103 research outputs found
The critical binary star separation for a planetary system origin of white dwarf pollution
The atmospheres of between one quarter and one half of observed single white
dwarfs in the Milky Way contain heavy element pollution from planetary debris.
The pollution observed in white dwarfs in binary star systems is, however, less
clear, because companion star winds can generate a stream of matter which is
accreted by the white dwarf. Here we (i) discuss the necessity or lack thereof
of a major planet in order to pollute a white dwarf with orbiting minor planets
in both single and binary systems, and (ii) determine the critical binary
separation beyond which the accretion source is from a planetary system. We
hence obtain user-friendly functions relating this distance to the masses and
radii of both stars, the companion wind, and the accretion rate onto the white
dwarf, for a wide variety of published accretion prescriptions. We find that
for the majority of white dwarfs in known binaries, if pollution is detected,
then that pollution should originate from planetary material.Comment: Accepted for publication in MNRA
Gaia 0007-1605: an old triple system with an inner brown dwarf-white dwarf binary and an outer white dwarf companion
We identify Gaia 0007–1605 A,C as the first inner brown dwarf–white dwarf binary of a hierarchical triple system in which the outer component is another white dwarf (Gaia 0007–1605 B). From optical/near-infrared spectroscopy obtained at the Very Large Telescope with the X-Shooter instrument and/or from Gaia photometry plus spectral energy distribution fitting, we determine the effective temperatures and masses of the two white dwarfs (12,018 ± 68 K and 0.54 ± 0.01 M¿ for Gaia 0007–1605 A and 4445 ± 116 K and 0.56 ± 0.05 M¿ for Gaia 0007–1605 B) and the effective temperature of the brown dwarf (1850 ± 50 K; corresponding to spectral type L3 ± 1). By analyzing the available TESS light curves of Gaia 0007–1605 A,C we detect a signal at 1.0446 ± 0.0015 days with an amplitude of 6.25 ppt, which we interpret as the orbital period modulated from irradiation effects of the white dwarf on the brown dwarf's surface. This drives us to speculate that the inner binary evolved through a common-envelope phase in the past. Using the outer white dwarf as a cosmochronometer and analyzing the kinematic properties of the system, we conclude that the triple system is about 10 Gyr old.Postprint (published version
The star formation history of Gaia white dwarf population through its colour-magnitude diagram
White dwarfs are the most common stellar remnants. Furthermore, as being old objects, their study could shed new light on different questions related to the history, formation and evolution of the Galaxy. Despite these objects have been broadly studied from a theoretical point of view, the observational data has been limited to a poor statistical sample, due to the intrinsic low luminosity of white dwarfs. However, thanks to the recent Gaia EDR3, for the first time, a significant sample of the White dwarf population of our Galaxy, containing around 13,000 objects up to 100 pc from the Sun, has been obtained. Such data, in particular its color-magnitude diagram, provides the ideal scenario for extracting the maximum information. This communication reports a work-in-progress of a widely applied technique for recovering the star formation history of galaxies through its color-magnitude diagram applied, in this case, for first time to the local White dwarf population.Peer ReviewedPostprint (published version
White dwarf Random Forest classification through Gaia spectral coefficients
The third data release of Gaia has provided approximately 220 million low
resolution spectra. Among these, about 100,000 correspond to white dwarfs. The
magnitude of this quantity of data precludes the possibility of performing
spectral analysis and type determination by human inspection. In order to
tackle this issue, we explore the possibility of utilising a machine learning
approach, based on a Random Forest algorithm. We aim to analyze the viability
of the Random Forest algorithm for the spectral classification of the white
dwarf population within 100 pc from the Sun, based on the Hermite coefficients
of Gaia spectra. We utilized the assigned spectral type from the Montreal White
Dwarf Database for training and testing our Random Forest algorithm. Once
validated, our algorithm model is applied to the rest of unclassified white
dwarfs within 100 pc. First, we started by classifying the two major spectral
type groups of white dwarfs: hydrogen-rich (DA) and hydrogen-deficient
(non-DA). Next, we explored the possibility of classifying the various spectral
subtypes, including in some cases the secondary spectral types. Our Random
