1,931 research outputs found
Nucleosynthesis in massive stars revisited
We have performed the first calculations to follow the evolution of all
stable nuclei and their radioactive progenitors in a finely-zoned stellar model
computed from the onset of central hydrogen burning through explosion as a Type
II supernova. Calculations were done for 15, 20, and 25 solar masses Pop I
stars using the most recently available set of experimental and theoretical
nuclear data, revised opacity tables, and taking into account mass loss due to
stellar winds. Here results are presented for one 15 solar masses model.Comment: 4 pages, 1 figure; needs espcrc1.sty; talk at "Nuclei in the Cosmos
2000", Aarhus, Denmark, June 2000; will appear in Nucl. Phys.
Nuclear Aspects of Nucleosynthesis in Massive Stars
Preliminary results of a new set of stellar evolution and nucleosynthesis
calculations for massive stars are presented. These results were obtained with
an extended reaction network up to Bi. The discussion focuses on the importance
of nuclear rates in pre- and post-explosive nucleosynthesis. The need for
further experiments to study specific reactions and nuclear properties (optical
alpha+nucleus potentials) is emphasized.Comment: 6 pages, 2 figures; invited talk, to appear in the Proceedings of the
Int. Conf. "Structure of the Nucleus at the Dawn of the Century", May 2000,
Bologna, Ital
Stellar evolution with rotation XIII: Predicted GRB rates at various Z
We present the evolution of rotation in models of massive single stars
covering a wide range of masses and metallicities. These models reproduce very
well observations during the early stages of the evolution (in particular WR
populations and ratio between type II and type Ib,c at different metallicities,
see Meynet & Maeder 2005).
Our models predict the production of fast rotating black holes. Models with
large initial masses or high metallicity end their life with less angular
momentum in their central remnant with respect to the break-up limit for the
remnant. Many WR star models satisfy the three main criteria (black hole
formation, loss of hydrogen-rich envelope and enough angular momentum to form
an accretion disk around the black hole) for gamma-ray bursts (GRB) production
via the collapsar model (Woosley 1993). Considering all types of WR stars as
GRB progenitors, there would be too many GRBs compared to observations. If we
consider only WO stars (type Ic supernovae as is the case for
SN2003dh/GRB030329, see Matheson et al. 2003) as GRBs progenitors, the GRBs
production rates are in much better agreement with observations. WO stars are
produced only at low metallicities in the present grid of models. This
prediction can be tested by future observations.Comment: ~16 pages, 14 figures, accepted by A&
Helium and Nitrogen Enrichment in Massive Main Sequence Stars: Mechanisms and Implications for the Origin of WNL Stars
The evolutionary paths taken by massive stars with remain substantially uncertain. They begin their lives as
main sequence (MS) O-stars. Depending on their masses, rotation rates, and
metallicities, they can then encounter a wide range of evolutionary states with
an equally broad set of possible surface compositions and spectral
classifications. We present a new grid of calculations for the evolution of
such stars that covers a broad range in mass, M/M to ,
rotation rate, to , metallicity,
to , and -element enhancement,
to . We show that rotating stars undergo
rotationally-induced dredge-up of nucleosynthetic products, mostly He and N, to
their surfaces while still on the MS. Non-rotating metal-rich stars also reveal
the products of nucleosynthesis on their surfaces because even modest amounts
of mass loss expose their "fossil" convective cores: regions that are no longer
convective, but which were part of the convective core at an early stage in the
star's evolution. Thus surface enhancement of He and N is expected for rotating
stars at all metallicities, and for non-rotating stars if they are relatively
metal-rich. We calculate a stellar atmosphere for a representative model from
our grid, properly accounting for He- and N-enhancement, and show that the
resulting spectrum provides a good match to observed WNL stars, strongly
suggesting that the physical mechanisms we have identified are the ultimate
cause of the WNL phase.Comment: 21 pages, 18 figures, 2 tables, accepted for publication in MNRAS, in
pres
Why a Single-Star Model Cannot Explain the Bipolar Nebula of Eta Carinae
I examine the angular momentum evolution during the 1837-1856 Great Eruption
of the massive star Eta Carinae. I find that the new estimate of the mass blown
during that eruption implies that the envelope of Eta Car substantially
spun-down during the 20 years eruption. Single-star models, most of which
require the envelope to rotate close to the break-up velocity, cannot account
for the bipolar nebula (the Homunculus) formed from matter expelled in that
