12,215 research outputs found
Might Carbon-Atmosphere White Dwarfs Harbour a New Type of Pulsating Star?
In the light of the recent and unexpected discovery of a brand new type of
white dwarfs, those with carbon-dominated atmospheres, we examine the
asteroseismological potential of such stars. The motivation behind this is
based on the observation that past models of carbon-atmosphere white dwarfs
have partially ionized outer layers that bear strong resemblance with those
responsible for mode excitation in models of pulsating DB (helium-atmosphere)
and pulsating DA (hydrogen-atmosphere) white dwarfs. Our exciting main result
is that, given the right location in parameter space, some carbon-atmosphere
white dwarfs are predicted to show pulsational instability against gravity
modes. We are eagerly waiting the results of observational searches for
luminosity variations in these stars.Comment: 4-page letter + 4 figure
Pulsation in carbon-atmosphere white dwarfs: A new chapter in white dwarf asteroseismology
We present some of the results of a survey aimed at exploring the
asteroseismological potential of the newly-discovered carbon-atmosphere white
dwarfs. We show that, in certains regions of parameter space, carbon-atmosphere
white dwarfs may drive low-order gravity modes. We demonstrate that our
theoretical results are consistent with the recent exciting discovery of
luminosity variations in SDSS J1426+5752 and some null results obtained by a
team of scientists at McDonald Observatory. We also present follow-up
photometric observations carried out by ourselves at the Mount Bigelow 1.6-m
telescope using the new Mont4K camera. The results of follow-up spectroscopic
observations at the MMT are also briefly reported, including the surprising
discovery that SDSS J1426+5752 is not only a pulsating star but that it is also
a magnetic white dwarf with a surface field near 1.2 MG. The discovery of
-mode pulsations in SDSS J1426+5752 is quite significant in itself as it
opens a fourth asteroseismological "window", after the GW Vir, V777 Her, and ZZ
Ceti families, through which one may study white dwarfs.Comment: 7 pages, 4 figures, to appear in Journal of Physics Conference
Proceedings for the 16th European White Dwarf Worksho
Towards an Empirical Determination of the ZZ Ceti Instability Strip
We present atmospheric parameters for a large sample of DA white dwarfs that
are known to be photometrically constant. For each star, we determine the
effective temperature and surface gravity by comparing high signal-to-noise
ratio optical spectra to the predictions of detailed model atmosphere
calculations. We also report the successful prediction and detection of
photometric variability in G232-38 based on similar Teff and log g
determinations. The atmospheric parameters derived for this sample of constant
stars as well as those for the known sample of bright ZZ Ceti stars (now
boosted to a total of 39) have been obtained in a highly homogeneous way. We
combine them to study the empirical red and blue edges as well as the purity of
the ZZ Ceti instability strip. We find that the red edge is rather well
constrained whereas there exists a rather large range of possibilities for the
slope of the blue edge. Furthermore, the ZZ Ceti instability strip that results
from our analysis contains no nonvariable white dwarfs. Our sample of constant
stars is part of a much broader spectroscopic survey of bright (V < 17) DA
white dwarfs, which we have recently undertaken. We also present here some
preliminary results of this survey. Finally, we revisit the analysis by Mukadam
et al. of the variable and nonvariable DA stars uncovered as part of the Sloan
Digital Sky Survey. Their erroneous conclusion of an instability strip
containing several nonvariable stars is traced back to the low signal-to-noise
ratio spectroscopic observations used in that survey.Comment: 43 pages, 2 tables, 14 figures, accepted for publication in the
Astrophysical Journa
Rare White dwarf stars with carbon atmospheres
White dwarfs represent the endpoint of stellar evolution for stars with
initial masses between approximately 0.07 msun and 8-10 msun, where msun is the
mass of the Sun (more massive stars end their life as either black holes or
neutron stars). The theory of stellar evolution predicts that the majority of
white dwarfs have a core made of carbon and oxygen, which itself is surrounded
by a helium layer and, for ~80 per cent of known white dwarfs, by an additional
hydrogen layer. All white dwarfs therefore have been traditionally found to
belong to one of two categories: those with a hydrogen-rich atmosphere (the DA
spectral type) and those with a helium-rich atmosphere (the non-DAs). Here we
report the discovery of several white dwarfs with atmospheres primarily
composed of carbon, with little or no trace of hydrogen or helium. Our analysis
shows that the atmospheric parameters found for these stars do not fit
satisfactorily in any of the currently known theories of post-asymptotic giant
branch evolution, although these objects might be the cooler counterpart of the
unique and extensively studied PG1159 star H1504+65. These stars, together with
H1504+65, might accordingly form a new evolutionary sequence that follow the
asymptotic giant branch.Comment: 7 pages, 1 figure, to appear in Nov 22nd 2007 edition of Natur
Improved determination of the atmospheric parameters of the pulsating sdB star Feige 48
As part of a multifaceted effort to exploit better the asteroseismological
potential of the pulsating sdB star Feige 48, we present an improved
spectroscopic analysis of that star based on new grids of NLTE, fully
line-blanketed model atmospheres. To that end, we gathered four high S/N
time-averaged optical spectra of varying spectral resolution from 1.0 \AA\ to
8.7 \AA, and we made use of the results of four independent studies to fix the
abundances of the most important metals in the atmosphere of Feige 48. The mean
atmospheric parameters we obtained from our four spectra of Feige 48 are :
Teff= 29,850 60 K, log = 5.46 0.01, and log N(He)/N(H) =
2.88 0.02. We also modeled for the first time the He II line at 1640
\AA\ from the STIS archive spectrum of the star and we found with this line an
effective temperature and a surface gravity that match well the values obtained
with the optical data. With some fine tuning of the abundances of the metals
visible in the optical domain we were able to achieve a very good agreement
between our best available spectrum and our best-fitting synthetic one. Our
derived atmospheric parameters for Feige 48 are in rather good agreement with
previous estimates based on less sophisticated models. This underlines the
relatively small effects of the NLTE approach combined with line blanketing in
the atmosphere of this particular star, implying that the current estimates of
the atmospheric parameters of Feige 48 are reliable and secure.Comment: Accepted for publication in ApJ, April 201
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