Forest classification presented a very high recall (>80%) for DA and DB white
dwarfs, and a very high precision (>90%) for DB, DQ and DZ white dwarfs. As a
result we have assigned a spectral type to 9,446 previously unclassified white
dwarfs: 4,739 DAs, 76 DBs (60 of them DBAs), 4,437 DCs, 132 DZs and 62 DQs (9
of them DQpec). Despite the low resolution of Gaia spectra, the Random Forest
algorithm applied to the Gaia spectral coefficients proves to be a highly
valuable tool for spectral classification.Comment: 15 pages, 17 figures, 3 tables. Submitted to Astronomy & Astrophysic
Exsolution process in white dwarf stars
White dwarf (WD) stars are considered cosmic laboratories to study the
physics of dense plasma. Furthermore, the use of WD stars as cosmic clocks to
date stellar populations and main sequence companions demands an appropriate
understanding of the WD physics in order to provide precise ages for these
stars. We aim at studying exsolution in the interior of WD stars, a process in
which a crystallized ionic binary mixture separates into two solid solutions
with different fractions of the constituents. Depending on the parent solid
mixture composition, this process can release or absorb heat, thus leading to a
delay or a speed-up of WD cooling. Relying on accurate phase diagrams for
exsolution, we have modeled this process in hydrogen-rich WDs with both
carbon-oxygen and oxygen-neon core composition, with masses ranging from 0.53
to 1.29Msun and from 1.10 to 1.29Msun, respectively. Exsolution is a slow
process that takes place at low luminosities (log(L/Lsun)-2.75) and
effective temperatures (Teff18 000K) in WDs. We find that exsolution
begins at brighter luminosities in CO than in ONe WDs of the same mass. Massive
WDs undergo exsolution at brighter luminosities than their lower-mass
counterparts. The net effect of exsolution on the WD cooling times depends on
the stellar mass and the exact chemical profile. For standard core chemical
profiles and preferred assumptions regarding miscibility gap microphysics, the
cooling delay can be as large as ~0.35 Gyrs at L/Lsun ~ -5. We have neglected a
chemical redistribution possibly associated with this process, which could lead
to a further cooling delay. Exsolution has a marginal effect on the WD cooling
times and, accordingly, we find no WD branches on the Gaia color magnitude
diagram associated with it. However, exsolution in massive WDs can alter the
faint end of the WD luminosity function, thus impacting WD cosmochronology.Comment: Accepted for publication in Astronomy & Astrophysic
The Gaia DR2 halo white dwarf population: the luminosity function, mass distribution and its star formation history
We analyze the volume-limited nearly complete 100 pc sample of 95 halo white
dwarf candidates identified by the second data release of Gaia. Based on a
detailed population synthesis model, we apply a method that relies on Gaia
astrometry and photometry to accurately derive the individual white dwarf
parameters (mass, radius, effective temperature, bolometric luminosity and
age). This method is tested with 25 white dwarfs of our sample for which we
took optical spectra and performed spectroscopic analysis. We build and analyse
the halo white dwarf luminosity function, for which we find for the first time
possible evidences of the cut-off at its faintest end, leading to an age
estimate of Gyr. The mass distribution of the sample peaks at
, with of the white dwarf masses below
and just two massive white dwarfs of more than
. From the age distribution we find three white dwarfs with
total ages above 12 Gyr, of which J1312-4728 is the oldest white dwarf known
with an age of Gyr. We prove that the star formation history is
mainly characterised by a burst of star formation that occurred from 10 to 12
Gyr in the past, but extended up to 8 Gyr. We also find that the peak of the
star formation history is centered at around 11 Gyr, which is compatible with
the current age of the Gaia-Enceladus encounter. Finally, of our halo
sample is contaminated by high-speed young objects (total age<7 Gyr). The
origin of these white dwarfs is unclear but their age distribution may be
compatible with the encounter with the Sagittarius galaxy.Comment: 15 pages, 9 figures, 2 tables; accepted for publication in MNRA
The age-metallicity relation from a sample of white dwarf-main sequence binarie
The age-metallicity relation (AMR) is a fundamental observational constraint for un-derstanding how the Galactic disc formed and evolved chemically in time. However, there is not yet an agreement on the observational properties of the AMR, primarily due to the difficulty inobtaining accurate ages for individual field stars. We have started an observational campaign for providing new observational input by using wide white dwarf-main sequence (WDMS) binaries.WDs are natural clocks and can be used to derive accurate ages. Metallicities can be obtained from the MS companions. Since the progenitors of WDs and the MS stars were born at the sametime, WDMS provide a unique opportunity to constrain in a robust way the properties of the AMR. We present the AMR derived from analysing a pilot sample of 23 WDMS and provide clear evidence for the lack of correlation between age and metallicity at young and intermediate ages.Peer ReviewedPostprint (published version
