23 research outputs found
Observations of the pulsating subdwarf B star Feige 48: Constraints on evolution and companions
Since pulsating subdwarf B (sdBV or EC14026) stars were first discovered
(Kilkenny et al, 1997), observational efforts have tried to realize their
potential for constraining the interior physics of extreme horizontal branch
(EHB) stars. Difficulties encountered along the way include uncertain mode
identifications and a lack of stable pulsation mode properties. Here we report
on Feige 48, an sdBV star for which follow-up observations have been obtained
spanning more than four years, which shows some stable pulsation modes.
We resolve the temporal spectrum into five stable pulsation periods in the
range 340 to 380 seconds with amplitudes less than 1%, and two additional
periods that appear in one dataset each. The three largest amplitude
periodicities are nearly equally spaced, and we explore the consequences of
identifying them as a rotationally split l=1 triplet by consulting with a
representative stellar model.
The general stability of the pulsation amplitudes and phases allows us to use
the pulsation phases to constrain the timescale of evolution for this sdBV
star. Additionally, we are able to place interesting limits on any stellar or
planetary companion to Feige 48.Comment: accepted for publication in MNRA
Understanding the Cool DA White Dwarf, G29-38
The white dwarfs are promising laboratories for the study of cosmochronology
and stellar evolution. Through observations of the pulsating white dwarfs, we
can measure their internal structures and compositions, critical to
understanding post main sequence evolution, along with their cooling rates,
allowing us to calibrate their ages directly. The most important set of white
dwarf variables to measure are the oldest of the pulsators, the cool DAVs,
which have not previously been explored through asteroseismology due to their
complexity and instability. Through a time-series photometry data set spanning
ten years, we explore the pulsation spectrum of the cool DAV, G29-38 and find
an underlying structure of 19 (not including multiplet components) normal-mode,
probably l=1 pulsations amidst an abundance of time variability and linear
combination modes. Modelling results are incomplete, but we suggest possible
starting directions and discuss probable values for the stellar mass and
hydrogen layer size. For the first time, we have made sense out of the
complicated power spectra of a large-amplitude DA pulsator. We have shown its
seemingly erratic set of observed frequencies can be understood in terms of a
recurring set of normal-mode pulsations and their linear combinations. With
this result, we have opened the interior secrets of the DAVs to future
asteroseismological modelling, thereby joining the rest of the known white
dwarf pulsators.Comment: 29 pages including 5 figures To appear in ApJ 1 Mar 9
Whole Earth Telescope observations of BPM 37093: a seismological test of crystallization theory in white dwarfs
BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which
has sufficient mass (~ 1.1 M_sun) to theoretically crystallize while still
inside the ZZ Ceti instability strip (T_eff ~ 12,000 K). As a consequence, this
star represents our first opportunity to test crystallization theory directly.
If the core is substantially crystallized, then the inner boundary for each
pulsation mode will be located at the top of the solid core rather than at the
center of the star, affecting mainly the average period spacing. This is
distinct from the "mode trapping" caused by the stratified surface layers,
which modifies the pulsation periods more selectively. In this paper we report
on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999.
Based on a simple analysis of the average period spacing we conclude that a
large fraction of the total stellar mass is likely to be crystallized.Comment: 7 pages, 3 figures, 3 tables, accepted for Astronomy & Astrophysic
The Whole Earth Telescope Arrives to Lithuania
A short description of participation of the Lithuanian astronomers in the Whole Earth Telescope program is given
The temporal spectrum of the sdB pulsating star HSâ2201+2610 at 2âms resolution
In this article we present the results of more than 180 hours of time-series
photometry on the low gravity (, â300âK,
by number) sdB pulsating star HSÂ 2201+2610, obtained between
September 2000 and August 2001.
The temporal spectrum is resolved and shows 5 close frequencies:
three main signals at 2860.94, 2824.10 and 2880.69âÎŒHz, with amplitudes
of about 1%, 0.5% and 0.1% respectively, are detected from
single run observations; two further peaks with very low amplitude
(<0.07%) at 2738.01 and 2921.82âÎŒHz are confirmed by phase analysis
on several independent runs.
Due to the small number of detected frequencies, it is not possible to obtain
a univocal identification of the excited modes and perform a detailed
seismological analysis of the star.
No clear signatures of rotational splitting are seen.
Nevertheless, the observed period spectrum is well inside the excited
period window obtained from pulsation calculations with nonadiabatic models
having effective temperature and surface gravity close to the spectroscopic
estimates.
Due to its relatively simple temporal spectrum, HSÂ 2201+2610 is a very good
candidate for trying to measure the secular variation of the pulsation periods
in time.
With this purpose a long-term monitoring of the star was started.
The results of the first 11Â months show amplitude variations up to ~20%
on time-scales of months, which are probably real, and allow us to measure the
pulsation frequencies with an unprecedented 0.02âÎŒHz resolution
Wet Observations of the DAV G185â32
The pulsating DAV white dwarf G185-32 was observed with the Whole Earth Telescope in 1992, during XCov8. We briefly report on the weighted Fourier transform of the data obtained
Observations of the pulsating white dwarf G 185-32
We observed the pulsating hydrogen atmosphere white dwarf G 185-32 with the Whole Earth Telescope in 1992. We report on a weighted Fourier transform of the data detecting 18 periodicities in its light curve. Using the Hubble Space Telescope Faint Object Spectrograph time resolved spectroscopy, and the wavelength dependence of the relative amplitudes, we identify the spherical harmonic degree ( ) for 14 pulsation signals. We also compare the determinations of effective temperature and surface gravity using the excited modes and atmospheric methods, obtaining 80 K, 0.04 and M=0.617