597 research outputs found
Evolution II - from the Main Sequence Through Core Helium Burning, M Equals 3M Sun Masses
Evolution of three solar mass star from main sequence to red gian
Morphological Properties of PPNs: Mid-IR and HST Imaging Surveys
We will review our mid-infrared and HST imaging surveys of the circumstellar
dust shells of proto-planetary nebulae. While optical imaging indirectly probes
the dust distribution via dust-scattered starlight, mid-IR imaging directly
maps the distribution of warm dust grains. Both imaging surveys revealed
preferencially axisymmetric nature of PPN dust shells, suggesting that
axisymmetry in planetary nebulae sets in by the end of the asymptotic giant
branch phase, most likely by axisymmetric superwind mass loss. Moreover, both
surveys yielded two morphological classes which have one-to-one correspondence
between the two surveys, indicating that the optical depth of circumstellar
dust shells plays an equally important role as the inclination angle in
determining the morphology of the PPN shells.Comment: 6 pages + 8 figures, to appear in the proceedings of the conference,
"Post-AGB Objects (proto-planetary nebulae) as a Phase of Stellar Evolution",
Torun, Poland, July 5-7, 2000, eds. R. Szczerba, R. Tylenda, and S.K. Gorny.
Figures have been degraded to minimize the total file siz
Stellar evolution and nucleosynthesis of Post-AGB Stars
I discuss recent new models of post-Asymptotic Giant Branch stellar
evolution. These models aim to clarify the evolutionary origin and status of a
variety of hydrogen-deficient post-AGB stars such as central stars of planetary
nebulae of Wolf-Rayet spectral type, PG1159 stars or Sakurai's object. Starting
with AGB models with overshoot such stars can evolve through one of four
distinct channels. Each of these channels has typical abundance patterns
depending on the relative timing of the departure from the AGB and the
occurrence of the last thermal pulse. I discuss the responsible mechanisms and
observational counterparts.Comment: 9 pages, 1 figure, conference paper, workshop "Post-AGB objects
(proto-planetary nebulae) as a phase of stellar evolution", Jul 5-7, 2000,
Torun, Poland, to appear in Ap&S
The population of close double white dwarfs in the Galaxy
We present a new model for the Galactic population of close double white
dwarfs. The model accounts for the suggestion of the avoidance of a substantial
spiral-in during mass transfer between a giant and a main-sequence star of
comparable mass and for detailed cooling models. It agrees well with the
observations of the local sample of white dwarfs if the initial binary fraction
is close to 50% and an ad hoc assumption is made that white dwarfs with mass
less than about 0.3 solar mass cool faster than the models suggest. About 1000
white dwarfs brighter than V=15 have to be surveyed for detection of a pair
which has total mass greater than the Chandrasekhar mass and will merge within
10 Gyr.Comment: 15 pages, 7 figures, to appear in Proc. ``The influence of binaries
on stellar population studies'', Brussels, August 2000 (Kluwer, D. Vanbeveren
ed.
An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate
There is wide agreement that Type Ia supernovae (used as standard candles for
cosmology) are associated with the thermonuclear explosions of white dwarf
stars. The nuclear runaway that leads to the explosion could start in a white
dwarf gradually accumulating matter from a companion star until it reaches the
Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in
a compact binary system. The X-ray signatures of these two possible paths are
very different. Whereas no strong electromagnetic emission is expected in the
merger scenario until shortly before the supernova, the white dwarf accreting
material from the normal star becomes a source of copious X-rays for ~1e7 yr
before the explosion. This offers a means of determining which path dominates.
