Galactic Population Research Articles

Weighing Milky Way satellites with LISA

ABSTRACT White dwarf stars are a well-established tool for studying Galactic stellar populations. Two white dwarfs in a tight binary system offer us an additional messenger – gravitational waves – for exploring the Milky Way and its immediate surroundings. Gravitational waves produced by double white dwarf (DWD) binaries can be detected by the future Laser Interferometer Space Antenna (LISA). Numerous and widespread DWDs have the potential to probe shapes, masses, and formation histories of the stellar populations in the Galactic neighbourhood. In this work we outline a method for estimating the total stellar mass of Milky Way satellite galaxies based on the number of DWDs detected by LISA. To constrain the mass we perform a Bayesian inference using binary population synthesis models and considering the number of detected DWDs associated with the satellite and the measured distance to the satellite as the only inputs. Based on a fiducial binary population synthesis model we find that for large satellites the stellar masses can be recovered to within (1) a factor 2 if the star formation history (SFH) is known and (2) an order of magnitude when marginalizing over different SFH models. For smaller satellites we can place upper limits on their stellar mass. Gravitational wave observations can provide mass measurements for large satellites that are comparable, and in some cases more precise, than standard electromagnetic observations.

PBjam: A Python Package for Automating Asteroseismology of Solar-like Oscillators*

Abstract Asteroseismology is an exceptional tool for studying stars using the properties of observed modes of oscillation. So far the process of performing an asteroseismic analysis of a star has remained somewhat esoteric and inaccessible to nonexperts. In this software paper we describe PBjam, an open-source Python package for analyzing the frequency spectra of solar-like oscillators in a simple but principled and automated way. The aim of PBjam is to provide a set of easy-to-use tools to extract information about the radial and quadropole oscillations in stars that oscillate like the Sun, which may then be used to infer bulk properties such as stellar mass, radius, age, or even structure. Asteroseismology and its data analysis methods are becoming increasingly important as space-based photometric observatories are producing a wealth of new data, allowing asteroseismology to be applied in a wide range of contexts such as exoplanet, stellar structure and evolution, and Galactic population studies.

The TESS-Keck Survey. II. An Ultra-short-period Rocky Planet and Its Siblings Transiting the Galactic Thick-disk Star TOI-561

Abstract We report the discovery of TOI-561, a multiplanet system in the galactic thick disk that contains a rocky, ultra-short-period planet. This bright (V = 10.2) star hosts three small transiting planets identified in photometry from the NASA TESS mission: TOI-561 b (TOI-561.02, P = 0.44 days, R p = 1.45 ± 0.11 R ⊕), c (TOI-561.01, P = 10.8 days, R p = 2.90 ± 0.13 R ⊕), and d (TOI-561.03, P = 16.3 days, R p = 2.32 ± 0.16 R ⊕). The star is chemically ([Fe/H] = −0.41 ± 0.05, [α/Fe] = +0.23 ± 0.05) and kinematically consistent with the galactic thick-disk population, making TOI-561 one of the oldest (10 ± 3 Gyr) and most metal-poor planetary systems discovered yet. We dynamically confirm planets b and c with radial velocities from the W. M. Keck Observatory High Resolution Echelle Spectrometer. Planet b has a mass and density of 3.2 ± 0.8 M ⊕ and g cm−3, consistent with a rocky composition. Its lower-than-average density is consistent with an iron-poor composition, although an Earth-like iron-to-silicates ratio is not ruled out. Planet c is 7.0 ± 2.3 M ⊕ and 1.6 ± 0.6 g cm−3, consistent with an interior rocky core overlaid with a low-mass volatile envelope. Several attributes of the photometry for planet d (which we did not detect dynamically) complicate the analysis, but we vet the planet with high-contrast imaging, ground-based photometric follow-up, and radial velocities. TOI-561 b is the first rocky world around a galactic thick-disk star confirmed with radial velocities and one of the best rocky planets for thermal emission studies.

