10,807 research outputs found
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
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
A NLTE model atmosphere analysis of the pulsating sdO star SDSS J1600+0748
We started a program to construct several grids of suitable model atmospheres
and synthetic spectra for hot subdwarf O stars computed, for comparative
purposes, in LTE, NLTE, with and without metals. For the moment, we use our
grids to perform fits on our spectrum of SDSS J160043.6+074802.9 (J1600+0748
for short), this unique pulsating sdO star. Our best fit is currently obtained
with NLTE model atmospheres including carbon, nitrogen and oxygen in solar
abundances, which leads to the following parameters for SDSS J1600+0748 : Teff
= 69 060 +/- 2080 K, log g = 6.00 +/- 0.09 and log N(He)/N(H) = -0.61 +/- 0.06.
Improvements are needed, however, particularly for fitting the available He II
lines. It is hoped that the inclusion of Fe will help remedy the situation.Comment: 4 pages, 4 figures, accepted in Astrophysics and Space Science
(24/02/2010), Special issue Hot sudbwarf star
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
Radiative levitation: a likely explanation for pulsations in the unique hot O subdwarf star SDSS J160043.6+074802.9
Context. SDSS J160043.6+074802.9 (J1600+0748 for short) is the only hot sdO star for which unambiguous multiperiodic luminosity variations have been reported so far. These rapid variations, with periods in the range from ~60 s to ~120 s, are best qualitatively explained in terms of pulsational instabilities, but the exact nature of the driving mechanism has remained a puzzle.
Aims. Our primary goal is to examine quantitatively how pulsation modes can be excited in an object such as J1600+0748. Given the failure of uniform-metallicity models as well documented in the recent Ph.D. thesis of C. RodrÃguez-López, we consider the effects of radiative levitation on iron as a means to boost the efficiency of the opacity-driving mechanism in models of J1600+0748.
Methods. We combine high sensitivity time-averaged optical spectroscopy and full nonadiabatic calculations to carry out our study. In the first instance, this is used to estimate the location of J1600+0748 in the log plane. Given this essential input, we pulsate stellar models consistent with these atmospheric parameters. We construct both uniform-metallicity models and structures in which the iron abundance is specified by the condition of diffusive equilibrium between gravitational settling and radiative levitation.
Results. On the basis of NTLE H/He synthetic spectra, we find that the target star has the following atmospheric parameters: log g = 5.93 0.11, = 71 070 2725 K, and log N(He)/N(H) = -0.85 0.08. This takes into account our deconvolution of the spectrum of J1600+0748 as it is polluted by the light of a main sequence companion. We confirm that uniform-metallicity stellar models with Z in the range from 0.02 to 0.10 cannot excite pulsation modes of the kind observed. On the other hand, we find that the inclusion of radiative levitation, as we implemented it, leads to pulsational instabilities in a period range that overlaps with, although it is narrower than, the observed range in J1600+0748. The excited modes correspond to low-order, low-degree p-modes.
Conclusions. We infer that radiative levitation is a likely essential ingredient in the excitation physics at work in J1600+0748
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
On The Evolution of Magnetic White Dwarfs
We present the first radiation magnetohydrodynamics simulations of the
atmosphere of white dwarf stars. We demonstrate that convective energy transfer
is seriously impeded by magnetic fields when the plasma-beta parameter, the
thermal to magnetic pressure ratio, becomes smaller than unity. The critical
field strength that inhibits convection in the photosphere of white dwarfs is
in the range B = 1-50 kG, which is much smaller than the typical 1-1000 MG
field strengths observed in magnetic white dwarfs, implying that these objects
have radiative atmospheres. We have then employed evolutionary models to study
the cooling process of high-field magnetic white dwarfs, where convection is
entirely suppressed during the full evolution (B > 10 MG). We find that the
inhibition of convection has no effect on cooling rates until the effective
temperature (Teff) reaches a value of around 5500 K. In this regime, the
standard convective sequences start to deviate from the ones without convection
owing to the convective coupling between the outer layers and the degenerate
reservoir of thermal energy. Since no magnetic white dwarfs are currently known
at the low temperatures where this coupling significantly changes the
evolution, effects of magnetism on cooling rates are not expected to be
observed. This result contrasts with a recent suggestion that magnetic white
dwarfs with Teff < 10,000 K cool significantly slower than non-magnetic
degenerates.Comment: 11 pages, 12 figures, accepted for publication in the Astrophysical
Journa
On a Conjecture of Rapoport and Zink
In their book Rapoport and Zink constructed rigid analytic period spaces
for Fontaine's filtered isocrystals, and period morphisms from PEL
moduli spaces of -divisible groups to some of these period spaces. They
conjectured the existence of an \'etale bijective morphism of
rigid analytic spaces and of a universal local system of -vector spaces on
. For Hodge-Tate weights and we construct in this article an
intrinsic Berkovich open subspace of and the universal local
system on . We conjecture that the rigid-analytic space associated with
is the maximal possible , and that is connected. We give
evidence for these conjectures and we show that for those period spaces
possessing PEL period morphisms, equals the image of the period morphism.
Then our local system is the rational Tate module of the universal
-divisible group and enjoys additional functoriality properties. We show
that only in exceptional cases equals all of and when the
Shimura group is we determine all these cases.Comment: v2: 48 pages; many new results added, v3: final version that will
appear in Inventiones Mathematica
Multiwavelength Observations of the Hot DB Star PG 0112+104
We present a comprehensive multiwavelength analysis of the hot DB white dwarf
PG 0112+104. Our analysis relies on newly-acquired FUSE observations, on
medium-resolution FOS and GHRS data, on archival high-resolution GHRS
observations, on optical spectrophotometry both in the blue and around Halpha,
as well as on time-resolved photometry. From the optical data, we derive a
self-consistent effective temperature of 31,300+-500 K, a surface gravity of
log g = 7.8 +- 0.1 (M=0.52 Msun), and a hydrogen abundance of log N(H)/N(He) <
-4.0. The FUSE spectra reveal the presence of CII and CIII lines that
complement the previous detection of CII transitions with the GHRS. The
improved carbon abundance in this hot object is log N(C)/N(He) = -6.15 +- 0.23.
No photospheric features associated with other heavy elements are detected. We
reconsider the role of PG 0112+104 in the definition of the blue edge of the
V777 Her instability strip in light of our high-speed photometry, and contrast
our results with those of previous observations carried out at the McDonald
Observatory.Comment: 10 pages in emulateapj, 9 figures, accepted for publication in Ap
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