eruption. The kinetic energy and momentum of the Homunculus further constrains
single-star models. I discuss how Eta Car can fit into a unified model for the
formation of bipolar lobes where two oppositely ejected jets inflate two lobes
(or bubbles). These jets are blown by an accretion disk, which requires stellar
companions in the case of bipolar nebulae around stellar objects.Comment: ApJ, in press. New references and segments were adde
Code dependencies of pre-supernova evolution and nucleosynthesis in massive stars: Evolution to the end of core helium burning
Massive stars are key sources of radiative, kinetic and chemical feedback in the Universe. Grids of massive star models computed by different groups each using their own codes, input physics choices and numerical approximations, however, lead to inconsistent results for the same stars. We use three of these 1D codes – genec, kepler and mesa – to compute non-rotating stellar models of 15, 20 and 25 M⊙ and compare their nucleosynthesis. We follow the evolution from the main sequence until the end of core helium burning. The genec and kepler models hold physics assumptions used in large grids of published models. The mesa code was set up to use convective core overshooting such that the CO core masses are consistent with those obtained by genec. For all models, full nucleosynthesis is computed using the NuGrid post-processing tool mppnp. We find that the surface abundances predicted by the models are in reasonable agreement. In the helium core, the standard deviation of the elemental overproduction factors for Fe to Mo is less than 30 per cent – smaller than the impact of the present nuclear physics uncertainties. For our three initial masses, the three stellar evolution codes yield consistent results. Differences in key properties of the models, e.g. helium and CO core masses and the time spent as a red supergiant, are traced back to the treatment of convection and, to a lesser extent, mass loss. The mixing processes in stars remain the key uncertainty in stellar modelling. Better constrained prescriptions are thus necessary to improve the predictive power of stellar evolution models
Dependence of X-Ray Burst Models on Nuclear Reaction Rates
X-ray bursts are thermonuclear flashes on the surface of accreting neutron
stars and reliable burst models are needed to interpret observations in terms
of properties of the neutron star and the binary system. We investigate the
dependence of X-ray burst models on uncertainties in (p,),
(,), and (,p) nuclear reaction rates using fully
self-consistent burst models that account for the feedbacks between changes in
nuclear energy generation and changes in astrophysical conditions. A two-step
approach first identified sensitive nuclear reaction rates in a single-zone
model with ignition conditions chosen to match calculations with a
state-of-the-art 1D multi-zone model based on the {\Kepler} stellar evolution
code. All relevant reaction rates on neutron deficient isotopes up to mass 106
were individually varied by a factor of 100 up and down. Calculations of the 84
highest impact reaction rate changes were then repeated in the 1D multi-zone
model. We find a number of uncertain reaction rates that affect predictions of
light curves and burst ashes significantly. The results provide insights into
the nuclear processes that shape X-ray burst observables and guidance for
future nuclear physics work to reduce nuclear uncertainties in X-ray burst
models.Comment: 24 pages, 13 figures, 4 tables, submitte
Discovery of Variability of the Progenitor of SN 2011dh in M51 Using the Large Binocular Telescope
We show that the candidate progenitor of the core-collapse SN 2011dh in M51
(8 Mpc away) was fading by 0.039 +- 0.006 mag/year during the three years prior
to the supernova, and that this level of variability is moderately unusual for
other similar stars in M 51. While there are uncertainties about whether the
true progenitor was a blue companion to this candidate, the result illustrates
that there are no technical challenges to obtaining fairly high precision light
curves of supernova progenitors using ground based observations of nearby (<10
Mpc) galaxies with wide field cameras on 8m-class telescopes. While other
sources of variability may dominate, it is even possible to reach into the
range of evolution rates required by the quasi-static evolution of the stellar
envelope. For M 81, where we have many more epochs and a slightly longer time
baseline, our formal 3 sigma sensitivity to slow changes is presently 3
millimag/year for a M_V ~= -8 mag star. In short, there is no observational
barrier to determining whether the variability properties of stars in their
last phases of evolution (post Carbon ignition) are different from earlier
phases.Comment: 17 pages, 5 figures, submitted to Ap
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