstrate how it works. The three galaxies have both clockwise and counter-clockwise magnetic fields, which implies that all three galaxies are not classified in simple Axis-Symmetric type but types of higher modes and that magnetic reversals commonly exist.

arXiv:1902.05738v1 [pdf, other]
Constraining Single Dark Matter Particle Mass by Primordial Gravitational Wave

We report on a new relation between dark matter (DM) particle mass, $m_\chi$, and the tensor-to-scalar ratio of cosmic microwave background (CMB) anisotropies, $r$. Once $r$ is determined in a future search of primordial gravitational wave (PGW), $m_\chi$ can be constrained by using this relation. Due to the resonance interplay between metric and DM density perturbations, a small local fluctuation of spacetime can be generated in a pair production of DM particles. Such fluctuations, arising from the DM pair productions, encode information of DM mass and slightly change the amplitude of metric perturbation. Within a general framework of cosmic reheating, we show that, these fluctuations are accumulated during DM production process, which amplifies the scalar modes of metric perturbation significantly and results in a strong suppression of $r$. Thus we establish a novel link between DM and PGW, which yields a new relation between $m_\chi$ and $r$. For illustration, in a toy model of cosmic reheating with $\alpha=10^{-8}$, we show that $m_\chi=10~\text{GeV}$ if PGW is detected at $r=0.3\times 10^{-2}$.

arXiv:1902.05744v1 [pdf, other]
The core and stellar mass functions in massive collapsing filaments
Comments: Accepted for publication by A&A

The connection between the pre-stellar core mass function (CMF) and the stellar initial mass function (IMF) lies at the heart of all star formation theories. In this paper, we study the earliest phases of star formation with a series of high-resolution numerical simulations that include the formation of sinks. In particular, we focus on the transition from cores to sinks within a massive molecular filament. We compare the CMF and IMF between magnetized and unmagnetized simulations, and between different resolutions. We find that selecting cores based on their kinematic virial parameter excludes collapsing objects because they host large velocity dispersions. Selecting only the thermally unstable magnetized cores, we observe that their mass-to-flux ratio spans almost two orders of magnitude for a given mass. We also see that, when magnetic fields are included, the CMF peaks at higher core mass values with respect to pure hydrodynamical simulations. Nonetheless, all models produce sink mass functions with a high-mass slope consistent with Salpeter. Finally, we examine the effects of resolution and find that, in isothermal simulations, even models with very high dynamical range fail to converge in the mass function. Our main conclusion is that, although the resulting CMFs and IMFs have similar slopes in all simulations, the cores have slightly different sizes and kinematical properties when a magnetic field is included. However, a core selection based on the mass-to-flux ratio alone is not enough to alter the shape of the CMF, if we do not take thermal stability into account. Finally, we conclude that extreme care should be given to resolution issues when studying sink formation with an isothermal equation of state.

arXiv:1902.05753v1 [pdf, other]
Ruprecht 147 DANCe I. Members, empirical isochrone, luminosity and mass distributions

Context. Ruprecht 147 is the oldest (2.5 Gyr) open cluster in the solar vicinity (< 300 pc), making it an important target for stellar evolution studies and exoplanet searches. Aims. Derive a census of members and the luminosity, mass, and spatial distributions of the cluster. Methods. We use an astro-photometric data set including all available information from the literature together with our own observations. We process the data with an updated version of an existent membership selection methodology. Results. We identify 259 high-probability candidate members, including 58 previously unreported. All these candidates cover the luminosity interval between G > 6 mag to i< 21 mag. The cluster luminosity and mass distributions are derived with an unprecedented level of details allowing us to recognize, among other features, the Wielen dip. The mass distribution in the low-mass regime drops sharply at 0.4 $M_{\odot}$ even though our data are sensitive to stellar masses down to 0.1 $M_{\odot}$, suggesting that most very-low-mass members left the cluster as the result of its dynamical evolution. In addition, the cluster is highly elongated (ellipticity $\sim$ 0.5) towards the galactic plane, and mass segregated. Conclusions. Our combined Gaia+DANCe data set allows us to obtain an extended list of cluster candidate members, and to derive luminosity, mass and projected spatial distributions in the oldest open cluster of the solar vicinity.