Here we report that the observed X-ray flux from six nearby elliptical galaxies
and galaxy bulges is a factor of ~30-50 less than predicted in the accretion
scenario, based upon an estimate of the supernova rate from their K-band
luminosities. We conclude that no more than ~5 per cent of Type Ia supernovae
in early type galaxies can be produced by white dwarfs in accreting binary
systems, unless their progenitors are much younger than the bulk of the stellar
population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs
make a significant contribution to the supernova rate.Comment: 10 pages, 1 tabl
The origin of HE0107-5240 and the production of O and Na in extremely metal-poor stars
We elaborate the binary scenario for the origin of HE0107-5240, the most
metal-poor star yet observed ([Fe/H] = -5.3), using current knowledge of the
evolution of extremely metal-poor stars. From the observed C/N value, we
estimate the binary separation and period. Nucleosynthesis in a helium
convective zone into which hydrogen has been injected allows us to discuss the
origin of surface O and Na as well as the abundance distribution of s-process
elements. We can explain the observed abundances of 12C, 13C, N, O, and Na and
predict future observations to validate the Pop III nature of HE0107-5240.Comment: 4 pages, 3 figures, proceedings of the conference, "Nuclei in the
Cosmos VIII", Nuclear Physics A in pres
Solar Neutrinos from CNO Electron Capture
The neutrino flux from the sun is predicted to have a CNO-cycle contribution
as well as the known pp-chain component. Previously, only the fluxes from beta+
decays of 13N, 15O, and 17F have been calculated in detail. Another neutrino
component that has not been widely considered is electron capture on these
nuclei. We calculate the number of interactions in several solar neutrino
detectors due to neutrinos from electron capture on 13N, 15O, and 17F, within
the context of the Standard Solar Model. We also discuss possible non-standard
models where the CNO flux is increased.Comment: 4 pages, 1 figure, submitted to Phys. Rev. C; v2 has minor changes
including integration over solar volume and addition of missing reference to
previous continuum electron capture calculation; v3 has minor changes
including addition of references and the correction of a small (about 1%)
numerical error in the table
Massive Star Mergers: Induced Mixing and Nucleosynthesis
We study the nucleosynthesis and the induced mixing during the merging of
massive stars inside a common envelope. The systems of interest are close
binaries, initially consisting of a massive red supergiant and a main-sequence
companion of a few solar masses. We apply parameterized results based on
hydrodynamical simulations to model the stream-core interaction and the
response of the star in a standard stellar-evolution code. Preliminary results
are presented illustrating the possibility of unusual nucleosynthesis and
post-merging dredge-up which can cause composition anomalies in the
supergiant's envelope.Comment: 4 pages. To be published in Ap&S
Double white dwarf mergers and elemental surface abundances in extreme helium and R Coronae Borealis stars
The surface abundances of extreme helium (EHe) and R Coronae Borealis (RCB)
stars are discussed in terms of the merger of a carbon-oxygen white dwarf with
a helium white dwarf. The model is expressed as a linear mixture of the
individual layers of both constituent white dwarfs, taking account of the
specific evolution of each star. In developing this recipe from previous
versions, particular attention has been given to the inter-shell abundances of
the asymptotic giant branch star which evolved to become the carbon-oxygen
white dwarf. Thus the surface composition of the merged star is estimated as a
function of the initial mass and metallicity of its progenitor. The question of
whether additional nucleosynthesis occurs during the white dwarf merger has
been examined.
The high observed abundances of carbon and oxygen must either originate by
dredge-up from the core of the carbon-oxygen white dwarf during a cold merger
or be generated directly by alpha-burning during a hot merger. The presence of
large quantities of O18 may be consistent with both scenarios, since a
significant O18 pocket develops at the carbon/helium boundary in a number of
our post-AGB models.
The production of fluorine, neon and phosphorus in the AGB intershell
produces n overabundance at the surface of the merged stars, but generally not
in sufficient quantity. However, the evidence for an AGB origin for these
elements points to progenitor stars with initial masses in the range 1.9 - 3
solar masses.
There is not yet sufficient information to discriminate the origin (fossil or
prompt) of all the abundance anomalies observed in EHe and RCB stars. Further
work is required on argon and s-process elements in the AGB intershell, and on
the predicted yields of all elements from a hot merger.Comment: 20 pages, 8 figures, 3 tables, MNRAS in pres
- âŠ