141 research outputs found
Metal abundances in PG1159 stars from Chandra and FUSE spectroscopy
We investigate FUSE spectra of three PG1159 stars and do not find any
evidence for iron lines. From a comparison with NLTE models we conclude a
deficiency of 1-1.5 dex. We speculate that iron was transformed into heavier
elements. A soft X-ray Chandra spectrum of the unique H- and He-deficient star
H1504+65 is analyzed. We find high neon and magnesium abundances and confirm
that H1504+65 is the bare core of either a C-O or a O-Ne-Mg white dwarf.Comment: To be published in: Proceedings 13th European Workshop on White
Dwarfs, NATO Science Series, 4 pages, 1 figur
Iron abundance in hot hydrogen-deficient central stars and white dwarfs from FUSE, HST, and IUE spectroscopy
We present a first systematic investigation of the iron abundance in very hot
(Teff>50,000K) hydrogen-deficient post-AGB stars. Our sample comprises 16
PG1159 stars and four DO white dwarfs. We use recent FUSE observations as well
as HST and IUE archival data to perform spectral analyses with line blanketed
NLTE model atmospheres. Iron is not detected in any PG1159 star. In most cases
this is compatible with a solar iron abundance due to limited quality of HST
and IUE data, although the tendency to an iron underabundance may be
recognized. However, the absence of iron lines in excellent FUSE spectra
suggests an underabundance by at least 1 dex in two objects (K1-16 NGC 7094). A
similar result has been reported recently in the [WC]-PG1159 transition object
Abell 78 (Werner et al. 2002). We discuss dust fractionation and s-process
neutron-captures as possible origins. We also announce the first identification
of sulfur in PG1159 stars.Comment: Accepted for publication in A&A, 10 pages, 9 figure
Dynamic masses for the close PG1159 binary SDSSJ212531.92-010745.9
SDSSJ212531.92-010745.9 is the first known PG1159 star in a close binary with
a late main sequence companion allowing a dynamical mass determination. The
system shows flux variations with a peak-to-peak amplitude of about 0.7 mag and
a period of about 6.96h. In August 2007, 13 spectra of SDSSJ212531.92-010745.9
covering the full orbital phase range were taken at the TWIN 3.5m telescope at
the Calar Alto Observatory (Alm\'{e}ria, Spain). These confirm the typical
PG1159 features seen in the SDSS discovery spectrum, together with the Balmer
series of hydrogen in emission (plus other emission lines), interpreted as
signature of the companion's irradiated side. A radial velocity curve was
obtained for both components. Using co-added radial-velocity-corrected spectra,
the spectral analysis of the PG1159 star is being refined.
The system's lightcurve, obtained during three seasons of photometry with the
G\"ottingen 50cm and T\"ubingen 80cm telescopes, was fitted with both the
NIGHTFALL and PHOEBE binary simulation programs. An accurate mass determination
of the PG1159 component from the radial velocity measurements requires to first
derive the inclination, which requires light curve modelling and yields further
constraints on radii, effective temperature and separation of the system's
components. From the analysis of all data available so far, we present the
possible mass range for the PG1159 component of SDSSJ212531.92-010745.9.Comment: 8 pages, in "White dwarfs", proceedings of the 16th European White
Dwarf Workshop, eds. E. Garcia-Berro, M. Hernanz, J. Isern, S. Torres, to be
published in J. Phys.: Conf. Se
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
A Grid of NLTE Line-Blanketed Model Atmospheres of O-type Stars
We have constructed a comprehensive grid of 690 metal line-blanketed, NLTE,
plane-parallel, hydrostatic model atmospheres for the basic parameters
appropriate to O-type stars. The OSTAR2002 grid considers 12 values of
effective temperatures, 27500K < Teff < 55000K with 2500K steps, 8 surface
gravities, 3.0 < log g< 4.75 with 0.25 dex steps, and 10 chemical compositions,
from metal-rich relative to the Sun to metal-free. The lower limit of log g for
a given effective temperature is set by an approximate location of the
Eddington limit. The selected chemical compositions have been chosen to cover a
number of typical environments of massive stars: the galactic center, the
Magellanic Clouds, Blue Compact Dwarf galaxies like I Zw 18, and galaxies at
high redshifts. The paper contains a description of the OSTAR2002 grid and some
illustrative examples and comparisons. The complete OSTAR2002 grid is available
at our website at http://tlusty.gsfc.nasa.gov.Comment: 57 pages, 16 figures; minor additions and corrections; to appear in
Astrophysical Journal Supplement, Vol. 146 (June 2003
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