46 articles on Friday, April 19

arXiv:1904.08406v1 [pdf, other]
Revealing the differences in the SMBH accretion rate distributions of starburst and non-starburst galaxies
Comments: 14 pages, 10 figures, accepted for publication in MNRAS

We infer and compare the specific X-ray luminosity distributions for a sample of massive (i.e. $\log_{10} (M*/M\odot) > 10.5$) galaxies split according to their far-infrared-derived star-forming properties (i.e., starburst and non-starburst) and redshift. We model each distribution as a power-law with an upper and lower turnover, and adopt a maximum likelihood method to include information from non-detections in the form of upper limits. When we use our inferred distributions to calculate the ratios of high to low sLx AGN (corresponding to above and below $0.1\lambda_{\text{Edd}}$, respectively) we find that starbursts have significantly higher proportions of high sLx AGN compared to their non-starburst counterparts. These findings help explain the increase in average X-ray luminosity in bins of increasing SFR reported by previous studies.

arXiv:1904.08425v1 [pdf, other]
Jet-cocoon outflows from neutron star mergers: structure, light curves, and fundamental physics
Comments: Submitted to ApJ. This version has some additional references w.r.t. the submitted one (which will be added to the revised version, when possible). If any reference is missing, please email the authors

The discovery of GW170817, the merger of a binary neutron star (NS) triggered by a gravitational wave detection by LIGO and Virgo, has opened a new window of exploration in the physics of NSs and their cosmological role. Among the important quantities to measure are the mass and velocity of the ejecta produced by the tidally disrupted NSs and the delay - if any - between the merger and the launching of a relativistic jet. These encode information on the equation of state of the NS, the nature of the merger remnant, and the jet launching mechanism, as well as yielding an estimate of the mass available for r-process nucleosynthesis. Here we derive analytic estimates for the structure of jets expanding in environments with different density, velocity, and radial extent. We compute the jet-cocoon structure and the properties of the broadband afterglow emission as a function of the ejecta mass, velocity, and time delay between merger and launch of the jet. We show that modeling of the afterglow light curve can constrain the ejecta properties and, in turn, the physics of neutron density matter. Our results increase the interpretative power of electromagnetic observations by allowing for a direct connection with the merger physics.

arXiv:1904.08427v1 [pdf, other]
Towards an accurate description of an accretion induced collapse and the associated ejected mass
Comments: No comment found

We revisit the accretion induced collapse (AIC) process, in which a white dwarf collapses into a neutron star. We are motivated by the persistent radio source associated with the fast radio burst 121102, which was explained by Waxman (2017) as a weak stellar explosion with a small ($\sim 10^{-5}M_{\odot}$) mass ejection. Since a typical supernova ejects much larger amount of mass, we study the possibility that an AIC caused the weak explosion. Additionally, the interaction of the relatively low ejected mass with a pre-collapse wind might be related to fast optical transients. The AIC is simulated with a one-dimensional, Lagrangian, Newtonian hydrodynamic code, and we put an emphasis on accurately treating the equation of state and the nuclear reaction network. We leave subjects such as neutrino physics and general relativity corrections for future work. Using an existing initial profile and our own initial profiles, we find that the ejected mass is $\sim 10^{-2}-10^{-1}M_{\odot}$ over a wide range of parameters, and construct a simple model to explain our results. Our results probably provide an upper limit to the ejected mass from AIC events.

arXiv:1904.08428v1 [pdf, other]
Disc Tearing and Bardeen-Petterson Alignment in GRMHD Simulations of Highly Tilted Thin Accretion Discs
Comments: 8 pages, 5 figures, submitted to MNRAS, see our YouTube playlist for animations: https://www.youtube.com/playlist?list=PLDO1oeU33GwlaPSME1TdCto1Y3P6yG91L

Luminous active galactic nuclei (AGN) and X-Ray binaries (XRBs) tend to be surrounded by geometrically thin, radiatively cooled accretion discs. According to both theory and observations, these are -- in many cases -- highly misaligned with the black hole spin axis. In this work we present the first general relativistic magnetohydrodynamic simulations of very thin ($h/r \sim 0.015-0.05$) accretion discs around rapidly spinning ($a \sim 0.9$) black holes and tilted by 45-65 degrees. We show that the inner regions of the discs with $h/r \lesssim 0.03$ align with the black hole equator, though at smaller radii than predicted by theoretical work. The inner aligned and outer misaligned disc regions are separated by a sharp break in tilt angle accompanied by a sharp drop in density. We find that frame-dragging by the spinning black hole overpowers the disc viscosity, which is self-consistently produced by magnetized turbulence, tearing the disc apart and forming a rapidly precessing inner sub-disc surrounded by a slowly precessing outer sub-disc. We find that at all tilt values the system produces a pair of relativistic jets. At small distances the jets precess rapidly together with the inner sub-disc, whereas at large distances they partially align with the outer sub-disc and precess more slowly. If the tearing radius can be modeled accurately in future work, emission model independent measurements of black hole spin based on precession-driven quasi-periodic oscillations may become possible.

arXiv:1904.08429v1 [pdf, other]
First detection of PSR B1259-63/LS 2883 in the Millimeter and Submillimeter Wavelengths with ALMA
Comments: 6 pages, 3 figures

We report Atacama Large Millimeter/submillimeter Array (ALMA) observations of the binary system containing the pulsar PSR B1259-63 orbiting around a Be star LS 2883 after the 2017 periastron passage. We detected radio continuum emission from the binary system in the millimeter/submillimeter wavelengths for the first time. At Band 3 (97 GHz), the flux 84 days after the periastron is almost the same as that 71 days after the periastron. Although the binary system showed intense GeV gamma-ray flares during our observations, the Band 3 flux did not indicate any time correlation with them. The Band 3 fluxes are consistent with an extrapolation of the radio spectrum at lower frequencies. Assuming that it is synchrotron emission, we constrain magnetic fields ($\lesssim 0.6$ G) and the high-energy cutoff of the electrons ($\gamma \gtrsim 360$). The flux at Band 7 (343 GHz) 69 days after the periastron shows a significant excess from the extrapolation of the radio spectrum at lower frequencies. The flux may be associated with the circumstellar disk around the Be star. We also present the results of Australian Telescope Compact Array (ATCA) observations at 94 GHz for the 2014 periastron passage, which show that the radio spectrum was relatively soft when the pulsar passed the disk.

arXiv:1904.08430v1 [pdf, other]
Dark Matter Strikes Back at the Galactic Center
Comments: 7+32 pages, 2+20 figures

Statistical evidence has previously suggested that the Galactic Center GeV Excess (GCE) originates largely from point sources, and not from annihilating dark matter. We examine the impact of unmodeled source populations on identifying the true origin of the GCE using non-Poissonian template fitting (NPTF) methods. In a proof-of-principle example with simulated data, we discover that unmodeled sources in the Fermi Bubbles can lead to a dark matter signal being misattributed to point sources by the NPTF. We discover striking behavior consistent with a mismodeling effect in the real Fermi data, finding that large artificial injected dark matter signals are completely misattributed to point sources. Consequently, we conclude that dark matter may provide a dominant contribution to the GCE after all.

arXiv:1904.08431v1 [pdf, other]
Origin of the Golden Mass of Galaxies and Black Holes
Comments: 9 pages, 12 figures

