2,499 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
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
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