The population of hot subdwarf stars studied with Gaia II. The Gaia DR2 catalogue of hot subluminous stars

Based on data from the ESA Gaia Data Release 2 (DR2) and several ground-based, multi-band photometry surveys we compiled an all-sky catalogue of $39\,800$ hot subluminous star candidates selected in Gaia DR2 by means of colour, absolute magnitude and reduced proper motion cuts. We expect the majority of the candidates to be hot subdwarf stars of spectral type B and O, followed by blue horizontal branch stars of late B-type (HBB), hot post-AGB stars, and central stars of planetary nebulae. The contamination by cooler stars should be about $10\%$. The catalogue is magnitude limited to Gaia $G<19\,{\rm mag}$ and covers the whole sky. Except within the Galactic plane and LMC/SMC regions, we expect the catalogue to be almost complete up to about $1.5\,{\rm kpc}$. The main purpose of this catalogue is to serve as input target list for the large-scale photometric and spectroscopic surveys which are ongoing or scheduled to start in the coming years. In the long run, securing a statistically significant sample of spectroscopically confirmed hot subluminous stars is key to advance towards a more detailed understanding of the latest stages of stellar evolution for single and binary stars.Comment: 13 pages, A&A, accepte

The Field White Dwarf Mass Distribution

We revisit the properties and astrophysical implications of the field white dwarf mass distribution in preparation of Gaia applications. Our study is based on the two samples with the best established completeness and most precise atmospheric parameters, the volume-complete survey within 20 pc and the Sloan Digital Sky Survey (SDSS) magnitude-limited sample. We explore the modelling of the observed mass distributions with Monte Carlo simulations, but find that it is difficult to constrain independently the initial mass function (IMF), the initial-to-final-mass relation (IFMR), the stellar formation history (SFH), the variation of the Galactic disk vertical scale height as a function of stellar age, and binary evolution. Each of these input ingredients has a moderate effect on the predicted mass distributions, and we must also take into account biases owing to unidentified faint objects (20 pc sample), as well as unknown masses for magnetic white dwarfs and spectroscopic calibration issues (SDSS sample). Nevertheless, we find that fixed standard assumptions for the above parameters result in predicted mean masses that are in good qualitative agreement with the observed values. It suggests that derived masses for both studied samples are consistent with our current knowledge of stellar and Galactic evolution. Our simulations overpredict by 40-50% the number of massive white dwarfs (M > 0.75 Msun) for both surveys, although we can not exclude a Salpeter IMF when we account for all biases. Furthermore, we find no evidence of a population of double white dwarf mergers in the observed mass distributions.Comment: 15 pages, 16 figures, accepted for publication in MNRA

The population of hot subdwarf stars studied with Gaia I. The catalogue of known hot subdwarf stars

In preparation for the upcoming all-sky data releases of the Gaia mission we compiled a catalogue of known hot subdwarf stars and candidates drawn from the literature and yet unpublished databases. The catalogue contains 5613 unique sources and provides multi-band photometry from the ultraviolet to the far infrared, ground based proper motions, classifications based on spectroscopy and colours, published atmospheric parameters, radial velocities and light curve variability information. Using several different techniques we removed outliers and misclassified objects. By matching this catalogue with astrometric and photometric data from the Gaia mission, we will develop selection criteria to construct a homogeneous, magnitude-limited all-sky catalogue of hot subdwarf stars based on Gaia data.Comment: 11 pages, A&A accepte

When flux standards go wild: white dwarfs in the age of Kepler

White dwarf stars have been used as flux standards for decades, thanks to their staid simplicity. We have empirically tested their photometric stability by analyzing the light curves of 398 high-probability candidates and spectroscopically confirmed white dwarfs observed during the original Kepler mission and later with K2 Campaigns 0-8. We find that the vast majority (>97 per cent) of non-pulsating and apparently isolated white dwarfs are stable to better than 1 per cent in the Kepler bandpass on 1-hr to 10-d timescales, confirming that these stellar remnants are useful flux standards. From the cases that do exhibit significant variability, we caution that binarity, magnetism, and pulsations are three important attributes to rule out when establishing white dwarfs as flux standards, especially those hotter than 30,000 K.Comment: Accepted for publication in MNRAS; 7 pages, 4 figures, 2 table

