10,941 research outputs found
Evolution, Explosion and Nucleosynthesis of Core Collapse Supernovae
We present a new set of presupernova evolutions and explosive yields of
massive stars of initial solar composition (Y=0.285, Z=0.02) in the mass range
13-35 Msun. All the models have been computed with the latest version (4.97) of
the FRANEC code that now includes a nuclear network extending from neutrons to
Mo98. The explosive nucleosynthesis has been computed twice: a first one with
an hydro code and a second one following the simpler radiation dominated shock
approximation (RDA).Comment: 20 pages, 10 figures, 12 tables. Accepted for publication on Ap
Presupernova evolution of accreting white dwarfs with rotation
We discuss the effects of rotation on the evolution of accreting
carbon-oxygen white dwarfs, with the emphasis on possible consequences in Type
Ia supernova (SN Ia) progenitors. Starting with a slowly rotating white dwarf,
we simulate the accretion of matter and angular momentum from a quasi-Keplerian
accretion disk. The role of the various rotationally induced hydrodynamic
instabilities for the transport of angular momentum inside the white dwarf is
investigated. We find that the dynamical shear instability is the most
important one in the highly degenerate core. Our results imply that accreting
white dwarfs rotate differentially throughout,with a shear rate close to the
threshold value for the onset of the dynamical shear instability. As the latter
depends on the temperature of the white dwarf, the thermal evolution of the
white dwarf core is found to be relevant for the angular momentum
redistribution. As found previously, significant rotation is shown to lead to
carbon ignition masses well above 1.4 Msun. Our models suggest a wide range of
white dwarf explosion masses, which could be responsible for some aspects of
the diversity observed in SNe Ia. We analyze the potential role of the bar-mode
and the r-mode instability in rapidly rotating white dwarfs, which may impose
angular momentum loss by gravitational wave radiation. We discuss the
consequences of the resulting spin-down for the fate of the white dwarf, and
the possibility to detect the emitted gravitational waves at frequencies of 0.1
>... 1.0 Hz in nearby galaxies with LISA. Possible implications of fast and
differentially rotating white dwarf cores for the flame propagation in
exploding white dwarfs are also briefly discussed.Comment: 22 pages, 16 figures, Accepted to A&
The spectroscopic Hertzsprung-Russell diagram
The Hertzsprung-Russell diagram is an essential diagnostic diagram for
stellar structure and evolution, which has now been in use for more than 100
years. Our spectroscopic Hertzsprung-Russell (sHR) diagram shows the inverse of
the flux-mean gravity versus the effective temperature. Observed stars whose
spectra have been quantitatively analyzed can be entered in this diagram
without the knowledge of the stellar distance or absolute brightness. Observed
stars can be as conveniently compared to stellar evolution calculations in the
sHR diagram as in the Hertzsprung-Russell diagram. However, at the same time,
our ordinate is proportional to the stellar mass-to-luminosity ratio, which can
thus be directly determined. For intermediate- and low-mass star evolution at
constant mass, we show that the shape of an evolutionary track in the sHR
diagram is identical to that in the Hertzsprung-Russell diagram. We also
demonstrate that for hot stars, their stellar Eddington factor can be directly
read off the sHR diagram. For stars near their Eddington limit, we argue that a
version of the sHR diagram may be useful where the gravity is exchanged by the
effective gravity. We discuss the advantages and limitations of the sHR
diagram, and show that it can be fruitfully applied to Galactic stars, but also
to stars with known distance, e.g., in the LMC or in galaxies beyond the Local
Group.Comment: 9 pages, 8 figures, Astronomy and Astrophysics, in pres
On the Collapsar Model of Long Gamma-Ray Bursts: Constraints from Cosmic Metallicity Evolution
We explore the consequences of new observational and theoretical evidence
that long gamma-ray bursts prefer low metallicity environments. Using recently
derived mass-metallicity correlations and the mass function from SDSS studies,
and adopting an average cosmic metallicity evolution from \citet{kewley2005}
and \citet{savaglio2005} we derive expressions for the the relative number of
massive stars formed below a given fraction of solar metallicity, ,
as function of redshift. We demonstrate that about 1/10th of all stars form
with . Therefore, a picture where the majority of GRBs form
with is not inconsistent with an empirical global SN/GRB ratio
of 1/1000. It implies that (1) GRB's peak at a significantly higher redshift
than supernovae; (2) massive star evolution at low metallicity may be
qualitatively different and; (3) the larger the low-metallicity bias of GRBs
the less likely binary evolution channels can be significant GRB producers.Comment: 12 pages, 2 figures; accepted as ApJ Lette
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