2,016 research outputs found
Precursors and Main-bursts of Gamma Ray Bursts in a Hypernova Scenario
We investigate a "hypernova" model for gamma-ray bursts (GRBs), i.e., massive
C+O star model with relativistic jets. In this model, non-thermal precursors
can be produced by the "first" relativistic shell ejected from the star. Main
GRBs are produced behind the "first"-shell by the collisions of several
relativistic shells. They become visible to distant observers after the
colliding region becomes optically thin. We examine six selected conditions
using relativistic hydrodynamical simulations and simple analyses.
Interestingly, our simulations show that sub-relativistic jets
from the central engine is sufficient to produce highly-relativistic shells. We find that the relativistic shells from such a star can
reproduce observed GRBs with certain conditions. Two conditions are especially
important. One is the sufficiently long duration of the central engine \gsim
100 sec. The other is the existence of a dense-shell somewhere behind the
"first"-shell. Under these conditions, both the existence and non-existence of
precursors, and long delay between precursors and main GRBs can be explained.Comment: 8 pages, 2 figures. Accepted for publication in the Astrophysical
Journal (Letters
Multipole expansion for magnetic structures: A generation scheme for symmetry-adapted orthonormal basis set in crystallographic point group
We propose a systematic method to generate a complete orthonormal basis set
of multipole expansion for magnetic structures in arbitrary crystal structure.
The key idea is the introduction of a virtual atomic cluster of a target
crystal, on which we can clearly define the magnetic configurations
corresponding to symmetry-adapted multipole moments. The magnetic
configurations are then mapped onto the crystal so as to preserve the magnetic
point group of the multipole moments, leading to the magnetic structures
classified according to the irreducible representations of crystallographic
point group. We apply the present scheme to pyrhochlore and hexagonal ABO3
crystal structures, and demonstrate that the multipole expansion is useful to
investigate the macroscopic responses of antiferromagnets
Novae as a Mechanism for Producing Cavities around the Progenitors of SN 2002ic and Other SNe Ia
We propose that a nova shell ejected from a recurrent nova progenitor system
created the evacuated region around the explosion center of SN 2002ic. In this
picture, periodic shell ejections due to nova explosions on a white dwarf sweep
up the slow wind from the binary companion, creating density variations and
instabilities that lead to structure in the circumstellar medium (CSM). Our
model naturally explains the observed gap between the supernova explosion
center and the CSM in SN 2002ic, accounts for the density variations observed
in the CSM, and resolves the coincidence problem of the timing of the explosion
of SN 2002ic with respect to the apparent cessation of mass-loss in the
progenitor system. We also consider such nova outburst sweeping as a generic
feature of Type Ia supernovae with recurrent nova progenitors.Comment: Accepted to ApJL. 11 pages, 1 tabl
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
Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars
We explore the effects on nucleosynthesis in Type II supernovae of various
parameters (mass cut, neutron excess, explosion energy, progenitor mass) in
order to explain the observed trends of the iron-peak element abundance ratios
([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of
metallicity for the range [Fe/H] . [Cr/Fe] and [Mn/Fe]
decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and
increases. We show that such a behavior can be explained by a variation of mass
cuts in Type II supernovae as a function of progenitor mass, which provides a
changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and
incomplete Si-burning. This explanation is consistent with the amount of
ejected Ni determined from modeling the early light curves of individual
supernovae. We also suggest that the ratio [H/Fe] of halo stars is mainly
determined by the mass of interstellar hydrogen mixed with the ejecta of a
single supernova which is larger for larger explosion energy and the larger
Str\"omgren radius of the progenitor.Comment: 17 pages, LaTeX, Accepted for publication in the Astrophysical
Journal, more discussion on the Galactic chemical evolutio
The Type Ic SN 2007gr: a census of the ejecta from late-time optical-infrared spectra
Nebular spectra of Supernovae (SNe) offer an unimpeded view of the inner
region of the ejecta, where most nucleosynthesis takes place. Optical spectra
cover most, but not all of the emitting elements, and therefore offer only a
partial view of the products of the explosion. Simultaneous optical-infrared
spectra, on the other hand, contain emission lines of all important elements,
from C and O through to the Intermediate Mass Elements (IME) Mg, Si, S, Ca, and
to Fe and Ni. In particular, Si and S are best seen in the IR. The availability
of IR data makes it possible to explore in greater detail the results of the
explosion. SN\,2007gr is the first Type Ic SN for which such data are
available. Modelling the spectra with a NLTE code reveals that the inner ejecta
contain \sim 1 \Msun of material within a velocity of \,\kms.