The population of white dwarf-main sequence binaries in the SDSS DR 12
We present a Monte Carlo population synthesis study of white dwarf-main sequence (WD+MS) binaries in the Galactic disc aimed at reproducing the ensemble properties of the entire population observed by the Sloan Digital Sky Survey (SDSS) Data Release 12. Our simulations take into account all known observational biases and use the most up-to-date stellar evolutionary models. This allows us to perform a sound comparison between the simulations and the observational data. We find that the properties of the simulated and observed parameter distributions agree best when assuming low values of the common envelope efficiency (0.2-0.3), a result that is in agreement with previous findings obtained by observational and population synthesis studies of close SDSSWD+MS binaries.We also show that all synthetic populations that result from adopting an initial mass ratio distribution with a positive slope are excluded by observations. Finally, we confirm that the properties of the simulated WD+MS binary populations are nearly independent of the age adopted for the thin disc, on the contribution of WD+MS binaries from the thick disc (0-17 per cent of the total population) and on the assumed fraction of the internal energy that is used to eject the envelope during the common envelope phase (0.1-0.5).Peer ReviewedPostprint (published version
White dwarf-main sequence binaries from LAMOST: the DR1 catalogue
Context. White dwarf-main sequence (WDMS) binaries are used to study several
different important open problems in modern astrophysics.
Aims. The Sloan Digital Sky Survey (SDSS) identified the largest catalogue of
WDMS binaries currently known. However, this sample is seriously affected by
selection effects and the population of systems containing cool white dwarfs
and early-type companions is under-represented.Here we search for WDMS binaries
within the spectroscopic data release 1 of the LAMOST (Large sky Area
Multi-Object fiber Spectroscopic Telescope) survey. LAMOST and SDSS follow
different target selection algorithms. Hence, LAMOST WDMS binaries may be drawn
from a different parent population and thus help in overcoming the selection
effects incorporated by SDSS on the current observed population.
Methods. We develop a fast and efficient routine based on the wavelet
transform to identify LAMOST WDMS binaries containing a DA white dwarf and a M
dwarf companion, and apply a decomposition/fitting routine to their LAMOST
spectra to estimate their distances and measure their stellar parameters,
namely the white dwarf effective temperatures, surface gravities and masses,
and the secondary star spectral types.
Results. We identify 121 LAMOST WDMS binaries, 80 of which are new
discoveries, and estimate the sample to be \sim90 per cent complete. The LAMOST
and SDSS WDMS binaries are found to be statistically different. However, this
result is not due to the different target selection criteria of both surveys,
but likely a simple consequence of the different observing conditions. Thus,
the LAMOST population is found at considerably shorter distances (\sim50-450
pc) and is dominated by systems containing early-type companions and hot white
dwarfs. (abridged)Comment: 14 pages, 8 figures, accepted for publication in A&
The kinematics of white dwarfs from the SDSS DR12
We use the Sloan Digital Sky Survey Data Release 12, which is the largest availablewhite dwarf catalogue to date, to study the evolution of the kinematical properties of the pop-ulation of white dwarfs of the Galactic disk. We derive masses, ages, photometric distances andradial velocities for all white dwarfs with hydrogen-rich atmospheres. For those stars for whichproper motions from the USNO-B1 catalogue are available, the three-dimensional componentsof the velocity are obtained. This subset of the original sample comprises 20,247 stars, makingit the largest sample of white dwarfs with measured three-dimensional velocities. The volumeprobed by our sample is large, allowing us to obtain relevant kinematical information. In partic-ular, our sample extends from a Galactocentric radial distanceRG=7.8 to 9.3 kpc, and verticaldistances from the Galactic plane ranging fromZ=+0.5to–0.5kpc.Peer ReviewedPostprint (published version
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