We address the origin of the golden mass and time for galaxy formation and the onset of rapid black-hole growth. The preferred dark-halo mass of ~$10^{12}M_\odot$ is translated to a characteristic epoch, z~2, at which the typical forming halos have a comparable mass. We put together a coherent picture based on existing and new simple analytic modeling and cosmological simulations. We describe how the golden mass arises from two physical mechanisms that suppress gas supply and star formation below and above the golden mass, supernova feedback and virial shock heating of the circum-galactic medium (CGM), respectively. Cosmological simulations reveal that these mechanisms are responsible for a similar favored mass for the dramatic events of gaseous compaction into compact star-forming "blue nuggets", caused by mergers, counter-rotating streams or other mechanisms. This triggers inside-out quenching of star formation, to be maintained by the hot CGM, leading to today's passive early-type galaxies. The blue-nugget phase is responsible for transitions in the galaxy structural, kinematic and compositional properties, e.g., from dark-matter to baryon central dominance and from prolate to oblate shape. The growth of the central black hole is suppressed by supernova feedback below the critical mass, and is free to grow once the halo is massive enough to lock the supernova ejecta by its deep potential well and the hot CGM. A compaction near the golden mass makes the black hole sink to the galactic center and triggers a rapid black-hole growth. This ignites feedback by the Active Galactic Nucleus that helps keeping the CGM hot and maintaining long-term quenching.

arXiv:1904.08434v1 [pdf, other]
The Herschel Dwarf Galaxy Survey: II. Physical conditions, origin of [CII] emission, and porosity of the multiphase low-metallicity ISM
Comments: Accepted in A&A. 20 pages, 12 figures, 5 tables plus appendices

The sensitive infrared telescopes, Spitzer and Herschel, have been used to target low-metallicity star-forming galaxies, allowing us to investigate the properties of their interstellar medium (ISM) in unprecedented detail. Interpretation of the observations in physical terms relies on careful modeling of those properties. We have employed a multiphase approach to model the ISM phases (HII region and photodissociation region) with the spectral synthesis code Cloudy. Our goal is to characterize the physical conditions (gas densities, radiation fields, etc.) in the ISM of the galaxies from the Herschel Dwarf Galaxy Survey. We are particularly interested in correlations between those physical conditions and metallicity or star-formation rate. Other key issues we have addressed are the contribution of different ISM phases to the total line emission, especially of the [CII]157um line, and the characterization of the porosity of the ISM. We find that the lower-metallicity galaxies of our sample tend to have higher ionization parameters and galaxies with higher specific star-formation rates have higher gas densities. The [CII] emission arises mainly from PDRs and the contribution from the ionized gas phases is small, typically less than 30% of the observed emission. We also find correlation - though with scatter - between metallicity and both the PDR covering factor and the fraction of [CII] from the ionized gas. Overall, the low metal abundances appear to be driving most of the changes in the ISM structure and conditions of these galaxies, and not the high specific star-formation rates. These results demonstrate in a quantitative way the increase of ISM porosity at low metallicity. Such porosity may be typical of galaxies in the young Universe.

arXiv:1904.08436v1 [pdf, other]
Constraining the Delay Time Distribution of Binary Neutron Stars with the Host Galaxies of Gravitational Wave Events
Comments: Submitted to ApJ Letters

The delay time distribution of (DTD) of binary neutron stars (BNS) remains poorly constrained, mainly by the small known population of Galactic binaries, the properties of short gamma-ray burst host galaxies, and inferences from $r$-process enrichment. In the new era of BNS merger detections through gravitational waves (GW), a new route to the DTD is the demographics of the host galaxies, localized through associated electromagnetic counterparts. This approach takes advantage of the correlation between star formation history (SFH) and galaxy mass, such that the convolution of the SFH and DTD impacts the BNS merger rate as a function of galaxy mass. Here we quantify this approach for a power law DTD governed by two parameters: the power law index ($\Gamma$) and a minimum delay time ($t_{\rm min}$). Under the reasonable assumption that EM counterparts are likely only detectable in the local universe, accessible by the current generation of GW detectors, we study how many host galaxies at $z\sim 0$ are required to constrain the DTD parameters. We find that the DTD is mainly imprinted in the statistics of massive galaxies (stellar mass of $M_*\gtrsim 10^{10.5}$ M$_\odot$, comparable to the host galaxy of GW170817). Taking account of relevant uncertainties we find that $\mathcal{O}(10^3)$ host galaxies are required to constrain the DTD; for a fixed value of $t_{\rm min}$, as done in previous analyses of the DTD, $\mathcal{O}(10^2)$ host galaxies will suffice. Such a sample will become available within the next two decades, prior to the advent of third-generation GW detectors.

arXiv:1904.08440v1 [pdf, other]
A New Radio Molecular Line Survey of Planetary Nebulae: HNC/HCN as a Diagnostic of Ultraviolet Irradiation
Comments: 17 pages, 17 figures, 6 tables, accepted for publication in Astronomy & Astrophysics

Certain planetary nebulae contain shells, filaments, or globules of cold gas and dust whose heating and chemistry are likely driven by UV and X-ray emission from their central stars and from wind-collision-generated shocks. We present the results of a survey of molecular line emission in the 88-236 GHz range from nine nearby (<1.5 kpc) planetary nebulae spanning a range of UV and X-ray luminosities, using the 30 m telescope of the Institut de Radioastronomie Millimetrique. Rotational transitions of thirteen molecules, including CO isotopologues and chemically important trace species, were observed and the results compared with and augmented by previous studies of molecular gas in PNe. Lines of the molecules HCO+, HNC, HCN, and CN, which were detected in most objects, represent new detections for five planetary nebulae in our study. Specifically, we present the first detections of 13CO (1-0, 2-1), HCO+, CN, HCN, and HNC in NGC 6445; HCO+ in BD+303639; 13CO (2-1), CN, HCN, and HNC in NGC 6853; and 13CO (2-1) and CN in NGC 6772. Flux ratios were analyzed to identify correlations between the central star and/or nebular UV and X-ray luminosities and the molecular chemistries of the nebulae. This analysis reveals a surprisingly robust dependence of the HNC/HCN line ratio on PN central star UV luminosity. There exists no such clear correlation between PN X-rays and various diagnostics of PN molecular chemistry. The correlation between HNC/HCN ratio and central star UV luminosity demonstrates the potential of molecular emission line studies of PNe for improving our understanding of the role that high-energy radiation plays in the heating and chemistry of photodissociation regions.

arXiv:1904.08442v1 [pdf, other]
Astro2020 Science White Paper: The Formation and Evolution of Multiple Star Systems
Comments: Science white paper submitted to the Astro2020 US Decadal Survey. 7 pages and 2 figures

Significant advances have been made over the past decade in the characterization of multiple protostar systems, enabled by the Karl G. Jansky Very Large Array (VLA), high-resolution infrared observations with the Hubble Space Telescope, and ground-based facilities. To further understand the mechanism(s) of multiple star formation, a combination of statistics, high-angular resolution radio/millimeter continuum imaging, characterization of kinematic structure, magnetic fields via polarimetry, and comparison with numerical simulations are needed. Thus, understanding the origin of stellar multiplicity in different regimes of companion separation will soon be within reach. However, to overcome challenges that studies in this field are now confronted with, a range of new capabilities are required: a new millimeter/centimeter wave facility with 10 mas resolution at {\lambda}=1 cm, space-based near to far-infrared observatories, continued development of low to high resolution spectroscopy on 3m to 10m class telescopes, and an ELT-class telescope with near to mid-infrared imaging/spectroscopic capability.

