146 research outputs found
Thick to Thin: The Evolutionary Connection Between PG 1159 Stars and the Thin Helium-Enveloped Pulsating White Dwarf GD 358
Seismological observations with the Whole Earth Telescope (WET) allow the
determination of the subsurface compositional structure of white dwarf stars.
The hot DO PG 1159 has a helium surface layer with a mass of 0.001 Msun, while
the cooler DB white dwarf GD 358 has a much thinner surface helium layer of
10^-6 Msun. These results imply that either there is no evolutionary relation
between these two stars, or that there is an unknown mass loss mechanism. To
investigate possible evolutionary links between these objects, we computed
evolutionary sequences of white dwarf models including time-dependent
diffusion. Our initial model is based on the PG~1159 pulsational data, and has
a surface composition of 30% helium, 35% carbon, and 35% oxygen. Below this is
a thin transition zone where the helium fraction falls to zero. As expected,
diffusion caused a separation of the elements; a thickening surface layer of
nearly pure helium overlays a deepening transition zone where the composition
returns to the original surface composition. When the model reached the
temperature range of GD~358 and the pulsating DB white dwarfs, this pure helium
surface layer was 3x10^-6 stellar masses deep. The resulting evolved model is
very similar to the model used by Bradley and Winget (1994) to match the
pulsation observations of GD 358. The pulsation periods of this model also show
a good fit to the WET observations. These results demonstrate the plausibility
of a direct evolutionary path from PG 1159 stars to the much cooler DB white
dwarfs by inclusion of time-dependent diffusion. A problem still remains in
that our models have no hydrogen, and thus must retain their DB nature while
their surface tempeture drops from 45,000K to 30,000K. Since there are no known
DB stars in this range, we plan to address this problem in future calculations.Comment: LaTeX, 10 pages, using AAS Macros. 2 PostScript figures. Accepted for
publication in The Astrophysical Journal Letters
Limits on the Halo White Dwarf Component of Baryonic Dark Matter from the {\em Hubble Deep Field}
The MACHO collaboration lensing event statistics suggest that a significant
fraction of the dark galactic halo can be comprised of baryonic matter in the
form of white dwarf stars with masses between 0.1 and 1.0 \Msun . Such a halo
white dwarf population, in order to have escaped detection by those who observe
the white dwarf luminosity function of the disk, must have formed from an old
population. The observations indicate that the number of halo white dwarfs per
cubic parsec per unit bolometric magnitude is less than at
\Lsun; the number must rise significantly at lower luminosities to
provide the needed baryonic halo mass. Such white dwarfs may easily escape
detection in most current and earlier surveys. Though it is limited in angular
extent, the {\em Hubble Deep Field} (HDF) probes a sufficient volume of the
galactic halo to provide interesting limits on the number of halo white dwarf
stars, and on the fraction of the halo mass that they can make up. If the HDF
field can be probed for stars down to then the MACHO result suggests
that there could be up to 12 faint halo white dwarfs visible in the HDF.
Finding (or not finding) these stars in turn places interesting constraints on
star formation immediately following the formation of the galaxy.Comment: 10 pages, AASTEX, 1 table, no figures, accepted for publication in
Ap.J. Letter
Pulsating sdB Stars: A New Approach to Probing their Interiors
Horizontal branch stars should show significant differential rotation with
depth. Models that assume systematic angular momentum exchange in the
convective envelope and local conservation of angular momentum in the core
produce HB models that preserve a rapidly rotating core. A direct probe of core
rotation is available. The nonradial pulsations of the EC14026 stars frequently
show rich pulsation spectra. Thus their pulsations probe the internal rotation
of these stars, and should show the effects of rapid rotation in their cores.
Using models of sdB stars that include angular momentum evolution, we explore
this possibility and show that some of the sdB pulsators may indeed have
rapidly rotating cores.Comment: 8 pages, 3 figures; paper presented at Keele Workshop on Extreme
Horizontal Branch Stars and Related Objects, June 2003 (ed. Pierre Maxsted
The internal rotation profile of the B-type star KIC10526294 from frequency inversion of its dipole gravity modes and statistical model comparison
The internal angular momentum distribution of a star is key to determine its
evolution. Fortunately, the stellar internal rotation can be probed through
studies of rotationally-split non-radial oscillation modes. In particular,
detection of non-radial gravity modes (g modes) in massive young stars has
become feasible recently thanks to the Kepler space mission. Our aim is to
derive the internal rotation profile of the Kepler B8V star KIC 10526294
through asteroseismology. We interpret the observed rotational splittings of
its dipole g modes using four different approaches based on the best seismic
models of the star and their rotational kernels. We show that these kernels can
resolve differential rotation the radiative envelope if a smooth rotational
profile is assumed and the observational errors are small. Based on Kepler
data, we find that the rotation rate near the core-envelope boundary is well
constrained to nHz. The seismic data are consistent with rigid
rotation but a profile with counter-rotation within the envelope has a
statistical advantage over constant rotation. Our study should be repeated for
other massive stars with a variety of stellar parameters in order to deduce the
physical conditions that determine the internal rotation profile of young
massive stars, with the aim to improve the input physics of their models.Comment: 52 pages, 32 figures, accepted for publication in The Astrophysical
Journa
Internal rotation of subdwarf B stars: limiting cases and asteroseismological consequences
Observations of the rotation rates of horizontal branch (HB) stars show
puzzling systematics. In particular, cooler HB stars often show rapid rotation
(with velocities in excess of 10 km/s), while hotter HB stars typically show
much smaller rotation velocities. Simple models of angular momentum evolution
of stars from the main sequence through the red giant branch fail to explain
these effects. In general, evolutionary models in all cases preserve a rapidly
rotating core. The observed angular velocities of HB stars require that some of
the angular momentum stored in the core reaches the surface.
To test the idea that HB stars contain such a core, one can appeal to
detailed computations of trace element abundences and rotational mixing.
However, a more direct probe is available to test these limiting cases of
angular momentum evolution. Some of the hottest horizontal branch stars are
members of the pulsating sdB class. They frequently show rich pulsation spectra
characteristic of nonradially pulsating stars. Thus their pulsations probe the
internal rotation of these stars, and should show the effects of rapid rotation
in their cores. Using models of sdB stars that include angular momentum
evolution, we explore this possibility and show that some of the sdB pulsators
may indeed have rapidly rotating cores.Comment: accepted for publication in The Astrophysical Journa
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