162 research outputs found
Atmospheric parameters and carbon abundance for hot DB white dwarfs
Atmospheric parameters for hot DB (helium atmosphere) white dwarfs near
effective temperatures of 25000K are extremely difficult to determine from
optical spectroscopy. This is particularly unfortunate, because this is the
range of variable DBV or V777 Her stars. Accurate atmospheric parameters are
needed to help or confirm the asteroseismic analysis of these objects. Another
important aspect is the new class of white dwarfs - the hot DQ - detected by
Dufour et al. (2007), with spectra dominated by carbon lines. The analysis
shows that their atmospheres are pure carbon. The origin of these stars is not
yet understood, but they may have an evolutionary link with the hotter DBs as
studied here. Our aim is to determine accurate atmospheric parameters and
element abundances and study the implications for the evolution white dwarfs of
spectral classes DB and hot DQ. High resolution UV spectra of five DBs are
studied with model atmospheres. We determine stellar parameters and abundances
or upper limits of C and Si. These objects are compared with cooler DBs below
20000K. We find photospheric C and no other heavy elements - with extremely
high limits on the C/Si ratio - in two of the five hot DBs. We compare various
explanations for this unusual composition, which have been proposed in the
literature: accretion of interstellar or circumstellar matter, radiative
levitation, carbon dredge-up from deeper interior below the helium layer, and a
residual stellar wind. None of these explanations is completely satisfactory,
and the problem of the origin of the hot DQ remains an open question
The frequency of planetary debris around young white dwarfs
(Abridged) We present the results of the first unbiased survey for metal
pollution among H-atmosphere (DA) white dwarfs with cooling ages of 20-200 Myr
and 17000K < Teff < 27000K, using HST COS in the far UV between 1130 and 1435
A. The atmospheric parameters and element abundances are determined using
theoretical models, which include the effects of element stratification due to
gravitational settling and radiative levitation.
We find 48 of the 85 DA white dwarfs studied, or 56% show traces of metals.
In 25 stars, the elements can be explained by radiative levitation alone,
although we argue that accretion has very likely occurred recently. The
remaining 23 white dwarfs (27%) must be currently accreting. Together with
previous studies, we find no accretion rate trend in cooling age from ~40 Myr
to ~2 Gyr. The median, main sequence progenitor of our sample corresponds to a
star of ~2 Msun, and we find 13 of 23 white dwarfs descending from 2-3 Msun
late B- and A-type stars to be currently accreting. Only one of 14 targets with
Mwd > 0.8 Msun is found to be currently accreting, which suggests a large
fraction are double-degenerate mergers, and the merger discs do not commonly
reform large planetesimals or otherwise pollute the remnant. We reconfirm our
previous finding that two white dwarf Hyads are currently accreting rocky
debris.
At least 27%, and possibly up to ~50%, of all white dwarfs with cooling ages
20-200 Myr are accreting planetary debris. At Teff > 23000K, the luminosity of
white dwarfs is likely sufficient to vaporize circumstellar dust, and hence no
stars with strong metal-pollution are found. However, planetesimal disruption
events should occur in this cooling age and Teff range as well, and likely
result in short phases of high mass transfer rates. It appears that the
formation of rocky planetary material is common around 2-3 Msun late B- and
A-type stars.Comment: Accepted by A&
The unbiased frequency of planetary signatures around single and binary white dwarfs using and
This paper presents combined IRAC and COS
results for a double-blind survey of 195 single and 22 wide binary white dwarfs
for infrared excesses and atmospheric metals. The selection criteria include
cooling ages in the range 9 to 300 Myr, and hydrogen-rich atmospheres so that
the presence of atmospheric metals can be confidently linked to ongoing
accretion from a circumstellar disc. The entire sample has infrared photometry,
whereas 168 targets have corresponding ultraviolet spectra. Three stars with
infrared excesses due to debris discs are recovered, yielding a nominal
frequency of per cent, while in stark contrast, the
fraction of stars with atmospheric metals is per cent. Thus, only one
out of 30 polluted white dwarfs exhibits an infrared excess at 3-4 m in
IRAC photometry, which reinforces the fact that atmospheric metal pollution is
the most sensitive tracer of white dwarf planetary systems. The corresponding
fraction of infrared excesses around white dwarfs with wide binary companions
is consistent with zero, using both the infrared survey data and an independent
assessment of potential binarity for well-established dusty and polluted stars.
