1,249 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
First Stars III Conference Summary
The understanding of the formation, life, and death of Population III stars,
as well as the impact that these objects had on later generations of structure
formation, is one of the foremost issues in modern cosmological research and
has been an active area of research during the past several years. We summarize
the results presented at "First Stars III," a conference sponsored by Los
Alamos National Laboratory, the Kavli Institute for Particle Astrophysics and
Cosmology, and the Joint Institute for Nuclear Astrophysics. This conference,
the third in a series, took place in July 2007 at the La Fonda Hotel in Santa
Fe, New Mexico, U.S.A.Comment: 11 pages, no figures; Conference summary for First Stars III, which
was held in Santa Fe, NM on July 15-20, 2007. To appear in "Proceedings of
First Stars III," Eds. Brian W. O'Shea, Alexander Heger & Tom Abe
Evidence from stable isotopes and Be-10 for solar system formation triggered by a low-mass supernova
About 4.6 billion years ago, some event disturbed a cloud of gas and dust,
triggering the gravitational collapse that led to the formation of the solar
system. A core-collapse supernova, whose shock wave is capable of compressing
such a cloud, is an obvious candidate for the initiating event. This hypothesis
can be tested because supernovae also produce telltale patterns of short-lived
radionuclides, which would be preserved today as isotopic anomalies. Previous
studies of the forensic evidence have been inconclusive, finding a pattern of
isotopes differing from that produced in conventional supernova models. Here we
argue that these difficulties either do not arise or are mitigated if the
initiating supernova was a special type, low in mass and explosion energy. Key
to our conclusion is the demonstration that short-lived Be-10 can be readily
synthesized in such supernovae by neutrino interactions, while anomalies in
stable isotopes are suppressed.Comment: 32 pages, 3 figures, to appear in Nature Communication
Conservative Initial Mapping For Multidimensional Simulations of Stellar Explosions
Mapping one-dimensional stellar profiles onto multidimensional grids as
initial conditions for hydrodynamics calculations can lead to numerical
artifacts, one of the most severe of which is the violation of conservation
laws for physical quantities such as energy and mass. Here we introduce a
numerical scheme for mapping one-dimensional spherically-symmetric data onto
multidimensional meshes so that these physical quantities are conserved. We
validate our scheme by porting a realistic 1D Lagrangian stellar profile to the
new multidimensional Eulerian hydro code CASTRO. Our results show that all
important features in the profiles are reproduced on the new grid and that
conservation laws are enforced at all resolutions after mapping.Comment: 7 pages, 5 figures, Proceeding for Conference on Computational
Physics (CCP 2011
Pulsational Analysis of the Cores of Massive Stars and its Relevance to Pulsar Kicks
The mechanism responsible for the natal kicks of neutron stars continues to
be a challenging problem. Indeed, many mechanisms have been suggested, and one
hydrodynamic mechanism may require large initial asymmetries in the cores of
supernova progenitor stars. Goldreich, Lai, & Sahrling (1997) suggested that
unstable g-modes trapped in the iron (Fe) core by the convective burning layers
and excited by the -mechanism may provide the requisite asymmetries.
We perform a modal analysis of the last minutes before collapse of published
core structures and derive eigenfrequencies and eigenfunctions, including the
nonadiabatic effects of growth by nuclear burning and decay by both neutrino
and acoustic losses. In general, we find two types of g-modes: inner-core
g-modes, which are stabilized by neutrino losses and outer-core g-modes which
are trapped near the burning shells and can be unstable. Without exception, we
find at least one unstable g-mode for each progenitor in the entire mass range
we consider, 11 M_{\sun} to 40 M_{\sun}. More importantly, we find that the
timescales for growth and decay are an order of magnitude or more longer than
the time until the commencement of core collapse. We conclude that the
-mechanism may not have enough time to significantly amplify core
g-modes prior to collapse.Comment: 32 pages including 12 color figures and 2 tables, submitted to Ap
Nucleosynthesis in Massive Stars With Improved Nuclear and Stellar Physics
We present the first calculations to follow the evolution of all stable
nuclei and their radioactive progenitors in stellar models computed from the
onset of central hydrogen burning through explosion as Type II supernovae.
Calculations are performed for Pop I stars of 15, 19, 20, 21, and 25 M_sun
using the most recently available experimental and theoretical nuclear data,
revised opacity tables, neutrino losses, and weak interaction rates, and taking
into account mass loss due to stellar winds. A novel ``adaptive'' reaction
network is employed with a variable number of nuclei (adjusted each time step)
ranging from about 700 on the main sequence to more than 2200 during the
explosion. The network includes, at any given time, all relevant isotopes from
hydrogen through polonium (Z=84). Even the limited grid of stellar masses
studied suggests that overall good agreement can be achieved with the solar
abundances of nuclei between 16O and 90Zr. Interesting discrepancies are seen
in the 20 M_sun model and, so far, only in that model, that are a consequence
of the merging of the oxygen, neon, and carbon shells about a day prior to core
collapse. We find that, in some stars, most of the ``p-process'' nuclei can be
produced in the convective oxygen burning shell moments prior to collapse; in
others, they are made only in the explosion. Serious deficiencies still exist
in all cases for the p-process isotopes of Ru and Mo.Comment: 53 pages, 17 color figures (3 as separate GIF images), slightly
extended discussion and references, accepted by Ap
Design and Gait Control of a Rollerblading Robot
We present the design and gait generation for an experimental ROLLERBLADER1. The ROLLERBLADER is a robot with a central platform mounted on omnidirectional casters and two 3 degree-of-freedom legs. A passive rollerblading wheel is attached to the end of each leg. The wheels give rise to nonholonomic constraints acting on the robot. The legs can be picked up and placed back on the ground allowing a combination of skating and walking gaits. We present two types of gaits for the robot. In the first gait, we allow the legs to be picked up and placed back on the ground while in the second, the wheels are constrained to stay on the ground at all times. Experimental gait results for a prototype robot are also presented
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