arXiv:1902.05792v1 [pdf, other]
Neutrinos below 100 TeV from the southern sky employing refined veto techniques to IceCube data
Comments: Preprint submitted to Astroparticle Physics, 19 pages, 17 figures

Many Galactic sources of gamma rays, such as supernova remnants, are expected to produce neutrinos with a typical energy cutoff well below 100 TeV. For the IceCube Neutrino Observatory located at the South Pole, the southern sky, containing the inner part of the Galactic plane and the Galactic Center, is a particularly challenging region at these energies, because of the large background of atmospheric muons. In this paper, we present recent advancements in data selection strategies for track-like muon neutrino events with energies below 100 TeV from the southern sky. The strategies utilize the outer detector regions as veto and features of the signal pattern to reduce the background of atmospheric muons to a level which, for the first time, allows IceCube searching for point-like sources of neutrinos in the southern sky at energies between 100 GeV and several TeV in the muon neutrino charged current channel. No significant clustering of neutrinos above background expectation was observed in four years of data recorded with the completed IceCube detector. Upper limits on the neutrino flux for a number of spectral hypotheses are reported for a list of astrophysical objects in the southern hemisphere.

arXiv:1902.05797v1 [pdf, other]
Nonthermal emission in the lobes of Fornax A
Comments: MNRAS, in press; 9 pages, 1 figure

Current measurements of the spectral energy distribution in radio, X-and-gamma-ray provide a sufficiently wide basis for determining basic properties of energetic electrons and protons in the extended lobes of the radio galaxy Fornax A. Of particular interest is establishing observationally, for the first time, the level of contribution of energetic protons to the extended emission observed by the Fermi satellite. Two recent studies concluded that the observed gamma-ray emission is unlikely to result from Compton scattering of energetic electrons off the optical radiation field in the lobes, and therefore that the emission originates from decays of neutral pions produced in interactions of energetic protons with protons in the lobe plasma, implying an uncomfortably high proton energy density. However, our exact calculation of the emission by energetic electrons in the magnetized lobe plasma leads to the conclusion that all the observed emission can, in fact, be accounted for by energetic electrons scattering off the ambient optical radiation field, whose energy density (which, based on recent observations, is dominated by emission from the central galaxy NGC 1316) we calculate to be higher than previously estimated.

arXiv:1902.05822v1 [pdf, other]
M31 circum-nuclear region: a molecular survey with the IRAM-interferometer
Comments: 21 pages, 25 figures Accepted for publication in Astronomy and Astrophysics

We analyse molecular observations performed at IRAM interferometer in CO(1-0) of the circum-nuclear region (within 250 pc) of Andromeda, with 2.9 arcsec = 11 pc resolution. We detect 12 molecular clumps in this region, corresponding to a total molecular mass of (8.4 +- 0.4) x 10**4 Msol. They follow the Larson's mass-size relation, but lie well above the velocity-size relation. We discuss that these clumps are probably not virialised, but transient agglomerations of smaller entities that might be virialised. Three of these clumps have been detected in CO(2-1) in a previous work, and we find temperature line ratio below 0.5. With a RADEX analysis, we show that this gas is in non local thermal equilibrium with a low excitation temperature (Tex = 5-9 K). We find a surface beam filling factor of order 5 percent and a gas density in the range 60-650 cm**{-3}, well below the critical density. With a gas-to-stellar mass fraction of 4 x 10**{-4} and dust-to-gas ratio of 0.01, this quiescent region has exhausted his gas budget. Its spectral energy distribution is compatible with passive templates assembled from elliptical galaxies. While weak dust emission is present in the region, we show that no star formation is present and support the previous results that the dust is heated by the old and intermediate stellar population. We study that this region lies formally in the low-density part of the Kennicutt-Schmidt law, in a regime where the SFR estimators are not completely reliable. We confirm the quiescence of the inner part of this galaxy known to lie on the green valley.