arXiv:1904.08443v1 [pdf, other]
Astro2020 Science White Paper: Measuring Protostar Masses: The Key to Protostellar Evolution
Comments: Science white paper submitted to the Astro2020 US Decadal Survey. 7 pages and 3 figures

Knowledge of protostellar evolution has been revolutionized with the advent of surveys at near-infrared to submillimeter wavelengths. This has enabled the bolometric luminosities and bolometric temperatures (traditional protostellar evolution diagnostics) to be measured for large numbers of protostars. However, further progress is difficult without knowing the masses of the central protostars. Protostar masses can be most accurately determined via molecular line kinematics from millimeter interferometers (i.e., ALMA). Theoretical investigations have predicted the protostellar mass function (PMF) for various protostellar mass accretion models, and it is now imperative to observationally constrain its functional form. While ALMA has enabled protostellar mass measurements, samples approaching 100 sources are necessary to constrain the functional form of the PMF, and upgrades to ALMA and/or a new mm/cm facility will increase the feasibility of measuring such a large number of protostar masses. The masses of protostars will enable their stellar structure (radius and intrinsic luminosity), evolution, and accretion histories to be better understood. This is made more robust when effective temperatures and accretion rates can be measured via ground/space-based near to mid-infrared spectroscopy. Furthermore, access to supercomputing facilities is essential to fit the protostar masses via radiative transfer modeling and updated theoretical/numerical modeling of stellar structure may also be required.

arXiv:1904.08446v1 [pdf, other]
Mars Obliquity History Constrained by Elliptic Crater Orientations
Comments: No comment found

The dynamics of Mars' obliquity are believed to be chaotic, and the historical ~3.5 Gyr (late-Hesperian onward) obliquity probability density function (PDF) is high uncertain and cannot be inferred from direct simulation alone. Obliquity is also a strong control on post-Noachian Martian climate, enhancing the potential for equatorial ice/snow melting and runoff at high obliquities (> 40{\deg}) and enhancing the potential for desiccation of deep aquifers at low obliquities (< 25{\deg}). We developed a new technique using the orientations of elliptical craters to constrain the true late-Hesperian-onward obliquity PDF. To do so, we developed a forward model of the effect of obliquity on elliptic crater orientations using ensembles of simulated Mars impactors and ~3.5 Gyr-long Mars obliquity simulations. In our model, the inclinations and speeds of Mars crossing objects bias the preferred orientation of elliptic craters which are formed by low-angle impacts. Comparison of our simulation predictions with a validated database of elliptic crater orientations allowed us to invert for best-fitting obliquity history. We found that since the onset of the late-Hesperian, Mars' mean obliquity was likely low, between ~10{\deg} and ~30{\deg}, and the fraction of time spent at high obliquities > 40{\deg} was likely < 20%.

arXiv:1904.08447v1 [pdf, other]
Action-based models for dwarf spheroidal galaxies and globular clusters
Comments: submitted to MNRAS

A new family of self-consistent DF-based models of stellar systems is explored. The stellar component of the models is described by a distribution function (DF) depending on the action integrals, previously used to model the Fornax dwarf spheroidal galaxy (dSph). The stellar component may cohabit with either a dark halo, also described by a DF, or with a massive central black hole. In all cases we solve for the model's self-consistent potential. Focussing on spherically symmetric models, we show how the stellar observables vary with the anisotropy prescribed by the DF, with the dominance and nature of the dark halo, and with the mass of the black hole. We show that precise fits to the observed surface brightness profiles of four globular clusters can be obtained for a wide range of prescribed velocity anisotropies. We also obtain precise fits to the observed projected densities of four dSphs. Finally, we present a three-component model of the Scupltor dSph with distinct DFs for the red and blue horizontal branch stars and the dark matter halo.

arXiv:1904.08460v1 [pdf, other]
The Tilt of the Local Velocity Ellipsoid as Seen by Gaia
Comments: MNRAS, submitted

The Gaia Radial Velocity Spectrometer (RVS) provides a sample of 7,224,631 stars with full six-dimensional phase space information. Bayesian distances of these stars are available from the catalogue of Sch\"onrich et al. (2019). We exploit this to map out the behaviour of the velocity ellipsoid within 5 kpc of the Sun. We find that the tilt of the disc-dominated RVS sample is accurately described by the relation $\alpha = (0.921 \pm 0.008)\arctan (|z|/R)$, where ($R,z$) are cylindrical polar coordinates. This corresponds to velocity ellipsoids close to spherical alignment (for which the normalising constant would be unity) and pointing towards the Galactic centre. Flattening of the tilt of the velocity ellipsoids is enhanced close to the plane and Galactic centre, whilst at high elevations far from the Galactic center the population is consistent with exact spherical alignment. Using the LAMOST catalogue cross-matched with Gaia DR2, we construct disc and halo samples of high purity based on metallicity. We find that the tilt of disc stars straddles $\alpha = (0.909-1.038)\arctan (|z|/R)$, and of halo stars straddles $\alpha = (0.927-1.063)\arctan (|z|/R)$. We caution against the use of reciprocal parallax for distances in studies of the tilt, as this can lead to serious artefacts.

arXiv:1904.08463v1 [pdf, other]
Fast W-Projection for Wide-field Imaging
Comments: submitted MNRAS

Wide-field imaging has become a major challenge for modern radio astronomy, which uses high sensitivity acquisition systems that deal with huge amounts of data. In this paper we investigate a fast wide-field imaging solution based on the w-projection algorithm, which is intended for modern astronomy systems. The core idea of the proposed method is to reduce the computational complexity of the convolution kernel generation step, specifically by replacing the standard two-dimensional FFT by the one-dimensional Hankel transform. Experimental results show that the optimised w-projection proposed here produces equivalent dirty image results in a circular image region, at a significantly lower computational cost than standard $w$-projection. One of the main advantages of the proposed solution is its slow scaling with the number of w-planes, thus enabling more accurate output results at a lower computational cost.

arXiv:1904.08472v1 [pdf, other]
Supergranular turbulence in a quiet Sun: Lagrangian coherent structures
Comments: Submitted to MNRAS on 17 April, 2019

The quiet Sun exhibits a wealth of magnetic activities that are fundamental for our understanding of solar and astrophysical magnetism. The magnetic fields in the quiet Sun are observed to evolve coherently, interacting with each other to form distinguished structures as they are advected by the horizontal photospheric flows. We study coherent structures in photospheric flows in a region of quiet Sun consisted of supergranules. Supergranular turbulence is investigated by detecting hyperbolic and elliptic Lagrangian coherent structures (LCS) using the horizontal velocity fields derived from Hinode intensity maps. Repelling/attracting LCS are found by computing the forward/backward finite-time Lyapunov exponent (FTLE). The Lagrangian centre of a supergranular cell is given by the local maximum of the forward FTLE; the Lagrangian boundaries of supergranular cells are given by the ridges of the backward FTLE. Objective velocity vortices are found by calculating the Lagrangian-averaged vorticity deviation, and false vortices are filtered by applying a criterion given by the displacement vector. The Lagrangian centres of neighboring supergranular cells are interconnected by ridges of the repelling LCS, which provide the transport barriers that allow the formation of vortices and the concentration of strong magnetic fields in the valleys of the repelling LCS. The repelling LCS also reveal the most likely sites for magnetic reconnection. We show that the ridges of the attracting LCS expose the locations of the sinks of photospheric flows at supergranular junctions, which are the preferential sites for the formation of kG magnetic flux tubes and persistent vortices.

