146 research outputs found
Private Possession of Child Pornography: The Tensions Between Stanley v. Georgia and New York v. Ferber
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
Stellar evolution with rotation VII: Low metallicity models and the blue to red supergiant ratio in the SMC
We calculate a grid of models with and without the effects of axial rotation
for massive stars in the range of 9 to 60 M and metallicity =
0.004 appropriate for the SMC. Remarkably, the ratios
of the angular velocity to the break-up angular
velocity grow strongly during the evolution of high mass stars, contrary to the
situation at = 0.020. The reason is that at low , mass loss is smaller
and the removal of angular momentum during evolution much weaker, also there is
an efficient outward transport of angular momentum by meridional circulation.
Thus, a much larger fraction of the stars at lower reach break-up
velocities and rotation may thus be a dominant effect at low . The models
with rotation well account for the long standing problem of the large numbers
of red supergiants observed in low galaxies, while current models with mass
loss were predicting no red supergiants. We discuss in detail the physical
effects of rotation which favour a redwards evolution in the HR diagram. The
models also predict large N enrichments during the evolution of high mass
stars. The predicted relative N-enrichments are larger at lower than solar
and this is in very good agreement with the observations for A-type supergiants
in the SMC.Comment: 18 pages, 16 figures, in press in Astronomy and Astrophysic
The thermonuclear production of F19 by Wolf-Rayet stars revisited
New models of rotating and non-rotating stars are computed for initial masses
between 25 and 120 Msun and for metallicities Z = 0.004, 0.008, 0.020 and 0.040
with the aim of reexamining the wind contribution of Wolf-Rayet (WR) stars to
the F19 enrichment of the interstellar medium. Models with an initial rotation
velocity vini = 300 km/s are found to globally eject less F19 than the
non-rotating models. We compare our new predictions with those of Meynet &
Arnould (2000), and demonstrate that the F19 yields are very sensitive to the
still uncertain F19(alpha,p)Ne22 rate and to the adopted mass loss rates. Using
the recommended mass loss rate values that take into account the clumping of
the WR wind and the NACRE reaction rates when available, we obtain WR F19
yields that are significantly lower than predicted by Meynet & Arnould (2000),
and that would make WR stars non-important contributors to the galactic F19
budget. In view, however, of the large nuclear and mass loss rate
uncertainties, we consider that the question of the WR contribution to the
galactic F19 remains quite largely open.Comment: 9 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
An evolutionary study of the pulsating subdwarf B eclipsing binary PG1336-018 (NY Vir)
The formation of subdwarf B (sdB) stars is not well understood within the
current framework of stellar single and binary evolution. In this study, we
focus on the formation and evolution of the pulsating sdB star in the very
short-period eclipsing binary PG1336-018. We aim at refining the formation
scenario of this unique system, so that it can be confronted with observations.
We probe the stellar structure of the progenitors of sdB stars in short-period
binaries using detailed stellar evolution calculations. Applying this to
PG1336-018 we reconstruct the common-envelope phase during which the sdB star
was formed. The results are interpreted in terms of the standard
common-envelope formalism (the alpha-formalism) based on the energy equation,
and an alternative description (the gamma-formalism) using the angular momentum
equation. We find that if the common-envelope evolution is described by the
alpha-formalism, the sdB progenitor most likely experienced a helium flash. We
then expect the sdB mass to be between 0.39 and 0.48 Msun, and the sdB
progenitor initial mass to be below ~2 Msun. However, the results for the
gamma-formalism are less restrictive, and a broader sdB mass range (0.3 - 0.8
Msun) is possible in this case. Future seismic mass determination will give
strong constraints on the formation of PG1336-018 and, in particular, on the CE
phase.Comment: 9 pages, 7 figures, 2 tables, accepted for publication in A&
Yields of rotating stars at solar metallicity
We present a new set of stellar yields obtained from rotating stellar models
at solar metallicity covering the massive star range (12-60 solar masses). The
stellar models were calculated with the latest version of the Geneva stellar
evolution code described in Hirschi et al (2004). Evolution and nucleosynthesis
are in general followed up to silicon burning. The yields of our non-rotating
models are consistent with other calculations and differences can be understood
in the light of the treatment of convection and the rate used for C12(a,g)O16.
This verifies the accuracy of our calculations and gives a safe basis for
studying the effects of rotation on the yields.
The contributions from stellar winds and supernova explosions to the stellar
yields are presented separately. We then add the two contributions to compute
the total stellar yields. Below about 30 solar masses, rotation increases the
total metal yields, Z, and in particular the yields of carbon and oxygen by a
factor of 1.5-2.5. As a rule of thumb, the yields of a rotating 20 solar masses
star are similar to the yields of a non-rotating 30 solar masses star, at least
for the light elements considered in this work. For very massive stars (around
60 solar masses), rotation increases the yield of helium but does not
significantly affect the yields of heavy elements.Comment: 11 pages, 4 figures; accepted for publication in A&
Stellar evolution with rotation XII: Pre-supernova models
We describe the latest developments of the Geneva stellar evolution code in
order to model the pre-supernova evolution of rotating massive stars. Rotating
and non-rotating stellar models at solar metallicity with masses equal to 12,
15, 20, 25, 40 and 60 solar masses were computed from the ZAMS until the end of
the core silicon burning phase. We took into account meridional circulation,
secular shear instabilities, horizontal turbulence and dynamical shear
instabilities.