%The spectrum is powered by \Nifs, in an amount (0.076 \Msun) consistent with
that %derived from the early-time data. The same mass of \Nifs\ derived from
the light curve peak (0.076 \Msun) was used to power the spectrum, yielding
consistent results. Oxygen is the dominant element, contributing \sim 0.8
\Msun. The C/O ratio is . IME account for \sim 0.1 \Msun. This
confirms that SN\,2007gr was the explosion of a low-mass CO core, probably the
result of a star of main-sequence mass \approx 15 \Msun. The ratios of the
\CaII\ lines, and of those of \FeII, are sensitive to the assumed degree of
clumping. In particular, the optical lines of [\FeII] become stronger, relative
to the IR lines, for higher degrees of clumping
Neutrino-driven Explosions
The question why and how core-collapse supernovae (SNe) explode is one of the
central and most long-standing riddles of stellar astrophysics. A solution is
crucial for deciphering the SN phenomenon, for predicting observable signals
such as light curves and spectra, nucleosynthesis, neutrinos, and gravitational
waves, for defining the role of SNe in the evolution of galaxies, and for
explaining the birth conditions and properties of neutron stars (NSs) and
stellar-mass black holes. Since the formation of such compact remnants releases
over hundred times more energy in neutrinos than the SN in the explosion,
neutrinos can be the decisive agents for powering the SN outburst. According to
the standard paradigm of the neutrino-driven mechanism, the energy transfer by
the intense neutrino flux to the medium behind the stagnating core-bounce
shock, assisted by violent hydrodynamic mass motions (sometimes subsumed by the
term "turbulence"), revives the outward shock motion and thus initiates the SN
blast. Because of the weak coupling of neutrinos in the region of this energy
deposition, detailed, multidimensional hydrodynamic models including neutrino
transport and a wide variety of physics are needed to assess the viability of
the mechanism. Owing to advanced numerical codes and increasing supercomputer
power, considerable progress has been achieved in our understanding of the
physical processes that have to act in concert for the success of
neutrino-driven explosions. First studies begin to reveal observational
implications and avenues to test the theoretical picture by data from
individual SNe and SN remnants but also from population-integrated observables.
While models will be further refined, a real breakthrough is expected through
the next Galactic core-collapse SN, when neutrinos and gravitational waves can
be used to probe the conditions deep inside the dying star. (abridged)Comment: Author version of chapter for 'Handbook of Supernovae,' edited by A.
Alsabti and P. Murdin, Springer. 54 pages, 13 figure
The Possible White Dwarf-Neutron Star Connection
The current status of the problem of whether neutron stars can form, in close
binary systems, by accretion-induced collapse (AIC) of white dwarfs is
examined. We find that, in principle, both initially cold C+O white dwarfs in
the high-mass tail of their mass distribution in binaries and O+Ne+Mg white
dwarfs can produce neutron stars. Which fractions of neutron stars in different
types of binaries (or descendants from binaries) might originate from this
process remains uncertain.Comment: 6 pages. To appear in "White Dwarfs", ed. J. Isern, M. Hernanz, and
E. Garcia-Berro (Dordrecht: Kluwer
Disc instability in RS Ophiuchi: a path to Type Ia supernovae?
We study the stability of disc accretion in the recurrent nova RS Ophiuchi.
We construct a one-dimensional time-dependent model of the binary-disc system,
which includes viscous heating and radiative cooling and a self-consistent
treatment of the binary potential. We find that the extended accretion disc in
this system is always unstable to the thermal-viscous instability, and
undergoes repeated disc outbursts on ~10-20yr time-scales. This is similar to
the recurrence time-scale of observed outbursts in the RS Oph system, but we
show that the disc's accretion luminosity during outburst is insufficient to
explain the observed outbursts. We explore a range of models, and find that in
most cases the accretion rate during outbursts reaches or exceeds the critical
accretion rate for stable nuclear burning on the white dwarf surface.
Consequently we suggest that a surface nuclear burning triggered by disc
instability may be responsible for the observed outbursts. This allows the
white dwarf mass to grow over time, and we suggest that disc instability in RS
Oph and similar systems may represent a path to Type Ia supernovae.Comment: 8 pages, 5 figures. Accepted for publication in MNRA
Carbon-poor stellar cores as supernova progenitors
Exploring stellar models which ignite carbon off-center (in the mass range of
about 1.05 - 1.25 Msun, depending on the carbon mass fraction) we find that
they may present an interesting SN I progenitor scenario, since whereas in the
standard scenario runaway always takes place at the same density of about 2 X
10^9 gr/cm^3, in our case, due to the small amount of carbon ignited, we get a
whole range of densities from 1 X 10^9 up to 6 X 10^9 gr/cm^3. These results
could contribute in resolving the emerging recognition that at least some
diversity among SNe I exists, since runaway at various central densities is
expected to yield various outcomes in terms of the velocities and composition
of the ejecta, which should be modeled and compared to observations.Comment: 49 pages, 20 figure
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