854 research outputs found
Temperature and gravity of the pulsating extreme helium star LSS 3184 (BX Cir) through its pulsation cycle
We report the analysis of optical spectra of the extreme helium star LSS 3184
(BX Cir) to determine its effective temperature and gravity throughout its
pulsation cycle. The spectra were also used to measure its chemical abundances.
We report rest gravity, log g = 3.38 +/- 0.02, and a chemical abundance
mixture consistent with those reported earlier in a study using an optical
spectrum with lower spectral resolution and a lower signal to noise ratio. Our
analysis decreases the upper limit for the H abundance to H < 6.0 (mass
fraction < 7.1 x 10^-7). Our gravity corresponds to stellar mass M = 0.47 +/-
0.03 M_sun.
We find that the effective log g varies through the pulsation cycle with an
amplitude of 0.28 dex. The effective gravity is smaller than the rest gravity
except when the star is very near its minimum radius. The change in effective
gravity is primarily caused by acceleration of the stellar surface.
Based on the optical spectra, we find the temperature varies with an
amplitude of 3450 K. We find a time averaged mean temperature, 23390 +/- 90 K,
consistent with that found in the earlier optical spectrum study. The mean
temperature is 1750 K hotter than that found using combined ultraviolet spectra
and V and R photometry and the variation amplitude is larger. This discrepancy
is similar to that found for the extreme helium star V652 Her.Comment: 7 pages, 6 figures, LaTeX, to be published in A&
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
The first binary star evolution model producing a Chandrasekhar mass white dwarf
Today, Type Ia supernovae are essential tools for cosmology, and recognized
as major contributors to the chemical evolution of galaxies. The construction
of detailed supernova progenitor models, however, was so far prevented by
various physical and numerical difficulties in simulating binary systems with
an accreting white dwarf component, e.g., unstable helium shell burning which
may cause significant expansion and mass loss. Here, we present the first
binary evolution calculation which models both stellar components and the
binary interaction simultaneously, and where the white dwarf mass grows up to
the Chandrasekhar limit by mass accretion. Our model starts with a 1.6 Msun
helium star and a 1.0 Msun CO white dwarf in a 0.124 day orbit. Thermally
unstable mass transfer starts when the CO core of the helium star reaches 0.53
Msun, with mass transfer rates of 1...8 times 10^{-6} Msun/yr. The white dwarf
burns the accreted helium steadily until the white dwarf mass has reached ~ 1.3
Msun and weak thermal pulses follow until carbon ignites in the center when the
white dwarf reaches 1.37 Msun. Although the supernova production rate through
this channel is not well known, and this channel can not be the only one as its
progenitor life time is rather short (~ 10^7 - 10^8 yr), our results indicate
that helium star plus white dwarf systems form a reliable route for producing
Type Ia supernovae.Comment: 4 pages, 5 figure
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
Spectroscopic Analyses of the "Blue Hook" Stars in omega Centauri: A Test of the Late Hot Flasher Scenario
omega Cen contains the largest population of very hot horizontal branch (HB)
stars known in a globular cluster. Recent UV observations (Whitney et al. 1998;
D'Cruz et al. 2000) show a significant population of hot stars below the
zero-age horizontal branch (``blue hook'' stars), which cannot be explained by
canonical stellar evolution. Stars which suffer unusually large mass loss on
the red giant branch and thus experience the helium core flash while descending
the white dwarf cooling curve could populate this region. Theory predicts that
these ``late hot flashers'' should show higher temperatures than the hottest
canonical HB stars and should have helium- and carbon-rich atmospheres. We
obtained and analysed medium resolution spectra of a sample of blue hook stars
to derive their atmospheric parameters. The blue hook stars are indeed both
hotter (Teff > 35,000K) and more helium-rich than classical extreme HB stars.
In addition we find indications for a large enhancement of the carbon abundance
relative to the cluster abundance.Comment: 8 pages, 5 figures, uses aa.cls (included), accepted for publication
in A&
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.
On the angular momentum evolution of merged white dwarfs
We study the angular momentum evolution of binaries containing two white
dwarfs which merge and become cool helium-rich supergiants. Our object is to
compare predicted rotation velocities with observations of highly evolved stars
believed to have formed from such a merger, including RCrB and extreme helium
stars. The principal study involves a binary containing a 0.6 solar mass CO
white dwarf, and a 0.3 solar mass He white dwarf. The initial condition for the
angular momentum distribution is defined where the secondary fills its Roche
Lobe. We assume conservation of angular momentum to compute the angular
momentum distribution in a collisionless disk and subsequently in the giant
envelope. At the end of shell-helium burning, the giant contracts to form a
white dwarf. We derive the surface rotation velocity during this contraction.
The calculation is repeated for a range of initial mass ratios, and also for
the case of mergers between two helium white dwarfs; the latter will contract
to the helium main-sequence rather than the white dwarf sequence. Assuming
complete conservation of angular momentum, we predict acceptable angular
rotation rates for cool giants and during the initial subsequent contraction.
However such stars will only survive spin-up to reach the white dwarf sequence
(CO+He merger) if the initial mass ratio is close to unity. He+He merger
products must lose angular momentum in order to reach the helium main sequence.
Minimum observed rotation velocities in extreme helium stars are lower than our
predictions by at least one half, indicating that CO+He mergers must lose at
least one half of their angular momentum.Comment: 11 pages, 11 figures, MNRAS in pres
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
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
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