2,209 research outputs found
Nebular Spectra of SN 1998bw Revisited: Detailed Study by One and Two Dimensional Models
Refined one- and two-dimensional models for the nebular spectra of the
hyper-energetic Type Ic supernova (SN) 1998bw, associated with the gamma-ray
burst GRB980425, from 125 to 376 days after B-band maximum are presented. One
dimensional, spherically symmetric spectrum synthesis calculations show that
reproducing features in the observed spectra, i.e., the sharply peaked [OI]
6300\AA doublet and MgI] 4570\AA emission, and the broad [FeII] blend around
5200\AA, requires the existence of a high-density O-rich core expanding at low
velocities (\lsim 8,000 km s) and of Fe-rich material moving faster
than the O-rich material. Synthetic spectra at late phases from aspherical
(bipolar) explosion models are also computed with a two-dimensional spectrum
synthesis code. The above features are naturally explained by the aspherical
model if the explosion is viewed from a direction close to the axis of symmetry
(), since the aspherical model yields a high-density O-rich
region confined along the equatorial axis. By examining a large parameter space
(in energy and mass), our best model gives following physical quantities: the
kinetic energy ergs \gsim 8 - 12 and the
main-sequence mass of the progenitor star M_{\rm ms} \gsim 30 - 35 \Msun. The
temporal spectral evolution of SN 1998bw also indicates mixing among Fe-, O-,
and C-rich regions, and highly clumpy structure.Comment: 38 pages, 22 figures. ApJ, 640 (01 April 2006 issue), in pres
Stability of the r-modes in white dwarf stars
Stability of the r-modes in rapidly rotating white dwarf stars is
investigated. Improved estimates of the growth times of the
gravitational-radiation driven instability in the r-modes of the observed DQ
Her objects are found to be longer (probably considerably longer) than 6x10^9y.
This rules out the possibility that the r-modes in these objects are emitting
gravitational radiation at levels that could be detectable by LISA. More
generally it is shown that the r-mode instability can only be excited in a very
small subset of very hot (T>10^6K), rather massive (M>0.9M_sun) and very
rapidly rotating (P_min<P<1.2P_min) white dwarf stars. Further, the growth
times of this instability are so long that these conditions must persist for a
very long time (t>10^9y) to allow the amplitude to grow to a dynamically
significant level. This makes it extremely unlikely that the r-mode instability
plays a significant role in any real white dwarf stars.Comment: 5 Pages, 5 Figures, revte
Type Ia supernovae and the formation history of early-type galaxies
Using the standard prescription for the rates of supernovae type II and type
Ia, we compare the predictions of a simple model of star formation in galaxies
with the observed radial gradients of abundance ratios in a sample of
early-type galaxies to infer the relative contribution of each type of
supernova. The data suggests a correlation between the fractional contribution
of Type Ia to the chemical enrichment of the stellar populations (1-xi) and
central velocity dispersion of order 1-xi ~ -0.16log sigma_0+0.40, so that the
type Ia contribution in stars ranges from a negligible amount in massive
galaxies up to 10% in low-mass systems. Our model is parametrized by a star
formation timescale (t_SF) which controls the duration of the starburst. A
correlation with galaxy radius as a power law (t_SF ~ r^beta) translates into a
radial gradient of the abundance ratios. The data implies a wide range of
formation scenarios for a simple model that fixes the luminosity profile,
ranging from inside-out (beta=2), to outside-in formation (beta=-1), as is
consistent with numerical simulations of elliptical galaxy formation. An
alternative scenario that links t_SF to the dynamical timescale favours
inside-out formation over a smaller range. In both cases, massive galaxies are
predicted to have undergone a more extended period of star formation in the
outer regions with respect to their low-mass counterparts.Comment: Accepted for publication in MNRAS. 8 pages with 5 embedded color EPS
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Electron-capture supernovae exploding within their progenitor wind
The most massive stars on the asymptotic giant branch (AGB), so called
super-AGB stars, are thought to produce supernovae (SNe) triggered by electron
captures in their degenerate O+Ne+Mg cores. Super-AGB stars are expected to
have slow winds with high mass-loss rates, so their wind density is high. The
explosions of super-AGB stars are therefore presumed to occur in this dense
wind. We provide the first synthetic light curves (LCs) for such events by
exploding realistic electron-capture supernova (ecSN) progenitors within their
super-AGB winds. We find that the early LC, i.e. before the recombination wave
reaches the bottom of the H-rich envelope of SN ejecta (the plateau phase), is
not affected by the dense wind. However, after the plateau phase, the
luminosity remains higher when the super-AGB wind is taken into account. We
compare our results to the historical LC of SN 1054, the progenitor of the Crab
Nebula, and show that the explosion of an ecSN within an ordinary super-AGB
wind can explain the LC features. We conclude that SN 1054 could have been a
Type IIn SN without any extra extreme mass loss which was previously suggested
to be necessary to account for its early high luminosity. We also show that our
LCs match Type IIn SNe with an early plateau phase (`Type IIn-P') and suggest
that they are ecSNe within super-AGB winds. Although some ecSNe can be bright
in the optical spectral range due to the large progenitor radius, their X-ray
luminosity from the interaction does not necessarily get as bright as other
Type IIn SNe whose optical luminosities are also powered by the interaction.
Thus, we suggest that optically-bright X-ray-faint Type IIn SNe can emerge from
ecSNe. Optically-faint Type IIn SNe, such as SN 2008S, can also originate from
ecSNe if their H-rich envelope masses are small. Some of them can be observed
as `Type IIn-b' SNe due to the small H-rich envelope mass.Comment: 8 pages, 6 figures, accepted by Astronomy & Astrophysics, abstract
abridge
A Wide Symbiotic Channel to Type Ia Supernovae
As a promising channel to Type Ia supernovae (SNe Ia), we have proposed a
symbiotic binary system consisting of a white dwarf (WD) and a low mass
red-giant (RG), where strong winds from the accreting WD play a key role to
increase the WD mass to the Chandrasekhar mass limit. Here we propose two new
evolutionary processes which make the symbiotic channel to SNe Ia much wider.
(1) We first show that the WD + RG close binary can form from a wide binary
even with such a large initial separation as . Such
a binary consists of an AGB star and a low mass main-sequence (MS) star, where
the AGB star is undergoing superwind before becoming a WD. If the superwind at
the end of AGB evolution is as fast as or slower than the orbital velocity, the
wind outflowing from the system takes away the orbital angular momentum
effectively. As a result the wide binary shrinks greatly to become a close
binary. Therefore, the WD + RG binary can form from much wider binaries than
our earlier estimate. (2) When the RG fills its inner critical Roche lobe, the
WD undergoes rapid mass accretion and blows a strong optically thick wind. Our
earlier analysis has shown that the mass transfer is stabilized by this wind
only when the mass ratio of RG/WD is smaller than 1.15. Our new finding is that
the WD wind can strip mass from the RG envelope, which could be efficient
enough to stabilize the mass transfer even if the RG/WD mass ratio exceeds
1.15. With the above two new effects (1) and (2), the symbiotic channel can
account for the inferred rate of SNe Ia in our Galaxy.Comment: 29 pages including 14 firgures, to be published in ApJ, 521, No.
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