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Register a new user$Gaia$ white dwarfs within 40 pc II: the volume-limited northern hemisphere sample
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We present an overview of the sample of northern hemisphere white dwarfs within 40 pc of the Sun detected from $Gaia$ Data Release 2 (DR2). We find that 521 sources are spectroscopically confirmed degenerate stars, 111 of which were first identified as white dwarf candidates from $Gaia$ DR2 and followed-up recently with the William Herschel Telescope and Gran Telescopio Canarias. Three additional white dwarf candidates remain spectroscopically unobserved and six unresolved binaries are known to include a white dwarf but were not in our initial selection of white dwarfs in the $Gaia$ DR2 Hertzsprung-Russell diagram (HRD). Atmospheric parameters are calculated from $Gaia$ and Pan-STARRS photometry for all objects in the sample, confirming most of the trends previously observed in the much smaller 20 pc sample. Local white dwarfs are overwhelmingly consistent with Galactic disc kinematics, with only four halo candidates. We find that DAZ white dwarfs are significantly less massive than the overall DA population ($overline{M}_mathrm{DAZ} = 0.59,mathrm{M}_odot$, $overline{M}_mathrm{DA} = 0.66,mathrm{M}_odot$). It may suggest that planet formation is less efficient at higher mass stars, producing more massive white dwarfs. We detect a sequence of crystallised white dwarfs in the mass range from $0.6 lesssim M/mathrm{M}_odot lesssim 1.0,$ and find that the vast majority of objects on the sequence have standard kinematic properties that correspond to the average of the sample, suggesting that their nature can be explained by crystallisation alone. We also detect 56 wide binaries including a white dwarf and 26 double degenerates.
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We analyse the 100pc Gaia white dwarf volume-limited sample by means of VOSA (Virtual Observatory SED Analyser) with the aim of identifying candidates for displaying infrared excesses. Our search focuses on the study of the spectral energy distribution (SED) of 3,733 white dwarfs with reliable infrared photometry and GBP-GRP colours below 0.8 mag, a sample which seems to be nearly representative of the overall white dwarf population. Our search results in 77 selected candidates, 52 of which are new identifications. For each target we apply a two-component SED fitting implemented in VOSA to derive the effective temperatures of both the white dwarf and the object causing the excess. We calculate a fraction of infrared-excess white dwarfs due to the presence of a circumstellar disk of 1.6+-0.2%, a value which increases to 2.6+-0.3% if we take into account incompleteness issues. Our results are in agreement with the drop in the percentage of infrared excess detections for cool (<8,000K) and hot (>20,000K) white dwarfs obtained in previous analyses. The fraction of white dwarfs with brown dwarf companions we derive is ~0.1-0.2%.
Using Gaia DR2 data, we present an up-to-date sample of white dwarfs within 20 pc of the Sun. In total we identified 139 systems in Gaia DR2, nine of which are new detections, with the closest of these located at a distance of 13.05 pc. We estimated atmospheric parameters for all stellar remnants based on the Gaia parallaxes and photometry. The high precision and completeness of the Gaia astrometry allowed us to search for wide binary companions. We re-identified all known binaries where both components have accurate DR2 astrometry, and established the binarity of one of the nine newly identified white dwarfs. No new companions were found to previously known 20 pc white dwarfs. Finally, we estimated the local white dwarf space-density to be $(4.49pm0.38)times10^{-3}$ pc$^{-3}$, having given careful consideration to the distance-dependent Gaia completeness, which misses known objects at short distances, but is close to complete for white dwarfs near 20 pc.
We use the data provided by the Gaia Early Data Release 3 to search for a highly-complete volume-limited sample of unresolved binaries consisting of a white dwarf and a main sequence companion (i.e. WDMS binaries) within 100pc. We select 112 objects based on their location within the Hertzsprung-Russell diagram, of which 97 are new identifications. We fit their spectral energy distributions (SED) with a two-body fitting algorithm implemented in VOSA (Virtual Observatory SED Analyser) to derive the effective temperatures, luminosities and radii (hence surface gravities and masses) of both components. The stellar parameters are compared to those from the currently largest catalogue of close WDMS binaries, from the Sloan Digital Sky Survey (SDSS). We find important differences between the properties of the Gaia and SDSS samples. In particular, the Gaia sample contains WDMS binaries with considerably cooler white dwarfs and main sequence companions (some expected to be brown dwarfs). The Gaia sample also shows an important population of systems consisting of cool and extremely low-mass white dwarfs, not present in the SDSS sample. Finally, using a Monte Carlo population synthesis code, we find that the volume-limited sample of systems identified here seems to be highly complete (~80+-9 per cent), however it only represents ~9 per cent of the total underlying population. The missing ~91 per cent includes systems in which the main sequence companions entirely dominate the SEDs. We also estimate an upper limit to the total space density of close WDMS binaries of ~(3.7+-1.9)x10^{-4} pc{-3}.
Many stars evolve into magnetic white dwarfs, and observations may help to understand when the magnetic field appears at the stellar surface, if and how it evolves during the cooling phase, and what are the mechanisms that generate it. After obtaining new spectropolarimetric observations and combining them with previous literature data, we have checked the population of about 152 white dwarfs within 20 pc from the Sun for the presence of magnetic fields, with a sensitivity that ranges from better than 1 kG for most of the stars of spectral class DA, to 1 MG for some of the featureless white dwarfs. We find that 33 white dwarfs of the local 20 pc volume are magnetic. Statistically the data are consistent with the possibility that the frequency of the magnetic field occurrence is similar in stars of all spectral classes, except that in the local 20 pc volume, either DQ stars are more frequently magnetic, or host much stronger fields than average. The distribution of the observed field strength ranges from 40 kG to 300 MG and is uniform per decade, in striking contrast to the field frequency distribution resulting from spectroscopic surveys. No fields weaker than 40 kG are found. We confirm that magnetic fields are more frequent in white dwarfs with higher than average mass. We find a marked deficiency of magnetic white dwarfs younger than 0.5 Gyr, and that the frequency of the occurrence of the magnetic field is significantly higher in white dwarfs that have undergone the process of core crystallisation than in white dwarfs with fully liquid core. There is no obvious evidence of field strength decay with time. We discuss the implications of our findings in relation to some of the proposals that have been put forward to explain the origin and evolution of magnetic fields in degenerate stars, in particular those that predict the presence of a dynamo acting during the crystallisation phase.
In the Gaia era, the majority of stars in the Solar neighbourhood have parallaxes and proper motions precisely determined while spectroscopic age indicators are still missing for a large fraction of low-mass young stars. In this work we select 756 overluminous late K and early M young star candidates in the southern sky and observe them over 64 nights with the ANU 2.3m Telescope at Siding Spring Observatory using the Echelle (R=24,000) and Wide Field spectrographs (WiFeS, R=3000-7000). Our selection is kinematically unbiased to minimize the preference against low-mass members of stellar associations that dissipate first, and to include potential members of diffuse components. We provide measurements of H$alpha$ and calcium H&K emission, as well as lithium absorption line, that enable identification of stars as young as $sim$10-30 Myr which is a typical age of a stellar association. We report on 346 stars showing a detectable lithium line, 318 of which are not found in the known catalogs of young stars. We also report 126 additional stars in our sample which have no detectable lithium but signs of stellar activity indicating youth. Radial velocities are determined for WiFeS spectra with a precision of 3.2 $mathrm{km;s^{-1}}$ and 1.5 $mathrm{km;s^{-1}}$ for the Echelle sample.
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