Constraining stellar population parameters from narrow band photometric surveys using convolutional neural networks

ABSTRACT Upcoming large-area narrow band photometric surveys, such as Javalambre Physics of the Accelerating Universe Astrophysical Survey (J-PAS), will enable us to observe a large number of galaxies simultaneously and efficiently. However, it will be challenging to analyse the spatially resolved stellar populations of galaxies from such big data to investigate galaxy formation and evolutionary history. We have applied a convolutional neural network (CNN) technique, which is known to be computationally inexpensive once it is trained, to retrieve the metallicity and age from J-PAS-like narrow-band images. The CNN was trained using synthetic photometry from the integral field unit spectra of the Calar Alto Legacy Integral Field Area survey and the age and metallicity obtained in a full spectral fitting on the same spectra. We demonstrate that our CNN model can consistently recover age and metallicity from each J-PAS-like spectral energy distribution. The radial gradients of the age and metallicity for galaxies are also recovered accurately, irrespective of their morphology. However, it is demonstrated that the diversity of the data set used to train the neural networks has a dramatic effect on the recovery of galactic stellar population parameters. Hence, future applications of CNNs to constrain stellar populations will rely on the availability of quality spectroscopic data from samples covering a wide range of population parameters.

The Swift bulge survey: motivation, strategy, and first X-ray results

ABSTRACT Very faint X-ray transients (VFXTs) are X-ray transients with peak X-ray luminosities (LX) of L$_X \lesssim 10^{36}$ erg s−1, which are not well understood. We carried out a survey of 16 deg2 of the Galactic Bulge with the Swift Observatory, using short (60 s) exposures, and returning every 2 weeks for 19 epochs in 2017–18 (with a gap from 2017 November to 2018 February, when the Bulge was in sun-constraint). Our main goal was to detect and study VFXT behaviour in the Galactic Bulge across various classes of X-ray sources. In this work, we explain the observing strategy of the survey, compare our results with the expected number of source detections per class, and discuss the constraints from our survey on the Galactic VFXT population. We detected 91 X-ray sources, 25 of which have clearly varied by a factor of at least 10. In total, 45 of these X-ray sources have known counterparts: 17 chromospherically active stars, 12 X-ray binaries, 5 cataclysmic variables (and 4 candidates), 3 symbiotic systems, 2 radio pulsars, 1 active galactic nuclei, and a young star cluster. The other 46 are of previously undetermined nature. We utilize X-ray hardness ratios, searches for optical/infrared counterparts in published catalogues, and flux ratios from quiescence to outburst to constrain the nature of the unknown sources. Of these 46, 7 are newly discovered hard transients, which are likely VFXT X-ray binaries. Furthermore, we find strong new evidence for a symbiotic nature of four sources in our full sample, and new evidence for accretion power in six X-ray sources with optical counterparts. Our findings indicate that a large subset of VXFTs is likely made up of symbiotic systems.

The Gaia-ESO Survey: Oxygen Abundance in the Galactic Thin and Thick Disks*

Abstract We analyze the oxygen abundances of a stellar sample representative of the two major Galactic populations: the thin and thick disks. The aim is to investigate the differences between members of the Galactic disks and contribute to the understanding of the origin of oxygen chemical enrichment in the Galaxy. The analysis is based on the [O i] = 6300.30 Å oxygen line in high-resolution spectra (R ∼ 52,500) obtained from the Gaia-ESO public spectroscopic Survey (GES). By comparing the observed spectra with a theoretical data set computed in LTE with the SPECTRUM synthesis and ATLAS12 codes, we derive the oxygen abundances of 516 FGK dwarfs for which we have previously measured carbon abundances. Based on kinematic, chemical, and dynamical considerations, we identify 20 thin and 365 thick disk members. We study the potential trends of both subsamples in terms of their chemistry ([O/H], [O/Fe], [O/Mg], and [C/O] versus [Fe/H] and [Mg/H]), age, and position in the Galaxy. The main results are that (a) [O/H] and [O/Fe] ratios versus [Fe/H] show systematic differences between thin and thick disk stars with an enhanced O abundance of thick disk stars with respect to thin disk members and a monotonic decrement of [O/Fe] with increasing metallicity, even at metal-rich regime; (b) there is a smooth correlation of [O/Mg] with age in both populations, suggesting that this abundance ratio can be a good proxy of stellar ages within the Milky Way; and (c) thin disk members with [Fe/H] ≃ 0 display a [C/O] ratio smaller than the solar value, suggesting a possibly outward migration of the Sun from lower Galactocentric radii.

