260,352 research outputs found
The Central Engines of Gamma-Ray Bursts
Leading models for the "central engine" of long, soft gamma-ray bursts (GRBs)
are briefly reviewed with emphasis on the collapsar model. Growing evidence
supports the hypothesis that GRBs are a supernova-like phenomenon occurring in
star forming regions, differing from ordinary supernovae in that a large
fraction of their energy is concentrated in highly relativistic jets. The
possible progenitors and physics of such explosions are discussed and the
important role of the interaction of the emerging relativistic jet with the
collapsing star is emphasized. This interaction may be responsible for most of
the time structure seen in long, soft GRBs. What we have called "GRBs" may
actually be a diverse set of phenomena with a key parameter being the angle at
which the burst is observed. GRB 980425/SN 1988bw and the recently discovered
hard x-ray flashes may be examples of this diversity.Comment: 8 pages, Proc. Woods Hole GRB meeting, Nov 5 - 9 WoodsHole
Massachusetts, Ed. Roland Vanderspe
Fallback and Black Hole Production in Massive Stars
The compact remnants of core collapse supernovae - neutron stars and black
holes - have properties that reflect both the structure of their stellar
progenitors and the physics of the explosion. In particular, the masses of
these remnants are sensitive to the density structure of the presupernova star
and to the explosion energy. To a considerable extent, the final mass is
determined by the ``fallback'', during the explosion, of matter that initially
moves outwards, yet ultimately fails to escape. We consider here the simulated
explosion of a large number of massive stars (10 to 100 \Msun) of Population I
(solar metallicity) and III (zero metallicity), and find systematic differences
in the remnant mass distributions. As pointed out by Chevalier(1989),
supernovae in more compact progenitor stars have stronger reverse shocks and
experience more fallback. For Population III stars above about 25 \Msun and
explosion energies less than erg, black holes are a common
outcome, with masses that increase monotonically with increasing main sequence
mass up to a maximum hole mass of about 35 \Msun. If such stars produce primary
nitrogen, however, their black holes are systematically smaller. For modern
supernovae with nearly solar metallicity, black hole production is much less
frequent and the typical masses, which depend sensitively on explosion energy,
are smaller. We explore the neutron star initial mass function for both
populations and, for reasonable assumptions about the initial mass cut of the
explosion, find good agreement with the average of observed masses of neutron
stars in binaries. We also find evidence for a bimodal distribution of neutron
star masses with a spike around 1.2 \Msun (gravitational mass) and a broader
distribution peaked around 1.4 \Msun.Comment: Accepted for publication in Ap
Very Low Energy Supernovae: Light Curves and Spectra of Shock Breakout
The brief transient emitted as a shock wave erupts through the surface of a
presupernova star carries information about the stellar radius and explosion
energy. Here the CASTRO code, which treats radiation transport using multigroup
flux-limited diffusion, is used to simulate the light curves and spectra of
shock breakout in very low-energy supernovae (VLE SNe), explosions in giant
stars with final kinetic energy much less than 10 erg. VLE SNe light
curves, computed here with the KEPLER code, are distinctively faint, red, and
long-lived, making them challenging to find with transient surveys. The
accompanying shock breakouts are brighter, though briefer, and potentially
easier to detect. Previous analytic work provides general guidance, but
numerical simulations are challenging due to the range of conditions and lack
of equilibration between color and effective temperatures. We consider previous
analytic work and extend discussions of color temperature and opacity to the
lower energy range explored by these events. Since this is the first
application of the CASTRO code to shock breakout, test simulations of normal
energy shock breakout of SN1987A are carried out and compared with the
literature. A set of breakout light curves and spectra are then calculated for
VLE SNe with final kinetic energies in the range ergs for
red supergiants with main sequence masses 15 Msun and 25 Msun. The importance
of uncertainties in stellar atmosphere model, opacity, and ambient medium is
discussed, as are observational prospects with current and forthcoming
missions.Comment: 19 pages; submitted to Astrophysical Journa
Delocalization of Wannier-Stark ladders by phonons: tunneling and stretched polarons
We study the coherent dynamics of a Holstein polaron in strong electric
fields. A detailed analytical and numerical analysis shows that even for small
hopping constant and weak electron-phonon interaction, polaron states can
become delocalized if a resonance condition develops between the original
Wannier-Stark states and the phonon modes, yielding both tunneling and
`stretched' polarons. The unusual stretched polarons are characterized by a
phonon cloud that {\em trails} the electron, instead of accompanying it. In
general, our novel approach allows us to show that the polaron spectrum has a
complex nearly-fractal structure, due to the coherent coupling between states
in the Cayley tree which describes the relevant Hilbert space. The eigenstates
of a finite ladder are analyzed in terms of the observable tunneling and
optical properties of the system.Comment: 7 pages, 4 figure
From antiferromagnetism to d-wave superconductivity in the 2D t-J model
We have found that the two dimensional t-J model, for the physical parameter
range J/t = 0.4 reproduces the main experimental qualitative features of
High-Tc copper oxide superconductors: d-wave superconducting correlations are
strongly enhanced upon small doping and clear evidence of off diagonal long
range order is found at the optimal doping \delta ~ 0.15. On the other hand
antiferromagnetic long range order, clearly present at zero hole doping, is
suppressed at small hole density with clear absence of antiferromagnetism at
\delta >~ 0.1.Comment: 4 pages, 5 figure
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