2,444 research outputs found
The origin of the Crab Nebula and the electron capture supernova in 8-10 M solar mass stars
The chemical composition of the Crab Nebula is compared with several presupernova models. The small carbon and oxygen abundances in the helium-rich nebula are consistent with only the presupernova model of the star whose main sequence mass was MMS approximately 8-9.5 M. More massive stars contain too much carbon in the helium layer and smaller mass stars do not leave neutron stars. The progenitor star of the Crab Nebula lost appreciable part of the hydrogen-rich envelope before the hydrogen-rich and helium layers were mixed by convection. Finally it exploded as the electron capture supernova; the O+Ne+Mg core collapsed to form a neutron star and only the extended helium-rich envelope was ejected by the weak shock wave
White dwarf models for type 1 supernovae and quiet supernovae, and presupernova evolution
Supernova mechanisms in accreting white dwarfs are considered with emphasis on deflagration as a plausible mechanism for producing Type I supernovae and electron captures to form quiet supernovae leaving neutron stars. These outcomes depend on accretion rate of helium, initial mass and composition of the white dwarf. The various types of hydrogen shell burning in the presupernova stage are also discussed
Nucleosynthesis in Type Ia Supernovae
Among the major uncertainties involved in the Chandrasekhar mass models for
Type Ia supernovae are the companion star of the accreting white dwarf (or the
accretion rate that determines the carbon ignition density) and the flame speed
after ignition. We present nucleosynthesis results from relatively slow
deflagration (1.5 - 3 % of the sound speed) to constrain the rate of accretion
from the companion star. Because of electron capture, a significant amount of
neutron-rich species such as ^{54}Cr, ^{50}Ti, ^{58}Fe, ^{62}Ni, etc. are
synthesized in the central region. To avoid the too large ratios of
^{54}Cr/^{56}Fe and ^{50}Ti/^{56}Fe, the central density of the white dwarf at
thermonuclear runaway must be as low as \ltsim 2 \e9 \gmc. Such a low central
density can be realized by the accretion as fast as \dot M \gtsim 1 \times
10^{-7} M_\odot yr^{-1}. These rapidly accreting white dwarfs might correspond
to the super-soft X-ray sources.Comment: 10 page LaTeX, 7 PostScript figures, to appear in Nuclear Physics A,
Vol. A621 (1997
Merging White Dwarfs and Thermonuclear Supernovae
Thermonuclear supernovae result when interaction with a companion reignites
nuclear fusion in a carbon-oxygen white dwarf, causing a thermonuclear runaway,
a catastrophic gain in pressure, and the disintegration of the whole white
dwarf. It is usually thought that fusion is reignited in near-pycnonuclear
conditions when the white dwarf approaches the Chandrasekhar mass. I briefly
describe two long-standing problems faced by this scenario, and our suggestion
that these supernovae instead result from mergers of carbon-oxygen white
dwarfs, including those that produce sub-Chandrasekhar mass remnants. I then
turn to possible observational tests, in particular those that test the absence
or presence of electron captures during the burning.Comment: 8 pages, 1 figures, accepted for publication in Phil. Tr. A, proc. of
New windows on transients across the Universe, ed. P. O'Brien et al.; v2
includes changes following comments by the 2 referee
Metallicity structure in X-ray bright galaxy groups
Using Chandra X-ray data of a sample of 15 X-ray bright galaxy groups, we
present preliminary results of a coherent study of the radial distribution of
metal abundances in the hot gas in groups. The iron content in group outskirts
is found to be lower than in clusters by a factor of ~2, despite showing mean
levels in the central regions comparable to those of clusters. The abundance
profiles are used to constrain the contribution from supernovae type Ia and II
to the chemical enrichment and thermal energy of the intragroup medium at
different group radii. The results suggest a scenario in which a substantial
fraction of the chemical enrichment of groups took place in filaments prior to
group collapse.Comment: 5 pages, 2 figures. To appear in the proceedings of ESO Astrophysics
Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V.
Ivanov, J. Burissova (Springer
Optical Emission from Aspherical Supernovae and the Hypernova SN 1998bw
A fully 3D Monte Carlo scheme is applied to compute optical bolometric light
curves for aspherical (jet-like) supernova explosion models. Density and
abundance distributions are taken from hydrodynamic explosion models, with the
energy varied as a parameter to explore the dependence. Our models show
initially a very large degree ( depending on model parameters) of
boosting luminosity toward the polar () direction relative to the equatorial
() plane, which decreases as the time of peak is approached. After the peak,
the factor of the luminosity boost remains almost constant () until
the supernova enters the nebular phase. This behavior is due mostly to the
aspherical Ni distribution in the earlier phase and to the disk-like
inner low-velocity structure in the later phase. Also the aspherical models
yield an earlier peak date than the spherical models, especially if viewed from
near the z-axis. Aspherical models with ejecta mass \sim 10\Msun are
examined, and one with the kinetic energy of the expansion ergs and a mass of Ni \sim 0.4\Msun yields a light
curve in agreement with the observed light curve of SN 1998bw (the prototypical
hyper-energetic supernova). The aspherical model is also at least qualitatively
consistent with evolution of photospheric velocities, showing large velocities
near the z-axis, and with a late-phase nebular spectrum. The viewing angle is
close to the z-axis, strengthening the case for the association of SN 1998bw
with the gamma ray burst GRB980425.Comment: Accepted by the Astrophysical Journal. 28 pages, 14 figure
The Connection between Gamma-Ray Bursts and Extremely Metal-Poor Stars as Nucleosynthetic Probes of the Early Universe
The connection between the long GRBs and Type Ic Supernovae (SNe) has
revealed the interesting diversity: (i) GRB-SNe, (ii) Non-GRB Hypernovae (HNe),
(iii) X-Ray Flash (XRF)-SNe, and (iv) Non-SN GRBs (or dark HNe). We show that
nucleosynthetic properties found in the above diversity are connected to the
variation of the abundance patterns of extremely-metal-poor (EMP) stars, such
as the excess of C, Co, Zn relative to Fe. We explain such a connection in a
unified manner as nucleosynthesis of hyper-aspherical (jet-induced) explosions
Pop III core-collapse SNe. We show that (1) the explosions with large energy
deposition rate, , are observed as GRB-HNe and their yields
can explain the abundances of normal EMP stars, and (2) the explosions with
small are observed as GRBs without bright SNe and can be
responsible for the formation of the C-rich EMP (CEMP) and the hyper metal-poor
(HMP) stars. We thus propose that GRB-HNe and the Non-SN GRBs (dark HNe) belong
to a continuous series of BH-forming stellar deaths with the relativistic jets
of different .Comment: 8 pages, 6 figures. To appear in "Massive Stars as Cosmic Engines",
Proceedings of IAU Symposium 250 (December 2007, Kauai), eds. F. Bresolin,
P.A. Crowther, & J. Puls (Cambridge Univ. Press
Comment on "Heavy element production in inhomogeneous big bang nucleosynthesis"
The work of Matsuura et al. [Phys. Rev. D 72, 123505 (2005);
astro-ph/0507439] claims that heavy nuclei could have been produced in a
combined p- and r-process in very high baryon density regions of an
inhomogeneous big bang. However, they do not account for observational
constraints and previous studies which show that such high baryon density
regions did not significantly contribute to big bang abundances.Comment: 2 pages, submitted to Phys. Rev. D on Feb 23, 200
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
- …