arXiv:1904.08485v1 [pdf, other]
Astro2020 Science White Paper: Are Supernovae the Dust Producer in the Early Universe?
Comments: Astro2020 Science White Paper: Dust Formation in Supernovae with US-ELT Perspective

Whether supernovae are a significant source of dust has been a long-standing debate. The large quantities of dust observed in high-redshift galaxies raise a fundamental question as to the origin of dust in the Universe since stars cannot have evolved to the AGB dust-producing phase in high-redshift galaxies. In contrast, supernovae occur within several millions of years after the onset of star formation. This white paper focuses on dust formation in supernova ejecta with US-Extremely Large Telescope (ELT) perspective during the era of JWST and LSST.

arXiv:1904.08526v1 [pdf, other]
The time-resolved spectra of photospheric emission from a structured jet for gamma-ray bursts
Comments: 18 pages, 17 figures, submitted to ApJ

The quasi-thermal components found in many Fermi gamma-ray bursts (GRBs) imply that the photosphere emission indeed contributes to the prompt emission of many GRBs. But whether the observed spectra empirically fitted by the Band function or cutoff power law, especially the spectral and peak energy ($E_{p}$) evolutions can be explained by the photosphere emission model alone needs further discussion. In this work, we investigate in detail the time-resolved spectra and $E_{p}$ evolutions of photospheric emission from a structured jet, with an inner-constant and outer-decreasing angular Lorentz factor profile. Also, a continuous wind with a time-dependent wind luminosity has been considered. We show that the photosphere spectrum near the peak luminosity is similar to the cutoff power-law spectrum. The spectrum can have the observed average low-energy spectral index $\alpha $ $ \sim -1$, and the distribution of the low-energy spectral index in our photosphere model is similar to that observed ($-2\lesssim $ $\alpha \lesssim 0$). Furthermore, the two kinds of spectral evolutions during the decay phase, separated by the width of the core ($\theta _{c}$), are consistent with the time-resolved spectral analysis results of several Fermi multi-pulse GRBs and single-pulse GRBs, respectively. Also, for this photosphere model we can reproduce the two kinds of observed $E_{p}$ evolution patterns rather well. Thus, by considering the photospheric emission from a structured jet, we reproduce the observations well for the GRBs best fitted by the cutoff power-law model for the peak-flux spectrum or the time-integrated spectrum.

arXiv:1904.08564v1 [pdf, other]
Kinematics of the OVI Circumgalactic Medium: Halo Mass Dependence and Outflow Signatures
Comments: 15 pages, 6 figures, 2 tables. Submitted to ApJ on April 17, 2019

We probe the high-ionization circumgalactic medium by examining absorber kinematics, absorber-galaxy kinematics, and average absorption profiles of 31 OVI absorbers from the "Multiphase Galaxy Halos" Survey as a function of halo mass, redshift, inclination, and azimuthal angle. The galaxies are isolated at $0.12<z_{\rm gal}<0.66$ and are probed by a background quasar within $D\approx 200$ kpc. Each absorber-galaxy pair has Hubble Space Telescope images and COS quasar spectra, and most galaxy redshifts have been accurately measured from Keck/ESI spectra. Using the pixel-velocity two-point correlation function (TPCF) method, we find that OVI absorber kinematics have a strong halo mass dependence. Absorbers hosted by $\sim L^{\ast}$ galaxies have the largest velocity dispersions, which we interpret to be that the halo virial temperature closely matches the temperature at which the collisionally ionized OVI fraction peaks. Lower mass galaxies and group environments have smaller velocity dispersions. Total column densities follow the same behavior, consistent with theoretical findings. After normalizing out the observed mass dependence, we studied absorber-galaxy kinematics with a modified TPCF and found non-virialized motions due to outflowing gas. Edge-on minor axis gas has large optical depths concentrated near the galaxy systemic velocity as expected for bipolar outflows, while face-on minor axis gas has a smoothly decreasing optical depth distribution out to large normalized absorber-galaxy velocities, suggestive of decelerating outflowing gas. Accreting gas signatures are not observed due to "kinematic blurring" in which multiple line-of-sight structures are observed. These results indicate that galaxy mass dominates OVI properties over baryon cycle processes.

arXiv:1904.08567v1 [pdf, other]
Search for nearby Earth analogs I. 15 planet candidates found in PFS data
Comments: 30 pages, 20 figures, 3 tables, accepted for publication in ApJS

The radial velocity method plays a major role in the discovery of nearby exoplanets. To efficiently find planet candidates from the data obtained in high precision radial velocity surveys, we apply a signal diagnostic framework to detect radial velocity signals that are statistically significant, consistent in time, robust to the choice of noise models, and not correlated with stellar activity. Based on the application of this approach to the survey data of the Planet Finder Spectrograph (PFS), we report fifteen planet candidates located in fourteen stellar systems. We find that the orbits of the planet candidates around HD 210193, 103949, 8326, and 71135 are consistent with temperate zones around these stars (where liquid water could exist on the surface). With periods of 7.76 and 15.14 days respectively, the planet candidates around star HIP 54373 form a 1:2 resonance system. These discoveries demonstate the feasibility of automated detection of exoplanets from large radial velocity surveys, which may provide a complete sample of nearby Earth analogs.

arXiv:1904.08579v1 [pdf, other]
Particle Energy Diffusion in Linear Magnetohydrodynamic Waves
Comments: 12 pages, 8 figures, accepted for publication in ApJ

In high-energy astronomical phenomena, the stochastic particle acceleration by turbulences is one of promising processes to generate non-thermal particles. In this paper, providing a temporally evolving wave ensemble, which consists of a single mode (Alfv\'en, fast or slow) of linear magnetohydrodynamic waves, we investigate the energy diffusion efficiency of relativistic particles. In addition to the gyroresonance with waves, the transit time damping (TTD) also contributes to the energy diffusion for fast and slow mode waves. While the resonance condition with the TTD has been considered to be fulfilled by very small fraction of particles, our simulations show that a significant fraction of particles are in the TTD resonance due to the resonance broadening by the mirror force, which non-resonantly diffuses the pitch angle of particles. When the cutoff scale in the turbulence spectrum is smaller than the Larmor radius of a particle, the gyroresonance is the main acceleration mechanism for all the three wave modes. For the fast mode, the coexistence of the gyroresonance and TTD resonance leads to anomalous energy diffusion. For a particle with its Larmor radius smaller than the cutoff scale, the gyroresonance is negligible, and the TTD becomes dominant mechanism to diffuse its energy. The energy diffusion by the TTD-only resonance with fast mode waves agrees with the hard-sphere-like acceleration suggested in some of high-energy astronomical phenomena.

arXiv:1904.08612v1 [pdf, other]
SkyMapper SEDs of nearby galaxies: quenching and bursting probed by a change index for star formation
Comments: 20 pages, 14 figures, submitted to PASA