Mapping the stellar age of the Milky Way bulge with the VVV

Context. A detailed study of the Galactic bulge stellar population necessarily requires an accurate representation of the interstellar extinction, particularly toward the Galactic plane and center, where severe and differential reddening is expected to vary on sub-arcmin scales. Although recent infrared surveys have addressed this problem by providing extinction maps across the whole Galactic bulge area, dereddened color-magnitude diagrams near the plane and center appear systematically undercorrected, prompting the need for higher resolution. These undercorrections affect any stellar study sensitive to color (e.g., star formation history analyses via color-magnitude diagram fitting), either making them inaccurate or limiting them to small and relatively stable extinction windows where this value is low and better constrained. Aims. This study is aimed at providing a high-resolution (2 arcmin to ∼10 arcsec) color excess map for the VVV bulge area in J − Ks color. Methods. We used the MW-BULGE-PSFPHOT catalogs, sampling ∼300 deg2 across the Galactic bulge (|l| < 10° and −10° < b < 5°) to isolate a sample of red clump and red giant branch stars, for which we calculated the average J − Ks color in a fine spatial grid in (l, b) space. Results. We obtained an E(J − Ks) map spanning the VVV bulge area of roughly 300 deg2, with the equivalent of a resolution between ∼1 arcmin for bulge outskirts (l < 6°) to below 20 arcsec within the central |l| < 1°, and below 10 arcsec for the innermost area (|l| < 1° and |b| < 3°).

The TeV Gamma-Ray Luminosity of the Milky Way and the Contribution of H.E.S.S. Unresolved Sources to Very High Energy Diffuse Emission

Abstract H.E.S.S. has recently completed the first systematic survey of the Galactic plane in the TeV energy domain. We analyze the flux, latitude, and longitude distributions of γ-ray sources observed by H.E.S.S. in order to infer the properties of the Galactic TeV source population. We show that the total Milky Way luminosity in the 1–100 TeV energy range is . Evaluating the cumulative flux expected at Earth by the considered population, we show that H.E.S.S. unresolved sources provide a relevant contribution to the diffuse Galactic emission. Finally, in the hypothesis that the majority of bright sources detected by H.E.S.S. are powered by pulsar activity, like, e.g., pulsar wind nebulae or TeV halos, we estimate the main properties of the pulsar population.

Magnetar Giant Flare Origin for GRB 200415A Inferred from a New Scaling Relation

Abstract Soft gamma-ray repeaters (SGRs) are a mainly Galactic population and originate from neutron stars with intense (B ≃ 1015 G) magnetic fields (magnetars). Occasionally, a giant flare occurs with enormous intensity, displaying a short, hard spike followed by a weaker, oscillatory phase that exhibits the rotational period of the neutron star. If the magnetar giant flares occur in nearby galaxies, they would appear as cosmic, short-hard gamma-ray bursts (GRBs) without detecting the weak oscillatory phase. Recently, a short-hard GRB named GRB 200415A was detected, with a position coincident with the Sculptor Galaxy (NGC 253), raising the question of whether it is a classic short GRB or a magnetar giant flare. Here we show that magnetar giant flares follow a scaling relation between the spectral peak energy and the isotropic energy in 1 keV–10 MeV, i.e., , and locate in a distinct region of the E p–E iso plane from that of classic short GRBs. The relation can be well understood in the model that giant flares arise from the photosphere emissions of relativistically expanding fireballs. GRB 200415A, together with two other candidate giant flares (GRB 051103 and GRB 070201) follow this relation, which strongly favors the giant flare origin of these GRBs. The GeV emission detected by Fermi/LAT from GRB 200415A at 18–285 s can also be explained in the giant flare scenario. The total energy in the GeV emission implies a baryon load of ∼1023 g in the giant flare fireball of GRB 200415A.

