22 research outputs found
Model atmosphere analysis of the extreme DQ white dwarf GSC2U J131147.2+292348
A new model atmosphere analysis for the peculiar DQ white dwarf discovered by
Carollo et al. (2002) is presented. The effective temperature and carbon
abundance have been estimated by fitting both the photometric data
(UBJ,VRF,IN,JHK) and a low resolution spectrum (3500<lambda<7500 A) with a new
model grid for helium-rich white dwarfs with traces of carbon (DQ stars). We
estimate Teff ~ 5120 +/- 200 K and log[C/He] ~ -5.8 +/- 0.5, which make GSC2U
J131147.2+292348 the coolest DQ star ever observed. This result indicates that
the hypothetical transition from C2 to C2H molecules around Teff = 6000 K,
which was inferred to explain the absence of DQ stars at lower temperatures,
needs to be reconsidered.Comment: 4 pages, 2 figures, accepted for publication in Astronomy and
Astrophysics Letter
Quasi-molecular Satellites of Lyman Beta in the Spectrum of the DA White Dwarf WOLF 1346
We present new FUV/UV observations of the DA white dwarf Wolf 1346 obtained
with the Hopkins Ultraviolet Telescope. The atmospheric parameters of this
object are estimated from a fit of model atmospheres to several optical spectra
to be Teff = 20000 K, log g = 7.90. From the optical spectrum this star is a
normal DA without any indications for chemical elements other than hydrogen.
The hydrogen line L beta, however, shows a very unusual shape, with a steep red
wing and two absorption features on this wing. The shape is reminiscent of the
effects of quasi-molecular line broadening, as observed in L alpha in cooler DA
white dwarfs. We show that this is indeed the correct explanation, by
identifying 4 quasi-molecular satellites caused through perturbations by the H+
ion (H2+ quasi-molecule). The steep red wing is caused by the exponential
decline of the line profile beyond the satellite most distant from the line
center at 1078 A.Comment: 11 pages Latex with aaspp4 style, 4 postscript figures, as compressed
tar file, ApJ Letters, in pres
Parameter-free Stark Broadening of Hydrogen Lines in DA White Dwarfs
We present new calculations for the Stark broadening of the hydrogen line
profiles in the dense atmospheres of white dwarf stars. Our improved model is
based on the unified theory of Stark broadening from Vidal, Cooper & Smith, but
it also includes non-ideal gas effects from the Hummer & Mihalas occupation
probability formalism directly inside the line profile calculations. This
approach improves upon previous calculations that relied on the use of an
ad-hoc free parameter to describe the dissolution of the line wing opacity in
the presence of high electric microfields in the plasma. We present here the
first grid of model spectra for hot Teff >~ 12,000 K DA white dwarfs that has
no free parameters. The atmospheric parameters obtained from optical and UV
spectroscopic observations using these improved models are shown to differ
substantially from those published in previous studies.Comment: 8 pages, 8 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
Analysis of IUE spectra of helium-rich white dwarf stars
We studied the class of DB white dwarf stars, using re-calibrated UV spectra
for thirty four DBs obtained with the IUE satellite. By comparing the observed
energy distributions with model atmospheres, we simultaneously determine
spectroscopic distances (d), effective temperature (Teff), and surface
gravities (log g). Using parallax measurements and previous determinations of
Teff and log g from optical spectra, we can study whether the atmospheres of
eleven DB stars are consistent with pure He or have a small amount of H
contamination. We also report on our observations of seventeen stars with Teff
close to the DB instability strip through time series photometry and found them
to be non variable within our detection limits.Comment: 8 pages, 5 figure
Towards a pure ZZ Ceti instability strip
We have observed again two stars inside the ZZ Ceti instability strip that
were previously classified as not-observed-to-vary (NOV) by Mukadam et al.
(2004) and found them to be low-amplitude variables. Some evidence points to a
pure ZZ Ceti instability strip; other evidence contests it. The two stars
previously classified as NOV have Sloan Digital Sky Survey (SDSS) spectroscopic
effective temperatures that place them inside the ZZ Ceti instability strip,
and they were "contaminating" the strip as constant stars, which could indicate
that the instability strip was no longer a simple evolutionary stage. A pure
instability strip indicates that pulsation is a normal phase which all DAs must
go through. We used effective temperatures derived from SDSS optical spectra by
comparing them with model atmospheres to look for pulsators through
time-resolved photometry and stars previously classified as NOV. Our new
results indicate, but do not prove, a pure instability strip, because there are
still other NOV stars that need to be observed again. Additionally, we have
discovered five other ZZ Ceti stars based on their effective temperatures.Comment: 12 pages, 3 figur
Two new young, wide magnetic + non-magnetic double-degenerate binary systems
Abridged: We report the discovery of two, new, rare, wide, double-degenerate
binaries that each contain a magnetic and a non-magnetic star. The components
of SDSSJ092646.88+132134.5 + J092647.00+132138.4 and SDSSJ150746.48+521002.1 +
J150746.80+520958.0 have angular separations of only 4.6 arcsec (a~650AU) and
5.1 arcsec (a~750AU), respectively. They also appear to share common proper
motions. Follow-up optical spectroscopy reveals each system to consist of a DA
and a H-rich high-field magnetic white dwarf (HFMWD). Our measurements of the
effective temperatures and the surface gravities of the DA components reveal
both to have larger masses than are typical of field white dwarfs. By assuming
that these degenerates have evolved essentially as single stars, due to their
wide orbital separations, we use them to place limits on the total ages of our
stellar systems. These argue that in each case the HFMWD is probably associated
with an early type progenitor (M_init > 2M_solar). We find that the cooling
time of SDSSJ150746.80+520958.0 (DAH) is somewhat lower than might be expected
had it followed the evolutionary path of a typical single star. This mild
discord is in the same sense as that observed for two of the small number of
other HFMWDs for which progenitor mass estimates have been made, REJ0317-853
and EG59. The mass of the other DAH, SDSSJ092646.88+132134.5, appears to be
smaller than expected on the basis of single star evolution. If this object
was/is a member of a hierarchical triple system it may have experienced greater
mass loss during an earlier phase of its life as a result of it having a close
companion. The large uncertainties on our estimates of the parameters of the
HFMWDs suggest a larger sample of these objects is required to firmly identify
any trends in their inferred cooling times and progenitor masses.Comment: Accepted for MNRA
Discovery of fourteen new ZZ Cetis with SOAR
We report the discovery of fourteen new ZZ Cetis with the 4.1m Southern
Astrophysical Research telescope, at Cerro Pachon, in Chile. The candidates
were selected from the SDSS (Sloan Digital Sky Survey) DA white dwarf stars
with Teff obtained from the optical spectra fit, inside the ZZ Ceti instability
strip. Considering these stars are multi-periodic pulsators and the pulsations
propagate to the nucleus of the star, they carry information on the structure
of the star and evolution of the progenitors. The ZZ Cetis discovered till 2003
are mainly within 100 pc from the Sun, and probe only the solar vicinity. The
recently discovered ones, and those reported here, may sample a distinct
population as they were selected mainly perpendicular to the galactic disk and
cover a distance up to ~400pc.Comment: 12 pages, 2 figure