The wish list of astronomers includes a tool that reveals quenching of star formation in galaxies directly as it proceeds. Here, we present a proof-of-concept for a new quenching-and-bursting diagnostic, a "change index" for star formation, that requires only photometric data, provided they include filters such as the violet $uv$ bands used by SkyMapper. The index responds mostly to changes in star-formation rate on a timescale of 20 to 500 Myr and is nearly insensitive to dust extinction. It works effectively to distances of 100 to 150 Mpc. We explore its application to eight example galaxies in SkyMapper DR2, including known E+A and Seyfert-1 galaxies. Owing to the degeneracies inherent in broad-band photometry, the change index can only be a qualitative indicator of changes in star-formation rate. But once the SkyMapper Southern Survey is complete, the change index will be available for every spatial resolution element of every galaxy in the Southern sky within its working distance range.

arXiv:1904.08660v1 [pdf, other]
The Fornax Deep Survey (FDS) with VST. VI. Optical properties of the dwarf galaxies in the Fornax cluster
Comments: Accepted to be published in Astronomy & Astrophysics

The Fornax Deep Survey Dwarf galaxy Catalog (FDSDC) includes 564 dwarf galaxies in the Fornax cluster and the in-falling Fornax A subgroup. We use the FDSDC galaxies for statistical analysis of the structural and stellar population differences in the range of galactic environments within the Fornax cluster. We present the standard scaling relations for the dwarfs and analyze trends as a function of cluster-centric radius. We find a different behavior for the bright dwarfs (-18.5 mag < M$_r$ < -16 mag) as compared to the fainter ones (M$_r$ > -16 mag): While considering galaxies in the same magnitude-bins, we find that, while for fainter dwarfs the g'-r' color is redder for lower surface brightness objects (as expected from fading stellar populations), for brighter dwarfs the color is redder for the higher surface brightness and higher S\'ersic n objects. The trend of the bright dwarfs might be explained by those galaxies being affected by harassment and by slower quenching of star formation in their inner parts. As the fraction of early-type dwarfs with respect to late-types increases toward the central parts of the cluster, the color-surface brightness trends are also manifested in the cluster-centric trends, confirming that it is indeed the environment that changes the galaxies. We also estimate the strengths of the ram-pressure stripping, tidal disruption, and harassment in the Fornax cluster, and find that our observations are consistent with the theoretically expected ranges of galaxy properties where each of those mechanisms dominate. We furthermore find that the luminosity function, color-magnitude relation, and axis-ratio distribution of the dwarfs in the center of the Fornax cluster are similar to those in the center of the Virgo cluster.

arXiv:1904.08693v1 [pdf, other]
Convolutional neural networks: a magic bullet for gravitational-wave detection?
Comments: The first two authors contributed equally

In the last few years, machine learning techniques, in particular convolutional neural networks, have been investigated as a method to replace or complement traditional matched filtering techniques that are used to detect the gravitational-wave signature of merging black holes. However, to date, these methods have not yet been successfully applied to the analysis of long stretches of data recorded by the Advanced LIGO and Virgo gravitational-wave observatories. In this work, we critically examine the use of convolutional neural networks as a tool to search for merging black holes. We identify the strengths and limitations of this approach, highlight some common pitfalls in translating between machine learning and gravitational-wave astronomy, and discuss the interdisciplinary challenges. In particular, we explain in detail why convolutional neural networks alone can not be used to claim a statistically significant gravitational-wave detection. However, we demonstrate how they can still be used to rapidly flag the times of potential signals in the data for a more detailed follow-up. Our convolutional neural network architecture as well as the proposed performance metrics are better suited for this task than a standard binary classifications scheme. A detailed evaluation of our approach on Advanced LIGO data demonstrates the potential of such systems as trigger generators. Finally, we sound a note of caution by constructing adversarial examples, which showcase interesting "failure modes" of our model, where inputs with no visible resemblance to real gravitational-wave signals are identified as such by the network with high confidence.

arXiv:1904.08713v1 [pdf, other]
Spot evolution on LQ Hya during 2006--2017: temperature maps based on SOFIN and FIES data
Comments: 9 pages, 5 figures

Context. LQ Hya is one of the most frequently studied young solar analogue stars. Recently, it has been observed to show intriguing behaviour by analysing long-term photometry: during 2003--2009, a coherent spot structure migrating in the rotational frame has been reported by various authors, but since that time the star has entered a chaotic state where coherent structures seem to have disappeared and rapid phase jumps of the photometric minima occur irregularly over time. Aims. LQ Hya is one of the stars included in the SOFIN/FIES long-term monitoring campaign extending over 25 years. Here we publish new temperature maps for the star during 2006--2017, covering the chaotic state of the star. Methods. We use a Doppler imaging technique to derive surface temperature maps from high-resolution spectra. Results. From the mean temperatures of the Doppler maps we see a weak but systematic increase in the surface temperature of the star. This is consistent with the simultaneously increasing photometric magnitude. During nearly all observing seasons we see a high-latitude spot structure which is clearly nonaxisymmetric. The phase behaviour of this structure is very chaotic but agrees reasonably well with the photometry. Equatorial spots are also frequently seen, but many of them we interpret to be artefacts due to the poor to moderate phase coverage. Conclusions. Even during the chaotic phase of the star, the spot topology has remained very similar to the higher activity epochs with more coherent and long-lived spot structures: we see high-latitude and equatorial spot activity, the mid latitude range still being most often void of spots. We interpret the erratic jumps and drifts in phase of the photometric minima to be caused by changes in the high-latitude spot structure rather than the equatorial spots.

arXiv:1904.08781v1 [pdf, other]
A line-binned treatment of opacities for the spectra and light curves from neutron star mergers
Comments: 27 pages, 22 figures. arXiv admin note: text overlap with arXiv:1702.02990

The electromagnetic observations of GW170817 were able to dramatically increase our understanding of neutron star mergers beyond what we learned from gravitational waves alone. These observations provided insight on all aspects of the merger from the nature of the gamma-ray burst to the characteristics of the ejected material. The ejecta of neutron star mergers are expected to produce such electromagnetic transients, called kilonovae or macronovae. Characteristics of the ejecta include large velocity gradients, relative to supernovae, and the presence of heavy $r$-process elements, which pose significant challenges to the accurate calculation of radiative opacities and radiation transport. For example, these opacities include a dense forest of bound-bound features arising from near-neutral lanthanide and actinide elements. Here we investigate the use of fine-structure, line-binned opacities that preserve the integral of the opacity over frequency. Advantages of this area-preserving approach over the traditional expansion-opacity formalism include the ability to pre-calculate opacity tables that are independent of the type of hydrodynamic expansion and that eliminate the computational expense of calculating opacities within radiation-transport simulations. Tabular opacities are generated for all 14 lanthanides as well as a representative actinide element, uranium. We demonstrate that spectral simulations produced with the line-binned opacities agree well with results produced with the more accurate continuous Monte Carlo Sobolev approach, as well as with the commonly used expansion-opacity formalism. Additional investigations illustrate the convergence of opacity with respect to the number of included lines, and elucidate sensitivities to different atomic physics approximations, such as fully and semi-relativistic approaches.

arXiv:1904.08786v1 [pdf, other]
Structured jets and X-ray plateaus in Gamma-ray Bursts
Comments: submitted to ApJ

