7,637 research outputs found
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
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
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
Supersoft X-ray Phase of Single Degenerate Type Ia Supernova Progenitors in Early Type Galaxies
In the single degenerate (SD) scenario for Type Ia supernova (SN Ia)
progenitors, an accreting white dwarf (WD) is expected to undergo a supersoft
X-ray source (SSS) phase. Recently, Gilfanov & Bogdan (2010, hereafter GB10)
claimed that observed X-ray fluxes of early type galaxies would be too low to
be consistent with the prediction of the SD scenario based on rather simple
assumptions. We present realistic evolutionary models of SD systems and
calculate durations of SSS phases. In most cases, accreting WDs spend a large
fraction of time in the optically thick wind phase and the recurrent nova phase
rather than the SSS phase. Thus the SSS phase lasts only for a few hundred
thousand years. This is by a factor of ~10 shorter than those adopted by GB10
where the SN~Ia progenitor WD was assumed to spend most of its life as a SSS.
The theoretical X-ray luminosity of the SSS has a large uncertainty because of
the uncertain atmospheric model of mass-accreting WDs and absorption of soft
X-rays by the companion star's cool wind material. We thus adopt an average of
the observed fluxes of existing symbiotic SSSs, i.e., ~0.4 x 10^{36} erg s^{-1}
for 0.3--0.7 keV. Using these SSS duration and soft X-ray luminosity, we show
that the observed X-ray flux obtained by GB10 is rather consistent with our
estimated flux in early type galaxies based on the SD scenario. This is a
strong support for the SD scenario as a main-contributor of SNe Ia in early
type galaxies.Comment: 5 pages, 3 figures, to appear in ApJ
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