247 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
On the very long term evolutionary behavior of hydrogen-accreting Low-Mass CO white dwarfs
Hydrogen-rich matter has been added to a CO white dwarf of initial mass 0.516
\msun at the rates and \msun \yrm1, and results are
compared with those for a white dwarf of the same initial mass which accretes
pure helium at the same rates. For the chosen accretion rates, hydrogen burns
in a series of recurrent mild flashes and the ashes of hydrogen burning build
up a helium layer at the base of which a He flash eventually occurs. In
previous studies involving accretion at higher rates and including initially
more massive WDs, the diffusion of energy inward from the H shell-flashing
region contributes to the increase in the temperature at the base of the helium
layer, and the mass of the helium layer when the He flash begins is
significantly smaller than in a comparison model accreting pure helium; the He
shell flash is not strong enough to develop into a supernova explosion. In
contrast, for the conditions adopted here, the temperature at the base of the
He layer becomes gradually independent of the deposition of energy by H shell
flashes, and the mass of the He layer when the He flash occurs is a function
only of the accretion rate, independent of the hydrogen content of the accreted
matter. When the He flash takes place, due to the high degeneracy at the base
of the He layer, temperatures in the flashing zone will rise without a
corresponding increase in pressure, nuclear burning will continue until nuclear
statistical equilibrium is achieved; the model will become a supernova, but not
of the classical type Ia variety.Comment: 14 pages and 3 Postscript figures, Accepted for publication on ApJ
Letter
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.
Hydrogen-Accreting Carbon-Oxygen White Dwarfs of Low Mass: Thermal and Chemical Behavior of Burning Shells
Numerical experiments have been performed to investigate the thermal behavior
of a cooled down white dwarf of initial mass M_{\rm WD} = 0.516 M_{\sun}
which accretes hydrogen-rich matter with Z = 0.02 at the rate
\msun \yrm1, typical for a recurrent hydrogen shell flash regime. The evolution
of the main physical quantities of a model during a pulse cycle is examined in
detail. From selected models in the mass range
\msunend, we derive the borders in the - plane of the
steady state accretion regime when hydrogen is burned at a constant rate as
rapidly as it is accreted. The physical properties during a hydrogen shell
flash in white dwarfs accreting hydrogen-rich matter with metallicities Z =
0.001 and Z = 0.0001 are also studied. For a fixed accretion rate, a decrease
in the metallicity of the accreted matter leads to an increase in the thickness
of the hydrogen-rich layer at outburst and a decrease in the hydrogen-burning
shell efficiency. In the - plane, the borders of the
steady state accretion band are critically dependent on the metallicity of the
accreted matter: on decreasing the metallicity, the band is shifted to lower
accretion rates and its width in is reduced.Comment: 31 pages and 10 Postscript figures; Accepted for publication on Ap
Carbon-Oxygen White Dwarfs Accreting CO-Rich Matter I: A Comparison Between Rotating and Non-Rotating Models
We investigate the lifting effect of rotation on the thermal evolution of CO
WDs accreting CO-rich matter. We find that rotation induces the cooling of the
accreting star so that the delivered gravitational energy causes a greater
expansion with respect to the standard non-rotating case. The increase in the
surface radius produces a decrease in the surface value of the critical angular
velocity and, therefore, the accreting WD becomes gravitationally unbound
(Roche instability). This occurrence is due to an increase in the total angular
momentum of the accreting WD and depends critically on the amount of specific
angular momentum deposited by the accreted matter. If the specific angular
momentum of the accreted matter is equal to that of the outer layers of the
accreting structure, the Roche instability occurs well before the accreting WD
can attain the physical conditions for C-burning. If the values of both initial
angular velocity and accretion rate are small, we find that the accreting WD
undergoes a secular instability when its total mass approaches 1.4 Msun. At
this stage, the ratio between the rotational and the gravitational binding
energy of the WD becomes of the order of 0.1, so that the star must deform by
adopting an elliptical shape. In this case, since the angular velocity of the
WD is as large as 1 rad/s, the anisotropic mass distribution induces the loss
of rotational energy and angular momentum via GWR. We find that, independent of
the braking efficiency, the WD contracts and achieves the physical conditions
suitable for explosive C-burning at the center so that a type Ia supernova
event is produced.Comment: 39 pages, 22 eps-figures; accepted for publication in Astrophysical
Journa
High-resolution spectroscopy of the R Coronae Borealis and Other Hydrogen Deficient Stars
High-resolution spectroscopy is a very important tool for studying stellar
physics, perhaps, particularly so for such enigmatic objects like the R Coronae
Borealis and related Hydrogen deficient stars that produce carbon dust in
addition to their peculiar abundances.
Examples of how high-resolution spectroscopy is used in the study of these
stars to address the two major puzzles are presented: (i) How are such rare
H-deficient stars created? and (ii) How and where are the obscuring soot clouds
produced around the R Coronae Borealis stars?Comment: 16 pages, 9 figures, Astrophysics and Space Science Proceedings,
Springer-Verlag, Berlin, 201
Element Abundance Determination in Hot Evolved Stars
The hydrogen-deficiency in extremely hot post-AGB stars of spectral class
PG1159 is probably caused by a (very) late helium-shell flash or a AGB final
thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden
intershell region. Thus, the photospheric element abundances of these stars
allow us to draw conclusions about details of nuclear burning and mixing
processes in the precursor AGB stars. We compare predicted element abundances
to those determined by quantitative spectral analyses performed with advanced
non-LTE model atmospheres. A good qualitative and quantitative agreement is
found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for
others (P, S, Fe) point at shortcomings in stellar evolution models for AGB
stars. Almost all of the chemical trace elements in these hot stars can only be
identified in the UV spectral range. The Far Ultraviolet Spectroscopic Explorer
and the Hubble Space Telescope played a crucial role for this research.Comment: To appear in: Recent Advances in Spectroscopy: Theoretical,
Astrophysical, and Experimental Perspectives, Proceedings, Jan 28 - 31, 2009,
Kodaikanal, India (Springer
Synthesis of the elements in stars: forty years of progress
Forty years ago Burbidge, Burbidge, Fowler, and Hoyle combined what we would now call fragmentary evidence from nuclear physics, stellar evolution and the abundances of elements and isotopes in the solar system as well as a few stars into a synthesis of remarkable ingenuity. Their review provided a foundation for forty years of research in all of the aspects of low energy nuclear experiments and theory, stellar modeling over a wide range of mass and composition, and abundance studies of many hundreds of stars, many of which have shown distinct evidence of the processes suggested by B2FH. In this review we summarize progress in each of these fields with emphasis on the most recent developments
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