The first multi-messenger detection of a binary system of neutron stars, GW170817, brought to the forefront the structured jet model as a way to explain multi-wavelength observations taken over one year. Here we show that high-latitude emission (HLE) from a structured jet can naturally produce an X-ray plateau in gamma-ray burst (GRB) light curves, independent of the radiation from an external shock. We calculate the radiation from a switched-off shell featuring an angular structure in both its relativistic bulk motion and intrinsic luminosity. Our model is able to explain the shallow decay phase (plateau) often observed in GRB X-ray light curves, and its spectral shapes. We discuss the possible contribution of the structured jet HLE to other distinctive features of GRB X-ray light curves, and its capability to explain their observed optical/X-ray temporal behaviour.

arXiv:1904.08797v1 [pdf, other]
Debris Disk Composition: A Diagnostic Tool for Planet Formation and Migration
Comments: Science white paper submitted to the Astro2020 Decadal Survey

Debris disks are exoplanetary systems containing planets, minor bodies (such as asteroids and comets) and debris dust. Unseen planets are presumed to perturb the minor bodies into crossing orbits, generating small dust grains that are detected via remote sensing. Debris disks have been discovered around main sequence stars of a variety of ages (from 10 Myr to several Gyr) and stellar spectral types (from early A-type to M-type stars). As a result, they serve as excellent laboratories for understanding whether the architecture and the evolution of our Solar System is common or rare. This white paper addresses two outstanding questions in debris disk science: (1) Are debris disk minor bodies similar to asteroids and comets in our Solar System? (2) Do planets separate circumstellar material into distinct reservoirs and/or mix material during planet migration? We anticipate that SOFIA/HIRMES, JWST, and WFIRST/CGI will greatly improve our understanding of debris disk composition, enabling the astronomical community to answer these questions. However, we note that despite their observational power, these facilities will not provide large numbers of detections or detailed characterization of cold ices and silicates in the Trans Neptunian zone. Origins Space Telescope is needed to revolutionize our understanding of the bulk composition and mixing in exoplanetary systems.

arXiv:1904.08803v1 [pdf, other]
A merger shock in Abell 1367
Comments: 5 pages, 1 figure, MNRAS Letters in press

Multi-wavelength observations show that Abell 1367 (A1367) is a dynamically young cluster, with at least two subclusters merging along the SE-NW direction. With the wide-field XMM-Newton mosaic of A1367, we discover a previously unknown merger shock at the NW edge of the cluster. We estimate the shock Mach number from the density and temperature jumps as $M_{\rho}=1.21\pm0.08$ and $M_T=1.60\pm0.07$, respectively. This shock region also corresponds to a radio relic discovered with the VLA and GBT, which could be produced by the shock re-acceleration of pre-existing seed relativistic electrons. We suggest that some of the seed relativistic electrons originate from late-type, star-forming galaxies in this region.

arXiv:1904.08815v1 [pdf, other]
Anomalous Cepheids discovered in a sample of Galactic short period Type II Cepheids
Comments: 23 pages, 10 figures

We revisited the short period Type II Cepheids (T2Cs), called the BL Herculis (BLHs), in the Galactic Field to derive a homogeneous analysis of their Fourier parameters. Only V-band data were compiled to make sure that it was directly comparable between the known variables of the OGLE-III catalogue and the 59 individual objects classified as short period Type II Cepheids in the General Catalogue of Variable Stars (GCVS) we had in our sample. The derived Fourier parameters were used to make the distinction between different classes of variables. From the 59 stars we found 19 BLHs, 19 fundamental mode Anomalous Cepheids (ACs) (8 of them were already known from the Catalina Sky Survey (CSS)), 1 first overtone AC, 2 were found to be possible peculiar W Virginis (pWVir), 11 classical Cepheids (DCEPs), and 7 stars were not pulsating variables at all. As a result we created a list of bright BLH stars in the Galactic Field, and separated the ACs, as well as other objects that were misclassified. The number of true BLHs decreased in our sample by more than 50%. We gathered the metallicity from spectroscopic measurements published in the literature. While the number of actual measurements is low, it is highly suggestive that ACs are metal poor. The mean metallicity from 8 measurements in 4 stars (UY Eri having 5 different [Fe/H] data points) is -1.12 dex, but if the higher value metallicity outliers of UY Eri are left out the mean metallicity becomes -1.88 dex, regardless if the AC is in the Milky Way itself or in a cluster. On the other hand, BLHs seem to have a Solar-like metallicity of 0 dex averaged from 21 measurements of 10 stars.

arXiv:1904.08821v1 [pdf, other]
Confirming interstellar C$_{60}^+$ using the Hubble Space Telescope
Comments: Accepted for publication in ApJL, April 2019

Recent advances in laboratory spectroscopy lead to the claim of ionized Buckminsterfullerene (C60+) as the carrier of two diffuse interstellar bands (DIBs) in the near-infrared. However, irrefutable identification of interstellar C60+ requires a match between the wavelengths and the expected strengths of all absorption features detectable in the laboratory and in space. Here we present Hubble Space Telescope (HST) spectra of the region covering the C60+ 9348, 9365, 9428 and 9577 {\AA} absorption bands toward seven heavily-reddened stars. We focus in particular on searching for the weaker laboratory C60+ bands, the very presence of which has been a matter for recent debate. Using the novel STIS-scanning technique to obtain ultra-high signal-to-noise spectra without contamination from telluric absorption that afflicted previous ground-based observations, we obtained reliable detections of the (weak) 9365, 9428 {\AA} and (strong) 9577 {\AA} C60+ bands. The band wavelengths and strength ratios are sufficiently similar to those determined in the latest laboratory experiments that we consider this the first robust identification of the 9428 {\AA} band, and a conclusive confirmation of interstellar C60+.

arXiv:1904.08834v1 [pdf, other]
Damping of Propagating Kink Waves in the Solar Corona
Comments: Accepted for publication in The Astrophysical Journal

Alfv\'enic waves have gained renewed interest since the existence of ubiquitous propagating kink waves were discovered in the corona. {It has long been suggested that Alfv\'enic} waves play an important role in coronal heating and the acceleration of the solar wind. To this effect, it is imperative to understand the mechanisms that enable their energy to be transferred to the plasma. Mode conversion via resonant absorption is believed to be one of the main mechanisms for kink wave damping, and is considered to play a key role in the process of energy transfer. This study examines the damping of propagating kink waves in quiescent coronal loops using the Coronal Multi-channel Polarimeter (CoMP). A coherence-based method is used to track the Doppler velocity signal of the waves, enabling us to investigate the spatial evolution of velocity perturbations. The power ratio of outward to inward propagating waves is used to estimate the associated damping lengths and quality factors. To enable accurate estimates of these quantities, {we provide the first derivation of a likelihood function suitable for fitting models to the ratio of two power spectra obtained from discrete Fourier transforms. Maximum likelihood estimation is used to fit an exponential damping model to the observed variation in power ratio as a function of frequency.} We confirm earlier indications that propagating kink waves are undergoing frequency dependent damping. Additionally, we find that the rate of damping decreases, or equivalently the damping length increases, for longer coronal loops that reach higher in the corona.

arXiv:1904.08836v1 [pdf, other]
All-Sky Near Infrared Space Astrometry
Comments: 7 pages

Gaia is currently revolutionizing modern astronomy. However, much of the Galactic plane, center and the spiral arm regions are obscured by interstellar extinction, rendering them inaccessible because Gaia is an optical instrument. An all-sky near infrared (NIR) space observatory operating in the optical NIR, separated in time from the original Gaia would provide microarcsecond NIR astrometry and millimag photometry to penetrate obscured regions unraveling the internal dynamics of the Galaxy.

