1,497 research outputs found
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
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
Carbon Ignition in Type Ia Supernovae: II. A Three-Dimensional Numerical Model
The thermonuclear runaway that culminates in the explosion of a Chandrasekhar
mass white dwarf as a Type Ia supernova begins centuries before the star
actually explodes. Here, using a 3D anelastic code, we examine numerically the
convective flow during the last minute of that runaway, a time that is crucial
in determining just where and how often the supernova ignites. We find that the
overall convective flow is dipolar, with the higher temperature fluctuations in
an outbound flow preferentially on one side of the star. Taken at face value,
this suggests an asymmetric ignition that may well persist in the geometry of
the final explosion. However, we also find that even a moderate amount of
rotation tends to fracture this dipole flow, making ignition over a broader
region more likely. Though our calculations lack the resolution to study the
flow at astrophysically relevant Rayleigh numbers, we also speculate that the
observed dipolar flow will become less organized as the viscosity becomes very
small. Motion within the dipole flow shows evidence of turbulence, suggesting
that only geometrically large fluctuations (~1 km) will persist to ignite the
runaway. We also examine the probability density function for the temperature
fluctuations, finding evidence for a Gaussian, rather than exponential
distribution, which suggests that ignition sparks may be strongly spatially
clustered.Comment: 16 pages, 9 figures, submitted to ApJ. A high resolution version of
this paper, as well as movies, can be found at
http://www.ucolick.org/~mqk/Carbo
Low Mach Number Modeling of Type Ia Supernovae. IV. White Dwarf Convection
We present the first three-dimensional, full-star simulations of convection
in a white dwarf preceding a Type Ia supernova, specifically the last few hours
before ignition. For these long-time calculations we use our low Mach number
hydrodynamics code, MAESTRO, which we have further developed to treat spherical
stars centered in a three-dimensional Cartesian geometry. The main change
required is a procedure to map the one-dimensional radial base state to and
from the Cartesian grid. Our models recover the dipole structure of the flow
seen in previous calculations, but our long-time integration shows that the
orientation of the dipole changes with time. Furthermore, we show the
development of gravity waves in the outer, stable portion of the star. Finally,
we evolve several calculations to the point of ignition and discuss the range
of ignition radii.Comment: 42 pages, some figures degraded to conserve space. Accepted to The
Astrophysical Journal (http://journals.iop.org/
On the Origin of the Type Ia Supernova Width-Luminosity Relation
Brighter Type Ia supernovae (SNe Ia) have broader, more slowly declining
B-band light curves than dimmer SNe Ia. We study the physical origin of this
width-luminosity relation (WLR) using detailed radiative transfer calculations
of Chandrasekhar mass SN Ia models. We find that the luminosity dependence of
the diffusion time (emphasized in previous studies) is in fact of secondary
relevance in understanding the model WLR. Instead, the essential physics
involves the luminosity dependence of the spectroscopic/color evolution of SNe
Ia. Following maximum-light, the SN colors are increasingly affected by the
development of numerous Fe II/Co II lines which blanket the B-band and, at the
same time, increase the emissivity at longer wavelengths. Because dimmer SNe Ia
are generally cooler, they experience an earlier onset of Fe III to Fe II
recombination in the iron-group rich layers of ejecta, resulting in a more
rapid evolution of the SN colors to the red. The faster B-band decline rate of
dimmer SNe Ia thus reflects their faster ionization evolution.Comment: 6 pages, submitted to Ap
Evolution and Explosion of Very Massive Primordial Stars
While the modern stellar IMF shows a rapid decline with increasing mass,
theoretical investigations suggest that very massive stars (>100 solar masses)
may have been abundant in the early universe. Other calculations also indicate
that, lacking metals, these same stars reach their late evolutionary stages
without appreciable mass loss. After central helium burning, they encounter the
electron-positron pair instability, collapse, and burn oxygen and silicon
explosively. If sufficient energy is released by the burning, these stars
explode as brilliant supernovae with energies up to 100 times that of an
ordinary core collapse supernova. They also eject up to 50 solar masses of
radioactive Ni56. Stars less massive than 140 solar masses or more massive than
260 solar masses should collapse into black holes instead of exploding, thus
bounding the pair-creation supernovae with regions of stellar mass that are
nucleosynthetically sterile. Pair-instability supernovae might be detectable in
the near infrared out to redshifts of 20 or more and their ashes should leave a
distinctive nucleosynthetic pattern.Comment: 7 pages, including 4 figures; in. proc. MPA/ESO/MPE/USM Joint
Astronomy Conference "Lighthouses of the Universe: The Most Luminous
Celestial Objects and their use for Cosmology
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