101 research outputs found
Variations in the Abundance Pattern of Extremely Metal-poor Stars and Nucleosynthesis in Population III Supernovae
We calculate nucleosynthesis in Population (Pop) III supernovae (SNe) and
compare the yields with various abundance patterns of extremely metal-poor
(EMP) stars. We assume that the observed EMP stars are the second generation
stars, which have the metal-abundance patterns of Pop III SNe. Previous
theoretical yields of Pop III SNe cannot explain the trends in the abundance
ratios among iron-peak elements (Mn, Co, Ni, Zn)/Fe as well as the large C/Fe
ratio observed in certain EMP stars with [Fe/H] <~ -2.5. In the present paper,
we show that if we introduce higher explosion energies and mixing-fallback in
the core-collapse SN models of M ~ 20 - 130 Msun, the above abundance features
of both typical and C-rich EMP stars can be much better explained. We suggest
that the abundance patterns of the [Fe/H] ~ -2.5 stars correspond to supernova
yields with normal explosion energies, while those of the carbon un-enhanced
([C/Fe] < 1) stars with [Fe/H] =~ -4 ~ - 3 correspond to high-energy supernova
yields. The abundance patterns of the C-rich ([C/Fe]>~ 2) and low [Fe/H] (=~ -5
\~ -3.5) stars can be explained with the yields of faint SNe that eject little
56Ni as observed in SN1997D. In the supernova-induced star formation model, we
can qualitatively explain why the EMP stars formed by the faint or energetic
supernovae have lower [Fe/H] than the EMP stars formed by normal supernovae. We
also examine how the abundance ratios among iron-peak elements depend on the
electron mole fraction Ye, and conclude that a large explosion energy is still
needed to realize the large Co/Fe and Zn/Fe ratios observed in typical EMP
stars with [Fe/H] <~ -3.5.Comment: 33 pages, 17 figures, 7 tables, To appear in the Astrophysical
Journal 2005, January 1
Evolution of Rotating Accreting White Dwarfs and the Diversity of Type Ia Supernovae
Type Ia supernovae (SNe Ia) have relatively uniform light curves and spectral
evolution, which make SNe Ia useful standard candles to determine cosmological
parameters. However, the peak brightness is not completely uniform, and the
origin of the diversity has not been clear. We examine whether the rotation of
progenitor white dwarfs (WDs) can be the important source of the diversity of
the brightness of SNe Ia. We calculate the structure of rotating WDs with an
axisymmetric hydrostatic code. The diversity of the mass induced by the
rotation is ~0.08 Msun and is not enough to explain the diversity of
luminosity. However, we found the following relation between the initial mass
of the WDs and their final state; i.e., a WD of smaller initial mass will
rotate more rapidly before the supernova explosion than that of larger initial
mass. This result might explain the dependence of SNe Ia on their host
galaxies.Comment: 7 pages, 6 figure
Electron-capture Supernovae of Super-AGB Stars: Sensitivity on Input Physics
Stars of M ~ 8â10 Mâ on their main sequence form strongly electron-degenerate oxygenâneonâmagnesium (ONeMg) cores and become superâasymptotic giant branch stars. If such an ONeMg core grows to 1.38 Mâ, electron captures on ²â°Ne(e, ν_e) ²â°F(e, ν_e) ²â°O take place and ignite OâNe deflagration around the center. In this work, we perform two-dimensional hydrodynamical simulations of the propagation of the OâNe flame to see whether such a flame triggers a thermonuclear explosion or induces a collapse of the ONeMg core due to subsequent electron capture behind the flame. We present a series of models to explore how the outcome depends on model parameters for a central density ranging between 10^(9.80) and 10^(10.20) g cmâťÂł, flame structures of both centered and off-centered ignition kernels, special and general relativistic effects, turbulent flame speed formulae, and the treatments of laminar burning phase. We obtain bifurcation between the electron-capture induced collapse and thermonuclear explosion depending mainly on the central density. We find that the ONeMg core obtained from stellar evolutionary models has a high tendency to collapse into a neutron star. We discuss the implications of the electron-capture supernovae in chemical evolution and the possible observational signals of this class of supernovae
A binary progenitor for the type IIb supernova 2011dh in M51
We perform binary stellar evolutionary calculations following the simultaneous evolution of both stars in the system to study a potential progenitor system for the Type IIb supernova 2011dh. Pre-explosion photometry as well as light-curve modeling has provided constraints on the physical properties of the progenitor system. Here, we present a close binary system (CBS) that is compatible with such constraints. The system is formed by stars of solar composition with 16 Mâ + 10 Mâ on a circular orbit with an initial period of 125 days. The primary star ends its evolution as a yellow supergiant with a mass of â4 Mâ, a final hydrogen content of â(3-5) Ă 10-3 Mâ, and with an effective temperature and luminosity in agreement with the Hubble Space Telescope (HST) pre-explosion observations of SN 2011dh. These results are nearly insensitive to the adopted accretion efficiency factor β. At the time of explosion, the companion star has an effective temperature of 22,000-40,000 K, depending on the value of β, and lies near the zero-age main sequence. Considering the uncertainties in the HST pre-SN photometry, the secondary star is only marginally detectable in the bluest observed band. CBSs, as opposed to single stars, provide a natural frame to explain the properties of SN 2011dh.Facultad de Ciencias AstronĂłmicas y GeofĂsica
nu-Process Nucleosynthesis in Population III Core-Collapse Supernovae
We investigate the effects of neutrino-nucleus interactions (the nu-process)
on the production of iron-peak elements in Population III core-collapse