arXiv:1904.08851v1 [pdf, other]
Integral constraints in spectroscopic surveys
Comments: 29 pages, 14 figures

Current clustering analyses of galaxy surveys rely on the knowledge of the survey selection function. Density fluctuations are estimated by comparing the galaxy density field to a random synthetic catalogue accounting for the survey density and geometry. However, this survey selection function is commonly partly inferred from the observed data itself, leading to so-called integral constraints. We present a new derivation of the global integral constraint effect, arising when the expected galaxy density is taken to be the measured one. We extend the formalism to the case where the full radial selection function is estimated from the data redshift distribution, as is often the case in the literature. We find that the radial integral constraint effect can be as significant as the window function correction at large scales. We model the radial integral constraint for a Redshift Space Distortions (RSD) analysis but we emphasise that it can be of paramount importance for large-scale studies of primordial non-Gaussianity. Its effect in configuration space cannot be safely ignored either. Finally, as a further application, we show that potential angular systematics can be mitigated by nulling the density contrast on a chosen angular scale. We model the subsequent loss of clustering by an angular integral constraint which can be combined with the radial one. We review the survey selection function normalisation, the shot noise contribution to the integral constraint corrections and wide-angle contributions.

arXiv:1904.08859v1 [pdf, other]
Redshift-weighted constraints on primordial non-Gaussianity from the clustering of the eBOSS DR14 quasars in Fourier space
Comments: 28 pages, 12 figures. Comments welcome!

We present constraints on local primordial non-Gaussianity (PNG), parametrized through $f^{\rm loc}_{\rm NL}$, using the Sloan Digital Sky Survey IV extended Baryon Oscillation Spectroscopic Survey Data Release 14 quasar sample. We measure and analyze the anisotropic clustering of the quasars in Fourier space, testing for the scale-dependent bias introduced by primordial non-Gaussianity on large scales. We derive and employ a power spectrum estimator using optimal weights that account for the redshift evolution of the PNG signal. We find constraints of $-51<f^{\rm loc}_{\rm NL}<21$ at 95% confidence level. These are amont the tightest constraints from Large Scale Structure (LSS) data. Our redshift weighting improves the error bar by 15% in comparison to the unweighted case. If quasars have lower response to PNG, the constraint degrades to $-81<f^{\rm loc}_{\rm NL}<26$, with a 40% improvement over the standard approach. We forecast that the full eBOSS dataset could reach $\sigma_{f^{\rm loc}_{\rm NL}}\simeq 5\text{-}8$ using optimal methods and full range of scales.

arXiv:1904.08860v1 [pdf, other]
Cool spots on the surface of the active giant PZ Mon
Comments: 5 pages, 7 figures

Based on the multiband (BVRIJHKL) photometric observations of the active red giant PZ Mon performed for the first time in the winter season of 2017-2018, we have determined the main characteristics of the spotted stellar surface in a parametric three-spot model. The unspotted surface temperature is Teff=4730 K, the temperature of the cool spots is Tspot=3500 K, their relative area is about 41%, and the temperature of the warm spots is Twarm=4500 K with a maximum relative area up to 20%. The distribution of spots over the stellar surface has been modeled. The warm spots have been found to be distributed at various longitudes in the hemisphere on the side of the secondary component and are most likely a result of its influence.

arXiv:1904.08867v1 [pdf, other]
A joint deconvolution algorithm to combine single dish and interferometer data for wideband multi-term and mosaic imaging
Comments: 15 pages, 19 figures. Accepted to The Astronomical Journal (16 April 2019)

Imaging in radio astronomy is usually carried out with a single-dish radio telescope doing a raster scan of a region of the sky or with an interferometer that samples the visibility function of the sky brightness. Mosaic observations are the current standard for imaging large fields of view with an interferometer and multi-frequency observations are now routinely carried out with both types of telescopes to increase the continuum imaging sensitivity and to probe spectral structure. This paper describes an algorithm to combine wideband data from these two types of telescopes in a joint iterative reconstruction scheme that can be applied to spectral cube or wideband multi-term imaging both for narrow fields of view as well as mosaics. Our results demonstrate the ability to prevent instabilities and error that typically arise when wide-band or joint mosaicing algorithms are presented with spatial and spectral structure that is inadequetely sampled by the interferometer alone. For comparable noise levels in the single dish and interferometer data, the numerical behaviour of this algorithm is expected to be similar to the idea of generating artificial visibilities from single dish data. However, our discussed implementation is simpler and more flexible in terms of applying relative data weighting schemes to match noise levels while preserving flux accuracy, fits within standard iterative image reconstruction frameworks, is fully compatible with wide-field and joint mosaicing gridding algorithms that apply corrections specific to the interferometer data and may be configured to enable spectral cube and wideband multi-term deconvolution for single-dish data alone.

arXiv:1904.08887v1 [pdf, other]
The accelerating rotation of the magnetic He-weak star HD 142990
Comments: 10 pages, 6 figures, 3 tables, accepted for publication in MNRAS

HD 142990 (V 913 Sco; B5 V) is a He-weak star with a strong surface magnetic field and a short rotation period ($P_{\rm rot} \sim 1$ d). While it is clearly a rapid rotator, recent determinations of $P_{\rm rot}$ are in formal disagreement. In this paper we collect magnetic and photometric data with a combined 40-year baseline in order to re-evaluate $P_{\rm rot}$ and examine its stability. Both period analysis of individual datasets and $O-C$ analysis of the photometric data demonstrate that $P_{\rm rot}$ has decreased over the past 30 years, violating expectations from magnetospheric braking models, but consistent with behaviour reported for 2 other hot, rapidly rotating magnetic stars, CU Vir and HD 37776. The available magnetic and photometric time series for HD 142990 can be coherently phased assuming a spin-up rate $\dot{P}$ of approximately $-0.6$ s/yr, although there is some indication that $\dot{P}$ may have slowed in recent years, possibly indicating an irregular or cyclic rotational evolution.

arXiv:1904.08896v1 [pdf, other]
Eigenfunctions of Galactic Phase Space Spirals from Dynamic Mode Decomposition
Comments: 8 pages, 6 figures

We introduce the method of Dynamic Mode Decomposition (DMD) to the field of galactic dynamics. In DMD the dynamics of non-linear systems is studied by determining the dominant modes of an approximate linear model for the evolution. Typically, the DMD algorithm is applied to simulation data. In this paper, we consider the evolution of a plane-symmetric collisionless system of massive particles (sheets) that evolve through their mutual gravitational interactions and through their interactions with an external potential. The system has been used extensively as a toy model for the vertical structure of the Galactic disc, which has been the subject of intense interest since the discovery of number count asymmetries, bulk motions, and phase spirals in the Galactic $z-v_z$ plane. We show that the DMD analysis allows us to write the distribution function as a linear combination of an equilibrium distribution, bending and breathing modes, and spiral modes. These DMD modes are essentially the eigenfunctions of a linear operator that is constructed from a sequence of simulation snapshots. The associated eigenvalues determine the lifetime and frequency of the modes. We outline how to apply the method to full three-dimensional simulations.

arXiv:1904.08898v1 [pdf, other]
Modelling the periodical variations in multiband polarisation and photometry for discs of binary Be stars
Comments: 20 pages, 19 figures; accepted for publication in MNRAS

