144 research outputs found
A search for variable white dwarfs in large area time domain surveys: a pilot study in SDSS Stripe 82
We present a method to reliably select variable white dwarfs from large area
time domain surveys and apply this method in a pilot study to search for
pulsating white dwarfs in the Sloan Digital Sky Survey Stripe 82. From a sample
400 high-confidence white dwarf candidates, we identify 24 which show
significant variability in their multi-epoch Stripe 82 data. Using colours, we
further selected a sample of pulsating white dwarf (ZZ Ceti) candidates and
obtained high cadence follow up for six targets. We confirm five of our
candidates as cool ZZ Cetis, three of which are new discoveries. Among our 24
candidates we also identify: one eclipsing binary, two magnetic white dwarfs
and one pulsating PG1159 star. Finally we discuss the possible causes for the
variability detected in the remaining targets. Even with sparse multi-epoch
data over the limited area of Stripe 82, we demonstrate that our selection
method can successfully identify various types of variable white dwarfs and
efficiently select high-confidence ZZ Ceti candidates.Comment: Accepted for publication in MNRAS, 14 pages, 11 figure
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
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 . The catalogue is magnitude limited to Gaia
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
. 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
Stellar and planetary remnants in large area surveys
The advent of large-area digital sky surveys marked a turning point for the entire field of astronomy. Today, with multi-band photometry for hundreds of millions of objects readily at hand, the ability to mine data for specific rare objects of interest has become of fundamental importance.
The aim of this work was to study white dwarfs and planetary remnants by, first of all, developing efficient selection algorithms to identify these objects in large area surveys. Using SDSS DR7 we developed a routine which relies on colours and proper motion to calculate probabilities of being a white dwarf (PWD) which, in turn, enables a flexible selection of white dwarf candidates without recourse to spectroscopy. The application of this selection method to SDSS DR10 lead to the creation of a catalogue of ≃ 66, 000 bright (g ≤ 19) objects with calculated PWD from which it is possible to select ≃ 23, 000 high-confidence white dwarf candidates . The reliability of the method was further tested using a sample of spectroscopic objects from the LAMOST survey. This independent test confirmed the robustness of our algorithm and lead to the identification of 290 new white dwarfs. We also applied our selection routine to the recently released ATLAS DR2 to construct a preliminary catalogue of ≃ 9000 ATLAS white dwarf candidates. This catalogue represents the first sample of white dwarfs candidates in the southern hemisphere.
We later exploited our catalogue in several science project. We developed a separate selection algorithm to identify variable white dwarfs in large area time-domain surveys. To test this method we carried out a pilot search for pulsating white dwarfs using 400 high-confidence white dwarfs candidates with available multi-epoch photometry in SDSS Stripe 82. This test proved the ability of our method to select different types of variable white dwarfs and allowed to identify 5 pulsating white dwarfs, 3 of which are new discoveries.
During the development of our catalogue, we also identified 64 new metal polluted white dwarfs. Recent studies have shown that the metal pollution in these objects is the result of accretion of remnants of planetary systems. In a few cases these planetary remnants form a circustellar debris disc which can be detected as an infrared excess. Here we present the results of high-resolution spectroscopic follow-up of 15 of the newly identified metalpolluted white dwarfs. Using accurate spectral analyses of the atmospheres of these white dwarfs we determined chemical compositions and masses of the accreted bodies, and discuss the impact of these finding on the current knowledge of extra-solar planetary systems. Using optical and infrared photometric data from various large-area surveys we carried out a search for infrared excess around our newly identified metal polluted white dwarfs, and high-confidence white dwarf candidates. We identified four metal polluted white dwarfs with possible debris discs and compiled a list of ≃ 300 white dwarfs candidates with infrared excess ready for future spectroscopic follow-up
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
Analysis of cool DO-type white dwarfs from the Sloan Digital Sky Survey Data Release 10
We report on the identification of 22 new cool DO-type white dwarfs (WD)
detected in Data Release 10 (DR10) of the Sloan Digital Sky Survey (SDSS).