Depletion of Bright Red Giants in the Galactic Center during Its Active Phases

Abstract Observations in the near-infrared domain showed the presence of the flat core of bright late-type stars inside ∼0.5 pc from the Galactic center supermassive black hole (Sgr A*), while young massive OB/Wolf–Rayet stars form a cusp. Several dynamical processes were proposed to explain this apparent paradox of the distribution of the Galactic center stellar populations. Given the mounting evidence on the significantly increased activity of Sgr A* during the past million years, we propose a scenario based on the interaction between the late-type giants and a nuclear jet, whose past existence and energetics can be inferred from the presence of γ-ray Fermi bubbles and bipolar radio bubbles. Extended, loose envelopes of red giant stars can be ablated by the jet with kinetic luminosity in the range of L j ≈ 1041– within the inner ∼0.04 pc of Sgr A* (S-cluster region), which would lead to their infrared luminosity decrease after several thousand jet–star interactions. The ablation of the atmospheres of red giants is complemented by the process of tidal stripping that operates at distances of ≲1 mpc, and by the direct mechanical interaction of stars with a clumpy disk at ≳0.04 pc, which can explain the flat density profile of bright late-type stars inside the inner half parsec from Sgr A*.

Characterizing the Evolved Stellar Population in the Galactic Foreground. I. Bolometric Magnitudes, Spatial Distribution and Period–Luminosity Relations

Abstract Radio campaigns using maser stellar beacons have provided crucial information to characterize Galactic stellar populations. Currently, the Bulge Asymmetries and Dynamical Evolution (BAaDE) project is surveying infrared (IR) color-selected targets for SiO masers. This provides a sample of evolved stars that can be used to study the inner, optically obscured Galaxy using line-of-sight velocities and possibly very long baseline interferometry proper motions. In order to use the BAaDE sample for kinematic studies, the stellar population should be characterized. In this study, the BAaDE targets have been cross-matched with IR (2MASS) and optical (Gaia) samples. By exploring the synergies of this cross-match together with Gaia parallaxes and extinction maps, the local (d < 2 kpc) asymptotic giant branch (AGB) stars can be characterized. We have defined a BAaDE–Gaia sample of 20,111 sources resulting from cross-matching BAaDE targets with IR and optical surveys. From this sample, a local sample of 1812 evolved stars with accurate parallax measurements, confirmed evolved stellar evolution stage, and within 2 kpc distance around the Sun was selected, for which absolute (bolometric) magnitudes are estimated. The evolved stellar population with Gaia counterparts that are variable seems to be predominantly associated with AGB stars with moderate luminosity ( ) and periods between 250 and 1250 days.

Stars Stripped in Binaries: The Living Gravitational-wave Sources

Abstract Binary interaction can cause stellar envelopes to be stripped, which significantly reduces the radius of the star. The orbit of a binary composed of a stripped star and a compact object can therefore be so tight that the gravitational radiation the system produces reaches frequencies accessible to the Laser Interferometer Space Antenna (LISA). Two such stripped stars in tight orbits with white dwarfs are known so far (ZTF J2130+4420 and CD−30°11223), but many more are expected to exist. These binaries provide important constraints for binary evolution models and may be used as LISA verification sources. We develop a Monte Carlo code that uses detailed evolutionary models to simulate the Galactic population of stripped stars in tight orbits with either neutron star or white dwarf companions. We predict 0–100 stripped star + white dwarf binaries and 0–4 stripped star + neutron star binaries with a signal-to-noise ratio >5 after 10 yr of observations with LISA. More than 90% of these binaries are expected to show large radial velocity shifts of ≳200 km s − 1 , which are spectroscopically detectable. Photometric variability due to tidal deformation of the stripped star is also expected and has been observed in ZTF J2130+4420 and CD−30°11223. In addition, the stripped star + neutron star binaries are expected to be X-ray bright with L X ≳ 1033–1036 erg s − 1 . Our results show that stripped star binaries are promising multimessenger sources for the upcoming electromagnetic and gravitational wave facilities.