The tidal interaction of a Be star with a binary companion forms two spiral arms that cause orbital modulation of the Be disc structure. The aim of this work is to identify observables in which this modulation is apparent. The structure of a Be disc in a coplanar circular binary system is computed with a smoothed-particle hydrodynamics code, and a radiation transfer code calculates the spectral energy distribution. Line depolarisation was confirmed, with polarisation profiles nearly reverse to emission-line profiles. The continuum flux maximizes for pole-on discs, but photometric variability maximizes for edge-on discs. The linear polarisation exhibits one or two maxima per orbital cycle. While polarisation variability in visible passbands is important only at low inclinations, infrared bands may demonstrate high orbital variability even at large inclinations. More evident is the modulation in the polarisation angle (PA) for low inclinations. The latter can be used to track azimuthal asymmetries for pole-on discs, where the spectroscopic variability in the violet-to-red (V/R) emission-component ratio disappears. PA reversals coincide with phases where V/R=1, tracking lines of sight directed towards regions where the approaching and receding arms overlap. Continuum flux and polarisation are mostly in phase for neighbouring wavelength regions. It is suggested that studies of non-symmetric discs distorted by tidal forces from a secondary star may be used to study disc variabilities of other origins.

arXiv:1904.08899v1 [pdf, other]
Non-ideal MHD simulation of HL Tau disk: formation of rings
Comments: 11 pages, 9 figures, submitted to ApJ

Recent high resolution observations unveil ring structures in circumstellar disks. The origin of these rings has been widely investigated under various theoretical scenarios. In this work we perform global 3D non-ideal MHD simulations including effects from both Ohmic resistivity and ambipolar diffusion (AD) to model the HL Tau disk. The non-ideal MHD diffusion profiles are calculated based on the global dust evolution calculation including sintering effects. Disk ionization structure changes dramatically across the snow line due to the change of dust size distribution close to snow line of major volatiles. We find that accretion is mainly driven by disk wind. Gaps and rings can be quickly produced from different accretion rates across snow line. Furthermore, ambipolar diffusion (AD) leads to highly preferential accretion at midplane, followed by magnetic reconnection. This results a local zone of decretion that drains of mass in the field reconnection area, which leaves a gap and an adjacent ring just outside it. Overall, under the favorable condition, both snow lines and non-ideal MHD effects can lead to gaseous gaps and rings in protoplanetary disks.

arXiv:1904.08907v1 [pdf, other]
Constraining the equation of state of high-density cold matter using nuclear and astronomical measurements
Comments: 32 pages, 12 figures, submitted to The Astrophysical Journal

The increasing richness of data related to cold dense matter, from laboratory experiments to neutron star observations, requires a framework for constraining the properties of such matter that makes use of all relevant information. Here we present a rigorous but practical Bayesian approach that can include diverse evidence, such as nuclear data and the inferred masses, radii, tidal deformabilities, moments of inertia, and gravitational binding energies of neutron stars. The method allows any parametrization of the equation of state to be used. We use a spectral parametrization to illustrate the implications of current measurements and show how future measurements in many domains could improve our understanding of cold catalyzed matter. In particular, we find that measurements of the masses of massive neutron stars such as PSR J0740+6620 (Cromartie et al. 2019) and multiple, high-precision measurements of tidal deformabilities will provide significant information about the equation of state of matter above ~5x nuclear saturation density.

arXiv:1904.08912v1 [pdf, other]
Axion-Dilaton Destabilization and the Hubble Tension
Comments: 4 pages, 2 figures

The discrepancy in measurements of the Hubble constant indicates new physics in dark energy, dark matter, or both. Drawing inspiration from string theory, where axions interact with the other moduli fields, including the dilaton, here we demonstrate that the dynamics of an interacting dilaton and axion naturally realizes the proposal of Early Dark Energy. In this setup, stabilization of the the dilaton is in part due to the axion, and in the early universe the dilaton contributes to dark energy. The combined axion-dilaton system is destabilized when the Hubble constant falls below the mass of the axion, triggering a phase of fast-roll evolution of the dilaton wherein its equation of state is $w=1$, and the early dark energy redshifts away as $a^{-6}$.

arXiv:1904.08917v1 [pdf, other]
Cosmic-ray transport from AMS-02 B/C data: benchmark models and interpretation
Comments: 16 pages, 4 figures

This article aims at establishing new benchmark scenarios for Galactic cosmic-ray propagation in the GV-TV rigidity range, based on fits to the AMS-02 B/C data with the USINE v3.5 propagation code. We employ a new fitting procedure, cautiously taking into account data systematic error correlations in different rigidity bins and considering Solar modulation potential and leading nuclear cross-section as nuisance parameters. We delineate specific low, intermediate, and high-rigidity ranges that can be related to both features in the data and peculiar microphysics mechanisms resulting in spectral breaks. We single out a scenario which yields excellent fits to the data and includes all the presumably relevant complexity, the BIG model. This model has two limiting regimes: (i) the SLIM model, a minimal diffusion-only setup, and (ii) the QUAINT model, a convection-reacceleration model where transport is tuned by non-relativistic effects. All models lead to robust predictions in the high-energy regime ($\gtrsim10$GV), i.e. independent of the propagation scenario: at $1\sigma$, the diffusion slope $\delta$ is $[0.43-0.53]$, whereas $K_{10}$, the diffusion coefficient at 10GV, is $[0.26-0.36]$kpc$^2$Myr$^{-1}$; we confirm the robustness of the high-energy break, with a typical value $\Delta_h\sim 0.2$. We also find a hint for a similar (reversed) feature at low rigidity around the B/C peak ($\sim 4$GV) which might be related to some effective damping scale in the magnetic turbulence.

arXiv:1904.08922v1 [pdf, other]
Blue, white, and red ocean planets - Simulations of orbital variations in flux and polarization colors
Comments: Accepted for publication in Astron. Astrophys

An exoplanet's habitability will depend strongly on the presence of liquid water. Flux and/or polarization measurements of starlight that is reflected by exoplanets could help to identify exo-oceans. We investigate which broadband spectral features in flux and polarization phase functions of reflected starlight uniquely identify exo-oceans. We compute total fluxes F and polarized fluxes Q of starlight reflected by cloud-free and (partly) cloudy exoplanets, for wavelengths from 350 to 865 nm. The ocean surface has waves composed of Fresnel reflecting wave facets and whitecaps, and scattering within the water body is included. Total flux F, polarized flux Q, and degree of polarization P of ocean planets change color from blue, through white, to red at phase angles alpha ranging from 134 -108 deg for F, and from 123-157 deg for Q, with cloud coverage fraction fc increasing from 0.0 to 1.0 for F, and to 0.98 for Q. The color change in P only occurs for fc ranging from 0.03-0.98, with the color crossing angle alpha ranging from 88-161 deg. The total flux F of a cloudy, zero surface albedo planet can also change color, and for fc=0.0, an ocean planet's F will not change color for surface pressures ps > 8 bars. Polarized flux Q of a zero surface albedo planet does not change color for any fc. The color change of P of starlight reflected by an exoplanet, from blue, through white, to red with increasing alpha above 88 deg, appears to identify a (partly) cloudy exo-ocean. The color change of polarized flux Q with increasing alpha above 123 deg appears to uniquely identify an exo-ocean, independent of surface pressure or cloud fraction. At the color changing phase angle, the angular distance between a star and its planet is much larger than at the phase angle where the glint appears in reflected light. The color change in polarization thus offers better prospects for detecting exo-oceans.