2,510 research outputs found
Thermonuclear Bursts with Short Recurrence Times from Neutron Stars Explained by Opacity-Driven Convection
Thermonuclear flashes of hydrogen and helium accreted onto neutron stars
produce the frequently observed Type I X-ray bursts. It is the current paradigm
that almost all material burns in a burst, after which it takes hours to
accumulate fresh fuel for the next burst. In rare cases, however, bursts are
observed with recurrence times as short as minutes. We present the first
one-dimensional multi-zone simulations that reproduce this phenomenon. Bursts
that ignite in a relatively hot neutron star envelope leave a substantial
fraction of the fuel unburned at shallow depths. In the wake of the burst,
convective mixing events driven by opacity bring this fuel down to the ignition
depth on the observed timescale of minutes. There, unburned hydrogen mixes with
the metal-rich ashes, igniting to produce a subsequent burst. We find burst
pairs and triplets, similar to the observed instances. Our simulations
reproduce the observed fraction of bursts with short waiting times of ~30%, and
demonstrate that short recurrence time bursts are typically less bright and of
shorter duration.Comment: 11 pages, 15 figures, accepted for publication in Ap
On the Progenitors of Collapsars
We study the evolution of stars that may be the progenitors of common
(long-soft) GRBs. Bare rotating helium stars, presumed to have lost their
envelopes due to winds or companions, are followed from central helium ignition
to iron core collapse. Including realistic estimates of angular momentum
transport (Heger, Langer, & Woosley 2000) by non-magnetic processes and mass
loss, one is still able to create a collapsed object at the end with sufficient
angular momentum to form a centrifugally supported disk, i.e., to drive a
collapsar engine. However, inclusion of current estimates of magnetic torques
(Spruit 2002) results in too little angular momentum for collapsars.Comment: 3 pages, 5 figures, in Proc. Woods Hole GRB meeting, ed. Roland
Vanderspe
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
New Two-Dimensional Models of Supernova Explosions by the Neutrino-Heating Mechanism: Evidence for Different Instability Regimes in Collapsing Stellar Cores
The neutrino-driven explosion mechanism for core-collapse supernovae in its
modern flavor relies on the additional support of hydrodynamical instabilities
in achieving shock revival. Two possible candidates, convection and the
so-called standing accretion shock instability (SASI), have been proposed for
this role. In this paper, we discuss new successful simulations of supernova
explosions that shed light on the relative importance of these two
instabilities. While convection has so far been observed to grow first in
self-consistent hydrodynamical models with multi-group neutrino transport, we
here present the first such simulation in which the SASI grows faster while the
development of convection is initially inhibited. We illustrate the features of
this SASI-dominated regime using an explosion model of a 27 solar mass
progenitor, which is contrasted with a convectively-dominated model of an 8.1
solar mass progenitor with subsolar metallicity, whose early post-bounce
behavior is more in line with previous 11.2 and 15 solar mass explosion models.
We analyze the conditions discriminating between the two different regimes,
showing that a high mass-accretion rate and a short advection time-scale are
conducive for strong SASI activity. We also briefly discuss some important
factors for capturing the SASI-driven regime, such as general relativity, the
progenitor structure, a nuclear equation of state leading to a compact
proto-neutron star, and the neutrino treatment. Finally, we evaluate possible
implications of our findings for 2D and 3D supernova simulations. Our results
show that a better understanding of the SASI and convection in the non-linear
regime is required.Comment: 12 pages, 13 figures; revised version accepted for publication in Ap
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
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