A Late-time Radio Survey of Short Gamma-ray Bursts at z < 0.5: New Constraints on the Remnants of Neutron-star Mergers

Abstract Massive, rapidly spinning magnetar remnants produced as a result of binary neutron-star (BNS) mergers may deposit a fraction of their energy into the surrounding kilonova ejecta, powering a synchrotron radio signal from the interaction of the ejecta with the circumburst medium. Here, we present 6.0 GHz Very Large Array (VLA) observations of nine, low-redshift short gamma-ray bursts (GRBs; z < 0.5) on rest-frame timescales of ≈2.4–13.9 yr following the bursts. We place 3σ limits on radio continuum emission of F ν ≲ 6–20 μJy at the burst positions, or L ν ≲ (0.6–8.3) × 1028 erg s−1 Hz−1. Comparing these limits with new light-curve modeling that properly incorporates relativistic effects, we obtain limits on the energy deposited into the ejecta of E ej ≲ (0.6–6.7) × 1052 erg ( erg) for an ejecta mass of 0.03 M ⊙ (0.1 M ⊙). We present a uniform reanalysis of 27 short GRBs with 5.5–6.0 GHz observations, and find that ≳50% of short GRBs did not form stable magnetar remnants in their mergers. Assuming short GRBs are produced by BNS mergers drawn from the Galactic BNS population plus an additional component of high-mass GW194025-like mergers in a fraction f GW190425 of cases, we place constraints on the maximum mass of a nonrotating neutron star (NS; Tolman–Oppenheimer–Volkoff mass; M TOV), finding for f GW190425 = 0.4; this limit increases for larger values of f GW190425. The detection (or lack thereof) of radio remnants in untargeted surveys such as the VLA Sky Survey (VLASS) could provide more stringent constraints on the fraction of mergers that produce stable remnants. If radio remnants are discovered in VLASS, this suggests that short GRBs are a biased population of BNS mergers in terms of the stability of the remnants they produce.

CARMENES input catalogue of M dwarfs

Context. The relevance of M dwarfs in the search for potentially habitable Earth-sized planets has grown significantly in the last years. Aims. In our on-going effort to comprehensively and accurately characterise confirmed and potential planet-hosting M dwarfs, in particular for the CARMENES survey, we have carried out a comprehensive multi-band photometric analysis involving spectral energy distributions, luminosities, absolute magnitudes, colours, and spectral types, from which we have derived basic astrophysical parameters. Methods. We have carefully compiled photometry in 20 passbands from the ultraviolet to the mid-infrared, and combined it with the latest parallactic distances and close-multiplicity information, mostly from Gaia DR2, of a sample of 2479 K5 V to L8 stars and ultracool dwarfs, including 2210 nearby, bright M dwarfs. For this, we made extensive use of Virtual Observatory tools. Results. We have homogeneously computed accurate bolometric luminosities and effective temperatures of 1843 single stars, derived their radii and masses, studied the impact of metallicity, and compared our results with the literature. The over 40 000 individually inspected magnitudes, together with the basic data and derived parameters of the stars, individual and averaged by spectral type, have been made public to the astronomical community. In addition, we have reported 40 new close multiple systems and candidates (ρ < 3.3 arcsec) and 36 overluminous stars that are assigned to young Galactic populations. Conclusions. In the new era of exoplanet searches around M dwarfs via transit (e.g. TESS, PLATO) and radial velocity (e.g. CARMENES, NIRPS+HARPS), this work is of fundamental importance for stellar and therefore planetary parameter determination.

Characterizing the continuous gravitational-wave signal from boson clouds around Galactic isolated black holes

Ultralight bosons can form large clouds around stellar-mass black holes via the superradiance instability. Through processes such as annihilation, these bosons can source continuous gravitational wave signals with frequencies within the range of LIGO and Virgo. If boson annihilation occurs, then the Galactic black hole population will give rise to many gravitational signals; we refer to this as the ensemble signal. We characterize the ensemble signal as observed by the gravitational-wave detectors; this is important because the ensemble signal carries the primary signature that a continuous wave signal has a boson annihilation origin. We explore how a broad set of black hole population parameters affects the resulting spin-0 boson annihilation signal and consider its detectability by recent searches for continuous gravitational waves. A population of $10^8$ black holes with masses up to $30\mathrm{M}_\odot$ and a flat dimensionless initial spin distribution between zero and unity produces up to a thousand signals loud enough to be in principle detected by these searches. For a more moderately spinning population the number of signals drops by about an order of magnitude, still yielding up to a hundred detectable signals for some boson masses. A non-detection of annihilation signals at frequencies between 100 and 1200 Hz disfavors the existence of scalar bosons with rest energies between $2\times10^{-13}$ and $2.5\times10^{-12}$ eV. Finally we show that, depending on the black hole population parameters, care must be taken in assuming that the continuous wave upper limits from searches for isolated signals are still valid for signals that are part of a dense ensemble: Between 200 and 300 Hz, we urge caution when interpreting a null result for bosons between 4 and $6\times10^{-13}$ eV.