supernovae. The nu-process and the following proton and neutron capture
reactions produce odd-Z iron-peak elements in complete and incomplete Si
burning region. This reaction sequence enhances the abundances of Sc, Mn, and
Co in the supernova ejecta. The supernova explosion models of 15 M_sol and 25
M_sol stars with the nu-process well reproduce the averaged Mn/Fe ratio
observed in extremely metal-poor halo stars. In order to reproduce the observed
Mn/Fe ratio, the total neutrino energy in the supernovae should be 3 - 9 x
10^{53} ergs. Stronger neutrino irradiation and other production sites are
necessary to reproduce the observed Sc/Fe and Co/Fe ratios, although these
ratios increase by the nu-process.Comment: 10 pages, 12 figures, Accepted for publication in The Astrophysical
Journa
Bipolar Supernova Explosions: Nucleosynthesis & Implication on Abundances in Extremely Metal-Poor Stars
Hydrodynamics and explosive nucleosynthesis in bipolar supernova explosions
are examined to account for some peculiar properties of hypernovae as well as
peculiar abundance patterns of metal-poor stars. The explosion is supposed to
be driven by bipolar jets which are powered by accretion onto a central
remnant. We explore the features of the explosions with varying progenitors'
masses and jet properties. The outcomes are different from conventional
spherical models. (1) In the bipolar models, Fe-rich materials are ejected at
high velocities along the jet axis, while O-rich materials occupy the central
region whose density becomes very high as a consequence of continuous accretion
from the side. This configuration can explain some peculiar features in the
light curves and the nebular spectra of hypernovae. (2) Production of Ni
tends to be smaller than in spherical thermal bomb models. To account for a
large amount of Ni observed in hypernovae, the jets should be initiated
when the compact remnant mass is still smaller than 2-3\msun, or the jets
should be very massive and slow. (3) Ejected isotopes are distributed as
follows in order of decreasing velocities: Zn, Co, Fe,
Ti, and He at the highest velocities, Mn, Cr,
S, and Si at the intermediate velocities, and Mg, O
at the lowest velocities. (4) The abundance ratios (Zn, Co)/Fe are enhanced
while the ratios (Mn, Cr)/Fe are suppressed. This can account for the abundance
pattern of extremely metal-poor stars. These agreements between the models and
observations suggest that hypernovae are driven by bipolar jets and have
significantly contributed to the early Galactic chemical evolution.Comment: Accepted version, to appear in the Astrophysical Journal. Additional
figures and an appendix. 58 pages including 21 figs and 9 table
Pulsational Pair-instability Supernovae. I. Pre-collapse Evolution and Pulsational Mass Ejection
We calculate the evolution of massive stars, which undergo pulsational pair-instability (PPI) when the O-rich core is formed. The evolution from the main sequence through the onset of PPI is calculated for stars with initial masses of 80â140 M_â and metallicities of Z = 10âťÂłâ1.0 Z_â. Because of mass loss, Z ⤠0.5 Z_â is necessary for stars to form He cores massive enough (i.e., mass >40 M_â) to undergo PPI. The hydrodynamical phase of evolution from PPI through the beginning of Fe-core collapse is calculated for He cores with masses of 40â62 M_ â and Z = 0. During PPI, electronâpositron pair production causes a rapid contraction of the O-rich core, which triggers explosive O-burning and a pulsation of the core. We study the mass dependence of the pulsation dynamics, thermodynamics, and nucleosynthesis. The pulsations are stronger for more massive He cores and result in a large amount of mass ejection such as 3â13 M_â for 40â62 M_â He cores. These He cores eventually undergo Fe-core collapse. The 64 M_â He core undergoes complete disruption and becomes a pair-instability supernova. The H-free circumstellar matter ejected around these He cores is massive enough to explain the observed light curve of Type I (H-free) superluminous supernovae with circumstellar interaction. We also note that the mass ejection sets the maximum mass of black holes (BHs) to be ~50 M_â, which is consistent with the masses of BHs recently detected by VIRGO and aLIGO
Early ultraviolet/optical emission of the type Ib SN 2008D
We propose an alternative explanation for the post-breakout emission of SN 2008D associated with the X-ray transient 080109. Observations of this object show a very small contrast of 0.35 dex between the light-curve minimum occurring soon after the breakout, and the main luminosity peak which is due to radioactive heating of the ejecta. Hydrodynamical models show that the cooling of a shocked Wolf-Rayet star leads to a much greater difference (âł 0.9 dex). Our proposed scenario is that of a jet produced during the explosion which deposits 56Ni-rich material in the outer layers of the ejecta. The presence of high-velocity radioactive material allows us to reproduce the complete luminosity evolution of the object. Without outer 56Ni it could be possible to reproduce the early emission purely from cooling of the shocked envelope by assuming a larger progenitor than a Wolf-Rayet star, but that would require an initial density structure significantly different from what is predicted by stellar evolution models. Analytic models of the cooling phase have been proposed reproduce the early emission of SN 2008D with an extended progenitor. However, we found that the models are valid only until 1.5 days after the explosion where only two data of SN 2008D are available. We also discuss the possibility of the interaction of the ejecta with a binary companion, based on published analytic expressions. However, the binary separation required to fit the early emission should be Ⲡ3 R â, which is too small for a system containing two massive stars.Facultad de Ciencias AstronĂłmicas y GeofĂsica
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