In contrast, the frequency of atmospheric pollution among the targets in wide
binaries is indistinct from apparently single stars, and moreover the
multiplicity of polluted white dwarfs in a complete and volume-limited sample
is the same as for field stars. Therefore, it appears that the delivery of
planetesimal material onto white dwarfs is ultimately not driven by stellar
companions, but by the dynamics of planetary bodies.Comment: 15 pages, 6 figures, 3 tables; accepted for publication in MNRA
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
HST Spectra of GW Librae: A Hot Pulsating White Dwarf in a Cataclysmic Variable
We have obtained Hubble Space Telescope UV spectra of the white dwarf in GW
Lib, the only known non-radially pulsating white dwarf in a cataclysmic
variable, and the first known DAZQ variable. The UV light curve reveals large
amplitude (10%) pulsationsin the UV with the same periods (646, 376 and 237 s)
as those seen at optical wavelengths, but the mean spectrum fits with an
average white dwarf temperature (14,700K for a 0.6M_{odot} white dwarf) that is
too hot to be in the normal instability strip for ZZ Ceti stars. A better fit
is achieved with a dual temperature model (with 63% of the white dwarf surface
at a temperature of 13300K and 37% at 17100K), and a higher mass (0.8M_{odot})
white dwarf with 0.1 solar metal abundance. Since the blue edge of the
instability strip moves to higher temperature with increasing mass, the lower
temperature of this model is within the instability strip. However, the
presence of accretion likely causes abundance and atmospheric temperature
differences in GW Lib compared to all known single white dwarf pulsators, and
the current models that have been capable of explaining ZZ Ceti stars may not
apply.Comment: 13 pages, 4 figure
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
Optical detection of the 1.1 day variability at the white dwarf GD 394 with TESS
Recent discoveries have demonstrated that planetary systems routinely survive the post-main-sequence evolution of their host stars, leaving the resulting white dwarf with a rich circumstellar environment. Among the most intriguing of such hosts is the hot white dwarf GD 394, exhibiting a unique 1.150 ± 0.003 day flux variation detected in Extreme Ultraviolet Explorer (EUVE) observations in the mid-1990s. The variation has eluded a satisfactory explanation, but hypotheses include channeled accretion producing a dark spot of metals, occultation by a gas cloud from an evaporating planet, or heating from a flux tube produced by an orbiting iron-cored planetesimal. We present observations obtained with the Transiting Exoplanet Survey Satellite (TESS) of GD 394. The space-based optical photometry demonstrates a 0.12 ± 0.01% flux variation with a period of 1.146 ± 0.001 days, consistent with the EUVE period and the first re-detection of the flux variation outside of the extreme ultraviolet. We describe the analysis of the TESS light curve and measurement of the optical variation, and discuss the implications of our results for the various physical explanations put forward for the variability of GD 394.Published versio
Detached cataclysmic variables are crossing the orbital period gap
A central hypothesis in the theory of cataclysmic variable (CV) evolution is
the need to explain the observed lack of accreting systems in the ~2-3 h
orbital period range, known as the period gap. The standard model, disrupted
magnetic braking (DMB), reproduces the gap by postulating that CVs transform
into inconspicuous detached white dwarf (WD) plus main sequence (MS) systems,
which no longer resemble CVs. However, observational evidence for this standard
model is currently indirect and thus this scenario has attracted some criticism
throughout the last decades. Here we perform a simple but exceptionally strong
test of the existence of detached CVs (dCVs). If the theory is correct dCVs
should produce a peak in the orbital period distribution of detached close
binaries consisting of a WD and an M4-M6 secondary star. We measured six new
periods which brings the sample of such binaries with known periods below 10 h
to 52 systems. An increase of systems in the ~2-3 h orbital period range is
observed. Comparing this result with binary population models we find that the
observed peak can not be reproduced by PCEBs alone and that the existence of
dCVs is needed to reproduce the observations. Also, the WD mass distribution in
the gap shows evidence of two populations in this period range, i.e. PCEBs and
more massive dCVs, which is not observed at longer periods. We therefore
conclude that CVs are indeed crossing the gap as detached systems, which
provides strong support for the DMB theory.Comment: 12 pages, 6 figures, 2 tables, accepted for publication in MNRA
Periodic optical variability and debris accretion in white dwarfs: a test for a causal connection
Recent Kepler photometry has revealed that about half of white dwarfs (WDs)
have periodic, low-level (~ 1e-4 - 1e-3), optical variations. Hubble Space
Telescope (HST) ultraviolet spectroscopy has shown that up to about one half of
WDs are actively accreting rocky planetary debris, as evidenced by the presence
of photospheric metal absorption lines. We have obtained HST ultraviolet
spectra of seven WDs that have been monitored for periodic variations, to test
the hypothesis that these two phenomena are causally connected, i.e. that the
optical periodic modulation is caused by WD rotation coupled with an
inhomogeneous surface distribution of accreted metals. We detect photospheric
metals in four out of the seven WDs. However, we find no significant
correspondence between the existence of optical periodic variability and the
detection of photospheric ultraviolet absorption lines. Thus the null
hypothesis stands, that the two phenomena are not directly related. Some other
source of WD surface inhomogeneity, perhaps related to magnetic field strength,
combined with the WD rotation, or alternatively effects due to close binary
companions, may be behind the observed optical modulation. We report the
marginal detection of molecular hydrogen in WD J1949+4734, only the fourth
known WD with detected H_2 lines. We also re-classify J1926+4219 as a
carbon-rich He-sdO subdwarf.Comment: MNRAS, in pres
The orbital period of the recurrent nova V2487 Oph revealed
We present the first reliable determination of the orbital period of the
recurrent nova V2487 Oph (Nova Oph 1998). We derived a value of d ( h) from the radial velocity curve of the intense He II
4686 emission line as detected in time-series X-shooter spectra. The
orbital period is significantly shorter than earlier claims, but it makes V2487
Oph one of the longest period cataclysmic variables known. The spectrum of
V2487 Oph is prolific in broad Balmer absorptions that resemble a white dwarf
spectrum. However, we show that they come from the accretion disc viewed at low
inclination. Although highly speculative, the analysis of the radial velocity
curves provides a binary mass ratio and a donor star mass M, assuming the reported white dwarf mass
M. A subgiant M-type star is tentatively suggested as the donor star.
We were lucky to inadvertently take some of the spectra when V2487 Oph was in a
flare state. During the flare, we detected high-velocity emission in the Balmer
and He II 4686 lines exceeding km s at close to orbital
phase 0.4. Receding emission up to km s at about phase 0.3 is
also observed. The similarities with the magnetic cataclysmic variables may
point to magnetic accretion on to the white dwarf during the repeating flares.Comment: Accepted for publication in MNRAS (October 9, 2023
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