Constraining Galactic Structure with the LISA White Dwarf Foreground

Abstract White dwarfs (WDs) comprise 95% of all stellar remnants, and are thus an excellent tracer of old stellar populations in the Milky Way. Current and planned telescopes are not able to directly probe the WD population in its entirety due to its inherently low luminosity. However, the Galactic population of double white dwarf binaries (DWDs) gives rise to a millihertz gravitational-wave foreground detectable by the Laser Interferometer Space Antenna (LISA). Here we show how characterizing the angular power of the WD foreground will enable probes of the Galactic structure in a novel way to determine whether the Galactic WD population traces the spatial distribution of young, bright stars, or traces a vertically heated spatial distribution associated with the Galaxy’s oldest stellar populations. We do this using a binary population synthesis study that incorporates different Galactic spatial distributions for the DWD population. We find that the level of anisotropy in the WD foreground’s angular power spectrum is dependent on the vertical scale height of the population, but show that multipole coefficients from the spherical harmonic decomposition must be considered individually because of LISA’s angular resolution. Finally, we show that LISA can probe the vertical scale height of the Galactic WD population with an accuracy of 300 pc, using the hexadecapole moment of the WD foreground.

A spectroscopic multiplicity survey of Galactic Wolf-Rayet stars

Context. It is now well established that the majority of massive stars reside in multiple systems. However, the effect of multiplicity is not sufficiently understood, resulting in a plethora of uncertainties about the end stages of massive-star evolution. In order to investigate these uncertainties, it is useful to study massive stars just before their demise. Classical Wolf-Rayet stars represent the final end stages of stars at the upper-mass end. The multiplicity fraction of these stars was reported to be ∼0.4 in the Galaxy but no correction for observational biases has been attempted. Aims. The aim of this study is to conduct a homogeneous radial-velocity survey of a magnitude-limited (V ≤ 12) sample of Galactic Wolf-Rayet stars to derive their bias-corrected multiplicity properties. The present paper focuses on 12 northern Galactic carbon-rich (WC) Wolf-Rayet stars observable with the 1.2 m Mercator telescope on the island of La Palma. Methods. We homogeneously measured relative radial velocities (RVs) for carbon-rich Wolf-Rayet stars using cross-correlation. Variations in the derived RVs were used to flag binary candidates. We investigated probable orbital configurations and provide a first correction of observational biases through Monte-Carlo simulations. Results. Of the 12 northern Galactic WC stars in our sample, seven show peak-to-peak RV variations larger than 10 km s−1, which we adopt as our detection threshold. This results in an observed spectroscopic multiplicity fraction of 0.58 with a binomial error of 0.14. In our campaign, we find a clear lack of short-period (P < ∼100 d), indicating that a large number of Galactic WC binaries likely reside in long-period systems. Finally, our simulations show that at the 10% significance level, the intrinsic multiplicity fraction of the Galactic WC population is at least 0.72.

Kraken reveals itself – the merger history of the Milky Way reconstructed with the E-MOSAICS simulations