Most of the differences between the pre-supernova structures obtained from
rotating and non-rotating stellar models have their origin in the effects of
rotation during the core hydrogen and helium burning phases.
The effects of rotation on pre-supernova models are significant between 15
and 30 solar masses. Indeed, rotation increases the core sizes (and the yields)
by a factor ~ 1.5. Above 20 solar masses, rotation may change the colour of the
supernova progenitors (blue instead of red supergiant) and the supernova type
(Ib instead of II). Rotation affects the lower mass limits for radiative core
carbon burning, for iron core collapse and for black hole formation. For
Wolf-Rayet stars (M > 30 solar masses), the pre-supernova structures are mostly
affected by the intensities of the stellar winds and less by rotational mixing.
Finally, the core of our rotating WR stars contain enough angular momentum to
produce GRBs.Comment: 23 pages, 23 figures, accepted for publication in A&
Non-resonant direct p- and d-wave neutron capture by 12C
Discrete gamma-rays from the neutron capture state of 13C to its low-lying
bound states have been measured using pulsed neutrons at En = 550 keV. The
partial capture cross sections have been determined to be 1.7+/-0.5,
24.2+/-1.0, 2.0+/-0.4 and 1.0+/-0.4 microb for the ground (1/2-), first (1/2+),
second (3/2-) and third (5/2+) excited states, respectively. From a comparison
with theoretical predictions based on the non-resonant direct radiative capture
mechanism, we could determine the spectroscopic factor for the 1/2+ state to be
0.80 +/- 0.04, free from neutron-nucleus interaction ambiguities in the
continuum. In addition we have detected the contribution of the non-resonant
d-wave capture component in the partial cross sections for transitions leading
to the 1/2- and 3/2- states. While the s-wave capture dominates at En < 100
keV, the d-wave component turns out to be very important at higher energies.
From the present investigation the 12C(n,gamma)13C reaction rate is obtained
for temperatures in the range 10E+7 - 10E+10 K.Comment: Accepted for publication in Phys. Rev. C. - 16 pages + 8 figure
Pulsations of massive ZZ Ceti stars with carbon/oxygen and oxygen/neon cores
We explore the adiabatic pulsational properties of massive white dwarf stars
with hydrogen-rich envelopes and oxygen/neon and carbon/oxygen cores. To this
end, we compute the cooling of massive white dwarf models for both core
compositions taking into account the evolutionary history of the progenitor
stars and the chemical evolution caused by time-dependent element diffusion. In
particular, for the oxygen/neon models, we adopt the chemical profile resulting
from repeated carbon-burning shell flashes expected in very massive white dwarf
progenitors. For carbon/oxygen white dwarfs we consider the chemical profiles
resulting from phase separation upon crystallization. For both compositions we
also take into account the effects of crystallization on the oscillation
eigenmodes. We find that the pulsational properties of oxygen/neon white dwarfs
are notably different from those made of carbon/oxygen, thus making
asteroseismological techniques a promising way to distinguish between both
types of stars and, hence, to obtain valuable information about their
progenitors.Comment: 11 pages, including 11 postscript figures. Accepted for publication
in Astronomy and Astrophysic
Evolution of a 3 \msun star from the main sequence to the ZZ Ceti stage: the role played by element diffusion
The purpose of this paper is to present new full evolutionary calculations
for DA white dwarf stars with the major aim of providing a physically sound
reference frame for exploring the pulsation properties of the resulting models
in future communications. Here, white dwarf evolution is followed in a
self-consistent way with the predictions of time dependent element diffusion
and nuclear burning. In addition, full account is taken of the evolutionary
stages prior to the white dwarf formation. In particular, we follow the
evolution of a 3 \msun model from the zero-age main sequence (the adopted
metallicity is Z=0.02) all the way from the stages of hydrogen and helium
burning in the core up to the thermally pulsing phase. After experiencing 11
thermal pulses, the model is forced to evolve towards its white dwarf
configuration by invoking strong mass loss episodes. Further evolution is
followed down to the domain of the ZZ Ceti stars on the white dwarf cooling
branch. Emphasis is placed on the evolution of the chemical abundance
distribution due to diffusion processes and the role played by hydrogen burning
during the white dwarf evolution. Furthermore, the implications of our
evolutionary models for the main quantities relevant for adiabatic pulsation
analysis are discussed. Interestingly, the shape of the Ledoux term is markedly
smoother as compared with previous detailed studies of white dwarfs. This is
translated into a different behaviour of the Brunt-Vaisala frequency.Comment: 11 pages, 11 figures, accepted for publication in MNRA
- âŠ