242 research outputs found
Extreme abundance ratios in the polluted atmosphere of the cool white dwarf NLTT 19868
We present an analysis of intermediate-dispersion spectra and photometric data of the newly identified cool, polluted white dwarf NLTT 19868. The spectra obtained with X-shooter on the Very Large Telescope-Melipal show strong lines of calcium, and several lines of magnesium, aluminium and iron. We use these spectra and the optical-to-near-infrared spectral energy distribution to constrain the atmospheric parameters of NLTT 19868. Our analysis shows that NLTT 19868 is iron poor with respect to aluminium and calcium. A comparison with other cool, polluted white dwarfs shows that the Fe to Ca abundance ratio (Fe/Ca) varies by up to approximately two orders of magnitudes over a narrow temperature range with NLTT 19868 at one extremum in the Fe/Ca ratio and, in contrast, NLTT 888 at the other extremum. The sample shows evidence of extreme diversity in the composition of the accreted material: in the case of NLTT 888, the inferred composition of the accreted matter is akin to iron-rich planetary core composition, while in the case of NLTT 19868 it is close to mantle composition depleted by subsequent chemical separation at the bottom of the convection zone
Optical observations and orbital parameters of the close DA plus dMe binaries BPM 71214, EUVE J0720-31.7, BPM 6502, and EC 13471-1258
We have obtained photometric and spectroscopic observations of the close DA plus dMe binaries BPM 6502 and EC 13471-1258, and spectroscopic observations of the close binaries BPM 71214 and EUVE J0720-31.7. We have updated the ephemerides of BPM 6502 and EUVE J0720-31.7 with the spectroscopic measurements and obtained periods of 0.20162 and 0.15074 days for BPM 71214 and EC 13471-1258, respectively, by measuring the shift in Hα emission. Photometric variations in R and I due to reprocessing of incoming radiation from the hot white dwarf were observed in BPM 6502. We have also observed eclipses and ellipsoidal variations in B and R for EC 13471-1258, suggesting that the secondary is nearly filling its Roche lobe
Evidence of enhanced magnetism in cool, polluted white dwarfs
We report the discovery of a new, polluted, magnetic white dwarf in the Luyten survey of high-proper motion stars. High-dispersion spectra of NLTT 7547 reveal a complex heavy element line spectrum in a cool (â5200 K) hydrogen-dominated atmosphere showing the effect of a surface averaged field of 163 kG, consistent with a 240 kG centred dipole, although the actual field structure remains uncertain. The abundance pattern shows the effect of accreted material with a distinct magnesium-rich flavour. Combined with earlier identifications, this discovery supports a correlation between the incidence of magnetism in cool white dwarfs and their contamination by heavy elements
Equilibrium abundances in hot DA white dwarfs as derived from self-consistent diffusion models I. Analysis of spectroscopic EUVE data
We present the first analysis of an EUV selected sample of hot DA white
dwarfs using a new type of atmospheric models. These models take into account
the interplay between gravitational settling and radiative acceleration to
predict the chemical stratification from an equilibrium between the two forces
while self-consistently solving for the atmospheric structure. In contrast to
atmospheric models with the assumption of chemical homogeneity, the number of
free parameters in the new models is reduced to the effective temperature and
surface gravity alone. The overall good reproduction of observed EUV spectra
reveals that these models are able to describe the physical conditions in hot
DA white dwarf atmospheres correctly. A comparison with previous analyses
highlights the improvements as well as the limits of our new models.Comment: 10 pages, 28 figures, accepted for publication by A&
On the empirical evidence for the existence of ultra-massive white dwarfs
We re-examine the evidence for the existence of ultra-massive (M > 1.1 M_sun)
white dwarfs based on gravitational redshift of white dwarfs in common proper
motion binaries or in clusters, on parallax measurements, on orbital solutions,
and, finally, on the analysis of hydrogen line profiles. We conclude that the
best evidence is largely based on the analysis of Balmer line profiles although
the companion to the A8V star HR 8210 is a compelling case made initially using
the large binary mass function and confirmed by an analysis of the Lyman line
spectrum. The confirmation and identification of high-mass white dwarfs, more
particularly non-DA white dwarfs, using parallax measurements may prove
critical in establishing the population fraction of these objects and in
constraining the high-end of empirical initial-mass to final-mass relations.
The existence of a substantial population of ultra-massive white dwarfs
supports the concept of a steeper initial-mass to final-mass relations linking
6 M_sun progenitors with approximately greater than 1.1 M_sun white dwarfs.Comment: Accepted for publication in MNRAS (with an updated discussion on
GD362
High ions towards white dwarfs: circumstellar line shifts and stellar temperature
Based on a compilation of OVI, CIV, SiIV and NV data from IUE, FUSE, GHRS,
STIS, and COS, we derive an anti- correlation between the stellar temperature
and the high ion velocity shift w.r.t. to the photosphere, with positive (resp.
negative) velocity shifts for the cooler (resp. hotter) white dwarfs. This
trend probably reflects more than a single process, however such a dependence
on the WD's temperature again favors a CS origin for a very large fraction of
those ion absorptions, previously observed with IUE, HST-STIS, HST-GHRS, FUSE,
and now COS, selecting objects for which absorption line radial velocities,
stellar effective temperature and photospheric velocity can be found in the
literature. Interestingly, and gas in near-equilibrium in the star vicinity. It
is also probably significant that the temperature that corresponds to a null
radial velocity, i.e. \simeq 50,000K, also corresponds to the threshold below
which there is a dichotomy between pure or heavy elements atmospheres as well
as some temperature estimates for and a form of balance between radiation
pressure and gravitation. This is consistent with ubiquitous evaporation of
orbiting dusty material. Together with the fact that the fraction of stars with
(red-or blue-) shifted lines and the fraction of stars known to possess heavy
species in their atmosphere are of the same order, such a velocity-temperature
relationship is consistent with quasi-continuous evaporation of orbiting CS
dusty material, followed by accretion and settling down in the photosphere. In
view of these results, ion measurements close to the photospheric or the IS
velocity should be interpreted with caution, especially for stars at
intermediate temperatures. While tracing CS gas, they may be erroneously
attributed to photospheric material or to the ISM, explaining the difficulty of
finding a coherent pattern of the high ions in the local IS 3D distribution.Comment: Accepted by A&A. Body of paper identical to v1. This submission has a
more appropriate truncation of the original abstrac
Mass-Radius Relation for Magnetic White Dwarfs
Recently, several white dwarfs with very strong surface magnetic fields have
been observed. In this paper we explore the possibility that such stars could
have sufficiently strong internal fields to alter their structure. We obtain a
revised white dwarf mass-radius relation in the presence of strong internal
magnetic fields. We first derive the equation of state for a fully degenerate
ideal electron gas in a magnetic field using an Euler-MacLaurin expansion. We
use this to obtain the mass-radius relation for magnetic He, C,
and Fe white dwarfs of uniform composition.Comment: 7 pages, 7 figures and 1 table, To appear in Ap
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