ABSTRACT Globular clusters (GCs) formed when the Milky Way experienced a phase of rapid assembly. We use the wealth of information contained in the Galactic GC population to quantify the properties of the satellite galaxies from which the Milky Way assembled. To achieve this, we train an artificial neural network on the E-MOSAICS cosmological simulations of the co-formation and co-evolution of GCs and their host galaxies. The network uses the ages, metallicities, and orbital properties of GCs that formed in the same progenitor galaxies to predict the stellar masses and accretion redshifts of these progenitors. We apply the network to Galactic GCs associated with five progenitors: Gaia-Enceladus, the Helmi streams, Sequoia, Sagittarius, and the recently discovered ‘low-energy’ GCs, which provide an excellent match to the predicted properties of the enigmatic galaxy ‘Kraken’. The five galaxies cover a narrow stellar mass range [M⋆ = (0.6–4.6) × 108 M⊙], but have widely different accretion redshifts ($\mbox{$z_{\rm acc}$}=0.57\!-\!2.65$). All accretion events represent minor mergers, but Kraken likely represents the most major merger ever experienced by the Milky Way, with stellar and virial mass ratios of $\mbox{$r_{M_\star }$}=1$:$31^{+34}_{-16}$ and $\mbox{$r_{M_{\rm h}}$}=1$:$7^{+4}_{-2}$, respectively. The progenitors match the z = 0 relation between GC number and halo virial mass, but have elevated specific frequencies, suggesting an evolution with redshift. Even though these progenitors likely were the Milky Way’s most massive accretion events, they contributed a total mass of only log (M⋆, tot/M⊙) = 9.0 ± 0.1, similar to the stellar halo. This implies that the Milky Way grew its stellar mass mostly by in-situ star formation. We conclude by organizing these accretion events into the most detailed reconstruction to date of the Milky Way’s merger tree.

Implications of a Fast Radio Burst from a Galactic Magnetar

Abstract A luminous radio burst was recently detected in temporal coincidence with a hard X-ray flare from the Galactic magnetar SGR 1935+2154 with a time and frequency structure consistent with cosmological fast radio bursts (FRBs) and a fluence within a factor of ≲10 of the least energetic extragalactic FRB previously detected. Although active magnetars are commonly invoked FRB sources, several distinct mechanisms have been proposed for generating the radio emission that make different predictions for the accompanying higher-frequency radiation. We show that the properties of the coincident radio and X-ray flares from SGR 1935+2154, including their approximate simultaneity and relative fluence , as well as the duration and spectrum of the X-ray emission, are consistent with extant predictions for the synchrotron maser shock model. Rather than arising from the inner magnetosphere, the X-rays are generated by (incoherent) synchrotron radiation from thermal electrons heated at the same internal shocks that produce the coherent maser emission as ultrarelativistic flare ejecta collides with a slower particle outflow (e.g., as generated by earlier flaring activity) on a radial scale of cm. Although the rate of SGR 1935+2154–like bursts in the local universe is not sufficient to contribute appreciably to the extragalactic FRB rate, the inclusion of an additional population of more active magnetars with stronger magnetic fields than the Galactic population can explain both the FRB rate and the repeating fraction, but only if the population of active magnetars are born at a rate that is at least 2 orders of magnitude lower than that of the SGR 1935+2154–like magnetars. This may imply that the more active magnetar sources are not younger magnetars formed in a similar way to the Milky Way population (e.g., via ordinary supernovae) but are instead formed through more exotic channels, such as superluminous supernovae, accretion-induced collapse, or neutron star mergers.

A Catalog of 531 White Dwarf Candidates in the Local Galactic Halo from Gaia Data Release 2

Abstract We present a catalog of 531 white dwarf candidates that have large apparent transverse motions relative to the Sun ( km s−1), thus making them likely members of the local Galactic halo population. The candidates were selected from the Gaia Data Release 2 and are located in a great circle with 20° width running across both Galactic poles and the Galactic center and anticenter, a zone that spans 17.3% of the sky. The selection used a combination of kinematic and photometric properties, derived primarily from Gaia proper motions, G magnitudes, and color, and including parallax whenever available. Additional validation of the white dwarf candidates is made using PanSTARRS photometric (gri) data. Our final catalog includes not only stars having full kinematic and luminosity estimates from reliable Gaia parallax, but also stars with presently unreliable or no available Gaia parallax measurements. We argue that our method of selecting local halo objects with and without reliable parallax data leads us to round up all possible halo white dwarfs in the Gaia catalog (in that particular section of the sky) with recorded proper motions mas yr−1 and that pass our km s−1 threshold requirement. We expect this catalog will be useful for the study of the white dwarf population of the local Galactic halo.