arXiv:1902.05824v1 [pdf, other]
Extra-tidal structures around the Gaia Sausage candidate globular cluster NGC6779 (M56)
Comments: 8 pages, 6 figures. Accepted for publication in Monthly Noticies of the Royal Astronomical Society

We present results on the stellar density radial profile of the outer regions of NGC6779, a Milky Way globular cluster recently proposed as a candidate member of the Gaia Sausage structure, a merger remnant of a massive dwarf galaxy with the Milky Way. Taking advantage of the Pan-STARRS PS1 public astrometric and photometric catalogue, we built the radial profile for the outermost cluster regions using horizontal branch and main sequence stars, separately, in order to probe for different profile trends because of difference stellar masses. Owing to its relatively close location to the Galactic plane, we have carefully treated the chosen colour-magnitude regions properly correcting them by the amount of interstellar extinction measured along the line-of-side of each star, as well as cleaned them from the variable field star contamination observed across the cluster field. In the region spanning from the tidal to the Jacobi radii the resulting radial profiles show a diffuse extended halo, with an average power law slope of -1. While analysing the relationships between the Galactocentric distance, the half-mass density, the half-light radius, the slope of the radial profile of the outermost regions, the internal dynamical evolutionary stage, among others, we found that NGC6779 shows structural properties similar to those of the remaining Gaia Sausage candidate globular clusters, namely, they are massive clusters (>10^5Mo) in a moderately early dynamical evolutionary stage, with observed extra-tidal structures.

arXiv:1902.05833v1 [pdf, other]
Submillimeter spectroscopy and astronomical searches of vinyl mercaptan, C$_2$H$_3$SH
Comments: 13 pages, 5 figures, 2 tables, 2 appendices. Accepted for publication in A&A 13th of February, 2019

We have extended the pure rotational investigation of the two isomers syn and anti vinyl mercaptan to the millimeter domain using a frequency-multiplication spectrometer. The species were produced by a radiofrequency discharge in 1,2-ethanedithiol. Additional transitions have been re-measured in the centimeter band using Fourier-transform microwave spectroscopy to better determine rest frequencies of transitions with low-$J$ and low-$K_a$ values. Experimental investigations were supported by quantum chemical calculations on the energetics of both the [C$_2$,H$_4$,S] and [C$_2$,H$_4$,O] isomeric families. Interstellar searches for both syn and anti vinyl mercaptan as well as vinyl alcohol were performed in the EMoCA (Exploring Molecular Complexity with ALMA) spectral line survey carried out toward Sagittarius (Sgr) B2(N2) with the Atacama Large Millimeter/submillimeter Array (ALMA). Highly accurate experimental frequencies (to better than 100 kHz accuracy) for both syn and anti isomers of vinyl mercaptan have been measured up to 250 GHz; these deviate considerably from predictions based on extrapolation of previous microwave measurements. Reliable frequency predictions of the astronomically most interesting millimeter-wave lines for these two species can now be derived from the best-fit spectroscopic constants. From the energetic investigations, the four lowest singlet isomers of the [C$_2$,H$_4$,S] family are calculated to be nearly isoenergetic, which makes this family a fairly unique test bed for assessing possible reaction pathways. Upper limits for the column density of syn and anti vinyl mercaptan are derived toward the extremely molecule-rich star-forming region Sgr B2(N2) enabling comparison with selected complex organic molecules.

arXiv:1902.05856v1 [pdf, other]
Optical detection of a GMRT-detected candidate high-redshift radio galaxy with 3.6-m Devasthal optical telescope
Comments: 7 pages, 2 figures, Accepted in Jr. of Astrophysics & Astronomy (JApA)