Among them, we found one more member of the so-called hot-wind DO WDs, which
show ultrahigh excitation absorption lines. Our non-LTE model atmosphere
analyses of these objects and two not previously analyzed hot-wind DO WDs,
revealed effective temperatures and gravities in the ranges Teff=45-80kK and
log g= 7.50-8.75. In eight of the spectra we found traces of C (0.001-0.01, by
mass). Two of these are the coolest DO WDs ever discovered that still show a
considerable amount of C in their atmospheres. This is in strong contradiction
with diffusion calculations, and probably, similar to what is proposed for DB
WDs, a weak mass-loss is present in DO WDs. One object is the most massive DO
WD discovered so far with a mass of 1.07 M_sun if it is an ONe-WD or 1.09 M_sun
if it is a CO-WD. We furthermore present the mass distribution of all known hot
non-DA (pre-) WDs and derive the hot DA to non-DA ratio for the SDSS DR7
spectroscopic sample. The mass distribution of DO WDs beyond the wind limit
strongly deviates from the mass distribution of the objects before the wind
limit. We address this phenomenon by applying different evolutionary input
channels. We argue that the DO WD channel may be fed by about 13% by
post-extreme-horizontal branch stars and that PG1159 stars and O(He) stars may
contribute in a similar extent to the non-DA WD channel.Comment: 13 pages, accepted for publication in A&
Can magnetic fields suppress convection in the atmosphere of cool white dwarfs? A case study on WD2105-820
Around 10% of white dwarfs exhibit global magnetic structures with fields
ranging from 1 kG to hundreds of MG. Recently, the first radiation
magnetohydrodynamics simulations of the atmosphere of white dwarfs showed that
convection should be suppressed in their photospheres for magnetic fields with
strengths B 50 kG. These predictions are in agreement with our
knowledge of stellar physics (e.g. energy transfer in strong magnetic field
regions of the solar photosphere), but have yet to be directly confirmed from
white dwarf observations. We obtained COS far-UV spectroscopy of the weakly
magnetic, hydrogen-atmosphere, white dwarf WD2105-820 and of three additional
non-magnetic, convective remnants (all in the range
9000-11,000 K). We fitted both the COS and the already available optical
spectra with convective and radiative atmospheric models. As expected, we find
that for two of the non-magnetic comparison stars only convective model fits
predict consistent values from both the optical and the FUV
spectra. In contrast, for WD2105-820 only the best fitting radiative model
produced consistent results.Comment: 8 pages, 7 figures, 1 table, accepted for publication in MNRA
A Word to the WISE: Confusion is Unavoidable for WISE-selected Infrared Excesses
Stars with excess infrared radiation from circumstellar dust are invaluable
for studies of exoplanetary systems, informing our understanding on processes
of planet formation and destruction alike. All-sky photometric surveys have
made the identification of dusty infrared excess candidates trivial, however,
samples that rely on data from WISE are plagued with source confusion, leading
to high false positive rates. Techniques to limit its contribution to
WISE-selected samples have been developed, and their effectiveness is even more
important as we near the end-of-life of Spitzer, the only facility capable of
confirming the excess. Here, we present a Spitzer follow-up of a sample of 22
WISE-selected infrared excess candidates near the faint-end of the WISE
detection limits. Eight of the 22 excesses are deemed the result of source
confusion, with the remaining candidates all confirmed by the Spitzer data. We
consider the efficacy of ground-based near-infrared imaging and astrometric
filtering of samples to limit confusion among the sample. We find that both
techniques are worthwhile for vetting candidates, but fail to identify all of
the confused excesses, indicating that they cannot be used to confirm
WISE-selected infrared excess candidates, but only to rule them out. This
result confirms the expectation that WISE-selected infrared excess samples will
always suffer from appreciable levels of contamination, and that care should be
taken in their interpretation regardless of the filters applied.Comment: 13 pages, 4 Figures; Accepted for publication in Ap
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