54 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
Constraining the Evolution of Zz Ceti
We report our analysis of the stability of pulsation periods in the DAV star (pulsating hydrogen atmosphere white dwarf) ZZ Ceti, also called R548. On the basis of observations that span 31 years, we conclude that the period 213.13 s observed in ZZ Ceti drifts at a rate dP/dt †(5:5 ± 1:9) x 10-15 s s-1, after correcting for proper motion. Our results are consistent with previous áč values for this mode and an improvement over them because of the larger time base. The characteristic stability timescale implied for the pulsation period is âžP / áč âž=âčâ„ 1:2 Gyr, comparable to the theoretical cooling timescale for the star. Our current stability limit for the period 213.13 s is only slightly less than the present measurement for another DAV, G117-B15A, for the period 215.2 s, establishing this mode in ZZ Ceti as the second most stable optical clock known, comparable to atomic clocks and more stable than most pulsars. Constraining the cooling rate of ZZ Ceti aids theoretical evolutionary models and white dwarf cosmochronology. The drift rate of this clock is small enough that we can set interesting limits on reflex motion due to planetary companions
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 Everchanging Pulsating White Dwarf GD358
We report 323 hours of nearly uninterrupted time series photometric
observations of the DBV star GD 358 acquired with the Whole Earth Telescope
(WET) during May 23rd to June 8th, 2000. We acquired more than 232 000
independent measurements. We also report on 48 hours of time-series photometric
observations in Aug 1996. We detected the non-radial g-modes consistent with
degree l=1 and radial order 8 to 20 and their linear combinations up to 6th
order.We also detect, for the first time, a high amplitude l=2 mode, with a
period of 796s. In the 2000 WET data, the largest amplitude modes are similar
to those detected with the WET observations of 1990 and 1994, but the highest
combination order previously detected was 4th order. At one point during the
1996 observations, most of the pulsation energy was transferred into the radial
order k=8 mode, which displayed a sinusoidal pulse shape in spite of the large
amplitude. The multiplet structure of the individual modes changes from year to
year, and during the 2000 observations only the k=9 mode displays clear normal
triplet structure. Even though the pulsation amplitudes change on timescales of
days and years, the eigenfrequencies remain essentially the same, showing the
stellar structure is not changing on any dynamical timescale.Comment: 34 pages, 14 figures, WET data, accepted to A&
Direct Impact Accretors: Evolutionary links between detached and semi-detached white dwarfs
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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
The everchanging pulsating white dwarf GD358
We report 323 hours of nearly uninterrupted time series photometric observations of the DBV star GD 358 acquired with the Whole Earth Telescope (WET) during May 23rd to June 8th, 2000. We acquired more than 232 000 independent measurements. We also report on 48 hours of time-series photometric observations in Aug 1996. We detected the non-radial g-modes consistent with degree l = 1 and radial order 8 to 20 and their linear combinations up to 6th order. We also detect, for the first time, a high amplitude l = 2 mode, with a period of 796 s. In the 2000 WET data, the largest amplitude modes are similar to those detected with the WET observations of 1990 and 1994, but the highest combination order previously detected was 4th order. At one point during the 1996 observations, most of the pulsation energy was transferred into the radial order k = 8 mode, which displayed a sinusoidal pulse shape in spite of the large amplitude. The multiplet structure of the individual modes changes from year to year, and during the 2000 observations only the k = 9 mode displays clear normal triplet structure. Even though the pulsation amplitudes change on timescales of days and years, the eigenfrequencies remain essentially the same, showing the stellar structure is not changing on any dynamical timescale
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