We report optical observations of TGSS J1054+5832, a candidate high-redshift ($z=4.8\pm2$) steep-spectrum radio galaxy, in $r$ and $i$ bands using the faint object spectrograph and camera mounted on 3.6-m Devasthal Optical Telescope (DOT). The source previously detected at 150 MHz from Giant Meterwave Radio Telescope and at 1420 MHz from Very Large Array has a known counterpart in near-infrared bands with $K$-band magnitude of AB 22. The source is detected in $i$-band with AB$24.3\pm0.2$ magnitude in the DOT images presented here. The source remains undetected in the $r$-band image at a 2.5$\sigma$ depth of AB 24.4 mag over an $1.2''\times1.2''$ aperture. An upper limit to $i-K$ color is estimated to be $\sim$2.3, suggesting youthfulness of the galaxy with active star formation. These observations highlight the importance and potential of the 3.6-m DOT for detections of faint galaxies.

arXiv:1902.05857v1 [pdf, other]
First-light images from low dispersion spectrograph-cum-imager on 3.6-meter Devasthal Optical Telescope
Comments: 8 pages, 6 figures, Accepted in Current Science (CSci; Indian Academy of Sciences)

A low dispersion spectrograph-cum-imager has been developed and assembled in ARIES, Nainital. The optical design of the spectrograph consists of a collimator and a focal reducer converting the f/9 beam from the 3.6-m Devasthal optical telescope to a nearly f/4.3 beam. The instrument is capable of carrying out broad-band imaging, narrow-band imaging and low-resolution ({\lambda}/{\Delta}{\lambda}<2000) slit spectroscopy in the wavelength range 350-1050 nm. A closed-cycle cryogenically cooled charge-coupled device camera, also assembled in ARIES, is used as the main imaging device for the spectrograph. The first images from the spectrograph on the telescope assert seeing-limited performance free from any significant optical aberration. An i-band image of the galaxy cluster Abell 370 made using the spectrograph shows faint sources down to ~25 mag. The quality and sensitivity of the optical spectrums of the celestial sources obtained from the spectrograph are as per the expectations from a 3.6-m telescope. Several new modes of observations such as polarimetry, fast-imaging, and monitoring of the atmospheric parameters are being included in the spectrograph. Using a test setup, single optical pulses from the Crab pulsar were detected from the telescope. The spectrograph is one of the main back-end instruments on the 3.6-m telescope for high sensitivity observations of celestial objects.

arXiv:1902.05859v1 [pdf, other]
Circular spectropolarimetric sensing of vegetation in the field; possibilities for the remote detection of extraterrestrial life

Homochirality is a generic and unique property of all biochemical life and the fractional circular polarization it induces therefore constitutes an unambiguous biosignature. However, while high-quality circular polarimetric spectra can be easily and quickly obtained in the laboratory, accurate measurements in the field are much more challenging due to large changes in illumination and target movement. In this study we have measured various targets in the field, up to distances of a few kilometers, using the dedicated circular spectropolarimeter TreePol. We show how photosynthetic life can readily be distinguished from abiotic matter. We underline the potential of circular polarization signals as a remotely accessible means to characterize terrestrial life and detecting the presence of extraterrestrial life.

arXiv:1902.05869v1 [pdf, other]
The variation of the magnetic field of the Ap star HD~50169 over its 29 year rotation period
Comments: 10 pages, 3 figures, accepted for publication in A&A

