29,850 research outputs found
Stark Broadening of Carbon and Oxygen Lines in Hot DQ White Dwarf Stars: Recent Results and Applications
White dwarf stars are traditionally found to have surface compositions made
primarily of hydrogen or helium. However, a new family has recently been
uncovered, the so-called Hot DQ white dwarfs, which have surface compositions
dominated by carbon and oxygen with little or no trace of hydrogen and helium
(Dufour et al. 2007, 2008, 2010). Deriving precise atmospheric parameters for
these objects (such as the effective temperature and the surface gravity)
requires detailed modeling of spectral line profiles. Stark broadening
parameters are of crucial importance in that context. We present preliminary
results from our new generation of model atmosphere including the latest Stark
broadening calculations for CII lines and discuss the implications as well as
future work that remains to be done.Comment: 6 pages, 1 figure, to appear in Baltic Astronom
A New Small-Amplitude Variable Hot DQ White Dwarf
We present the discovery of photometric variations in the carbon-dominated
atmosphere (hot DQ) white dwarf star SDSS J133710.19-002643.6. We find evidence
for two low-amplitude, harmonically-related periodicities at 169 s and 339 s,
making it the fastest and smallest amplitude of the four known hot DQ variables
and the only variable whose spectrum suggests the presence of hydrogen. The
star's fundamental and harmonic amplitudes are roughly equal, and its pulse
shape is similar to the other two members of the class with detected harmonics.
Like the other variables, it appears relatively stable in frequency and
amplitude.Comment: Accepted for publication in the Astrophysical Journal Letters. 5
pages, 2 figures, uses emulateap
Follow-up Observations of the Second and Third Known Pulsating Hot DQ White Dwarfs
We present follow-up time-series photometric observations that confirm and
extend the results of the significant discovery made by Barlow et al.(2008)
that the Hot DQ white dwarfs SDSS J220029.08-074121.5 and SDSS
J234843.30-094245.3 are luminosity variable. These are the second and third
known members of a new class of pulsating white dwarfs, after the prototype
SDSS J142625.71+575218.3 (Montgomery et al. 2008). We find that the light curve
of SDSS J220029.08-074121.5 is dominated by an oscillation at 654.397+-0.056 s,
and that the light pulse folded on that period is highly nonlinear due to the
presence of the first and second harmonic of the main pulsation. We also
present evidence for the possible detection of two additional pulsation modes
with low amplitudes and periods of 577.576+-0.226 s and 254.732+-0.048 s in
that star. Likewise, we find that the light curve of SDSS J234843.30-094245.3
is dominated by a pulsation with a period of 1044.168+-0.012 s, but with no
sign of harmonic components. A new oscillation, with a low amplitude and a
period of 416.919+-0.004 s, is also probably detected in that second star. We
argue, on the basis of the very different folded pulse shapes, that SDSS
J220029.08-074121.5 is likely magnetic, while SDSS J234843.30-094245.3 is
probably not.Comment: 12 pages, 19 figures, accepted for publication in Ap
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Stark Broadening and White Dwarfs
White dwarf and pre-white dwarf atmospheres are one of the best examples for
the application of Stark broadening research results in astrophysics, due to
plasma conditions very favorable for this line broadening mechanism. For
example in hot hydrogen-deficient (pre-) white dwarf stars Teff = 75 000 K -
180 000 K and log g = 5.5-8 [cgs]. Even for much cooler DA and DB white dwarfs
with typical effective temperatures of 10 000 K - 20 000 K, Stark broadening is
usually the dominant broadening mechanism. In this review, Stark broadening in
white dwarf spectra is considered and the attention is drawn to the STARK-B
database (http://stark-b.obspm.fr/), containing Stark broadening parameters
needed for white dwarf spectra analysis and synthesis, as well as to the new
search facilities which will provide the collective effort to develop Virtual
Atomic and Molecular Data Center (VAMDC - http://vamdc.org/)
Follow-up Studies of the Pulsating Magnetic White Dwarf SDSS J142625.71+575218.3
We present a follow-up analysis of the unique magnetic luminosity-variable
carbon-atmosphere white dwarf SDSS J142625.71+575218.3. This includes the
results of some 106.4 h of integrated light photometry which have revealed,
among other things, the presence of a new periodicity at 319.720 s which is not
harmonically related to the dominant oscillation (417.707 s) previously known
in that star. Using our photometry and available spectroscopy, we consider the
suggestion made by Montgomery et al. (2008) that the luminosity variations in
SDSS J142625.71+575218.3 may not be caused by pulsational instabilities, but
rather by photometric activity in a carbon-transferring analog of AM CVn. This
includes a detailed search for possible radial velocity variations due to rapid
orbital motion on the basis of MMT spectroscopy. At the end of the exercise, we
unequivocally rule out the interacting binary hypothesis and conclude instead
that, indeed, the luminosity variations are caused by g-mode pulsations as in
other pulsating white dwarfs. This is in line with the preferred possibility
put forward by Montgomery et al. (2008).Comment: 11 pages in emulateApJ, 12 figures, accepted for publication in Ap
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