Accretion of a giant planet onto a white dwarf star

The detection of a dust disc around G29-38 and transits from debris orbiting WD1145+017 confirmed that the photospheric trace metals found in many white dwarfs arise from the accretion of tidally disrupted planetesimals. The composition of these planetesimals is similar to that of rocky bodies in the inner solar system. Gravitationally scattering planetesimals towards the white dwarf requires the presence of more massive bodies, yet no planet has so far been detected at a white dwarf. Here we report optical spectroscopy of a $\simeq27\,750$K hot white dwarf that is accreting from a circumstellar gaseous disc composed of hydrogen, oxygen, and sulphur at a rate of $\simeq3.3\times10^9\,\mathrm{g\,s^{-1}}$. The composition of this disc is unlike all other known planetary debris around white dwarfs, but resembles predictions for the makeup of deeper atmospheric layers of icy giant planets, with H$_2$O and H$_2$S being major constituents. A giant planet orbiting a hot white dwarf with a semi-major axis of $\simeq15$ solar radii will undergo significant evaporation with expected mass loss rates comparable to the accretion rate onto the white dwarf. The orbit of the planet is most likely the result of gravitational interactions, indicating the presence of additional planets in the system. We infer an occurrence rate of spectroscopically detectable giant planets in close orbits around white dwarfs of $\simeq10^{-4}$.Comment: Nature, December 5 issu

Partly burnt runaway stellar remnants from peculiar thermonuclear supernovae

We report the discovery of three stars that, along with the prototype LP40-365, form a distinct class of chemically peculiar runaway stars that are the survivors of thermonuclear explosions. Spectroscopy of the four confirmed LP 40-365 stars finds ONe-dominated atmospheres enriched with remarkably similar amounts of nuclear ashes of partial O- and Si-burning. Kinematic evidence is consistent with ejection from a binary supernova progenitor; at least two stars have rest-frame velocities indicating they are unbound to the Galaxy. With masses and radii ranging between 0.20-0.28 Msun and 0.16-0.60 Rsun, respectively, we speculate these inflated white dwarfs are the partly burnt remnants of either peculiar Type Iax or electron-capture supernovae. Adopting supernova rates from the literature, we estimate that ~20 LP40-365 stars brighter than 19 mag should be detectable within 2 kpc from the Sun at the end of the Gaia mission. We suggest that as they cool, these stars will evolve in their spectroscopic appearance, and eventually become peculiar O-rich white dwarfs. Finally, we stress that the discovery of new LP40-365 stars will be useful to further constrain their evolution, supplying key boundary conditions to the modelling of explosion mechanisms, supernova rates, and nucleosynthetic yields of peculiar thermonuclear explosions.Comment: 22 pages, 14 figures, 6 tables. Accepted for publication on MNRA

White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Linewidths: Kepler Observations of 27 Pulsating DA White Dwarfs Through K2 Campaign 8

We present photometry and spectroscopy for 27 pulsating hydrogen-atmosphere white dwarfs (DAVs, a.k.a. ZZ Ceti stars) observed by the Kepler space telescope up to K2 Campaign 8, an extensive compilation of observations with unprecedented duration (>75 days) and duty cycle (>90%). The space-based photometry reveals pulsation properties previously inaccessible to ground-based observations. We observe a sharp dichotomy in oscillation mode linewidths at roughly 800 s, such that white dwarf pulsations with periods exceeding 800 s have substantially broader mode linewidths, more reminiscent of a damped harmonic oscillator than a heat-driven pulsator. Extended Kepler coverage also permits extensive mode identification: We identify the spherical degree of 61 out of 154 unique radial orders, providing direct constraints of the rotation period for 20 of these 27 DAVs, more than doubling the number of white dwarfs with rotation periods determined via asteroseismology. We also obtain spectroscopy from 4m-class telescopes for all DAVs with Kepler photometry. Using these homogeneously analyzed spectra we estimate the overall mass of all 27 DAVs, which allows us to measure white dwarf rotation as a function of mass, constraining the endpoints of angular momentum in low- and intermediate-mass stars. We find that 0.51-to-0.73-solar-mass white dwarfs, which evolved from 1.7-to-3.0-solar-mass ZAMS progenitors, have a mean rotation period of 35 hr with a standard deviation of 28 hr, with notable exceptions for higher-mass white dwarfs. Finally, we announce an online repository for our Kepler data and follow-up spectroscopy, which we collect at http://www.k2wd.org.Comment: 33 pages, 31 figures, 5 tables; accepted for publication in ApJS. All raw and reduced data are collected at http://www.k2wd.or

Stellar archaeology with Gaia: the Galactic white dwarf population

Gaia will identify several 1e5 white dwarfs, most of which will be in the solar neighborhood at distances of a few hundred parsecs. Ground-based optical follow-up spectroscopy of this sample of stellar remnants is essential to unlock the enormous scientific potential it holds for our understanding of stellar evolution, and the Galactic formation history of both stars and planets.Comment: Summary of a talk at the 'Multi-Object Spectroscopy in the Next Decade' conference in La Palma, March 2015, to be published in ASP Conference Series (editors Ian Skillen & Scott Trager