Context. The Ap stars that rotate extremely slowly, with periods of decades to centuries, represent one of the keys to the understanding of the processes leading to the differentiation of stellar rotation. Aims. We characterise the variations of the magnetic field of the Ap star HD 50169 and derive constraints about its structure. Methods. We combine published measurements of the mean longitudinal field <Bz> of HD 50169 with new determinations of this field moment from circular spectropolarimetry obtained at the 6-m telescope BTA of the Special Astrophysical Observatory of the Russian Academy of Sciences. For the mean magnetic field modulus <B>, literature data are complemented by the analysis of ESO spectra, both newly acquired and from the archive. Radial velocities are also obtained from these spectra. Results. We present the first determination of the rotation period of HD 50169, Prot = (29.04+/-0.82) y. HD 50169 is currently the longest-period Ap star for which magnetic field measurements have been obtained over more than a full cycle. The variation curves of both <Bz> and <B> have a significant degree of anharmonicity, and there is a definite phase shift between their respective extrema. We confirm that HD 50169 is a wide spectroscopic binary, refine its orbital elements, and suggest that the secondary is probably a dwarf star of spectral type M. Conclusions. The shapes and mutual phase shifts of the derived magnetic variation curves unquestionably indicate that the magnetic field of HD 50169 is not symmetric about an axis passing through its centre. Overall, HD 50169 appears similar to the bulk of the long-period Ap stars.

arXiv:1902.05891v1 [pdf, other]
The blue straggler population of the old open cluster Berkeley 17
Comments: 6 pages, 6 figures, Accepted for publication in Astronomy and Astrophysics

Blue Straggler Stars (BSSs) are observed in Galactic globular clusters and old open clusters. The radial distribution of BSSs has been used to diagnose the dynamical evolution of globular clusters. For the first time, with a reliable sample of BSSs identified with Gaia DR2, we conduct such an analysis for an open cluster. We identify members, including BSSs, of the oldest known Galactic open cluster Berkeley 17 with the Gaia DR2 proper motions and parallaxes. We study the radial distribution of the BSS population to understand the dynamical evolution of the cluster. We select cluster members to populate the colour magnitude diagram in the Gaia filters. Cluster parameters are derived using the brightest members. The BSSs and giant branch stars are identified, and their radial distributions are compared. The segregation of BSSs is also evaluated with respect to the giant branch stars using the Minimum Spanning Tree analysis. We determine Berkeley 17 to be at $3138.6^{+285.5}_{-352.9}$ pc. We find 23 BSS cluster members, only two of which were previously identified. We find a bimodal radial distribution of BSSs supported by findings from the MST method. The bimodal radial distribution of BSSs in Berkeley 17 indicates that they have just started to sink towards the cluster center, placing Berkeley 17 with globular clusters of intermediate dynamical age. This is the first such determination for an open cluster.

arXiv:1902.05894v1 [pdf, other]
Viable inflationary magnetogenesis with helical coupling

We consider helical coupling to electromagnetism and present a simple model of evolution of the coupling function leading to a viable inflationary magnetogenesis. In this scenario, helical magnetic fields of any desirable strength of order up to $10^{- 7}\,\text{G}$ at the current epoch can be generated in a narrow spectral band centered at any reasonable wavenumber by adjusting the model parameters.

arXiv:1902.05895v1 [pdf, other]
The eye of Gaia on globular clusters kinematics: internal rotation
Comments: 19 pages, 7 figures, accepted for publication by MNRAS

We derived the three-dimensional velocities of individual stars in a sample of 62 Galactic globular clusters using proper motions from the second data release of the Gaia mission together with the most comprehensive set of line-of-sight velocities with the aim of investigating the rotation pattern of these stellar systems. We detect the unambiguous signal of rotation in 15 clusters at amplitudes which are well above the level of random and systematic errors. For these clusters, we derived the position and inclination angle of the rotation axis with respect to the line of sight and the overall contribution of rotation to the total kinetic energy budget. The rotation strengths are weakly correlated with the half-mass radius, the relaxation time and anticorrelated with the destruction rate, while no significant alignment of the rotation axes with the orbital poles has been observed. This evidence points toward a primordial origin of the systemic rotation in these stellar systems.

arXiv:1902.05917v1 [pdf, other]