623 research outputs found
Trans-Relativistic Supernovae, Circumstellar Gamma-Ray Bursts, and Supernova 1998bw
Supernova (SN) 1998bw and gamma-ray burst (GRB) 980425 offer the first direct
evidence that supernovae are the progenitors of some GRBs. However, this burst
was unusually dim, smooth and soft compared to other bursts with known
afterglows. Whether it should be considered a prototype for cosmological GRBs
depends largely on whether the supernova explosion and burst were asymmetrical
or can be modeled as spherical. We address this question by treating the
acceleration of the supernova shock in the outermost layers of the stellar
envelope, the transition to relativistic flow, and the subsequent expansion
(and further acceleration) of the ejecta into the surrounding medium. We find
that GRB 980425 could plausibly have been produced by a collision between the
relativistic ejecta from SN 1998bw and the star's pre-supernova wind; the model
requires no significant asymmetry. This event therefore belongs to a dim
subclass of GRBs and is not a prototype for jet-like cosmological GRBs.Comment: 5 pages, 2 figures, to appear in Gamma 2001, eds. S. Ritz, N.
Gehrels, and C. Shrade
Bipolar molecular outflows driven by hydromagnetic protostellar winds
We demonstrate that magnetically-collimated protostellar winds will sweep
ambient material into thin, radiative, momentum-conserving shells whose
features reproduce those commonly observed in bipolar molecular outflows. We
find the typical position-velocity and mass-velocity relations to occur in
outflows in a wide variety of ambient density distributions, regardless of the
time histories of their driving winds.Comment: 4 pages, 1 figure, submitted to ApJ
The Trans-Relativistic Blast Wave Model for SN 1998bw and GRB 980425
The spatiotemporal coincidence of supernova (SN) 1998bw and gamma-ray burst
(GRB) 980425 and this supernova's unusual optical and radio properties have
prompted many theoretical models that produce GRBs from supernovae. We review
the salient features of our simple, spherical model in which an energetic
supernova explosion shock accelerates a small fraction of the progenitor's
stellar envelope to mildly relativistic velocities. This material carries
sufficient energy to produce a weak GRB and a bright radio supernova through an
external shock against a dense stellar wind.Comment: 3 pages, To appear in Proceedings of the 20th Texas Symposium on
Relativistic Astrophysics, eds. J. C. Wheeler & H. Martel, AI
Collisions of strings with Y junctions
We study the dynamics of Nambu--Goto strings with junctions at which three
strings meet. In particular, we exhibit one simple exact solution and examine
the process of intercommuting of two straight strings, in which they exchange
partners but become joined by a third string. We show that there are important
kinematical constraints on this process. The exchange cannot occur if the
strings meet with very large relative velocity. This may have important
implications for the evolution of cosmic superstring networks and non-abelian
string networks.Comment: 4 pages, 1 figure, uses revtex 4. Clarifying comments added to
correct a conceptual error, reference updated. Version accepted by Phys Rev
Letters, with additional references and minor change
Global Models for the Evolution of Embedded, Accreting Protostellar Disks
Most analytic work to date on protostellar disks has focused on those in
isolation from their environments. However, observations are now beginning to
probe the earliest, most embedded phases of star formation, during which disks
are rapidly accreting from their parent cores and cannot be modeled in
isolation. We present a simple, one-zone model of protostellar accretion disks
with high mass infall rates. Our model combines a self-consistent calculation
of disk temperatures with an approximate treatment of angular momentum
transport via two mechanisms. We use this model to survey the properties of
protostellar disks across a wide range of stellar masses and evolutionary
times, and make predictions for disks' masses, sizes, spiral structure, and
fragmentation that will be directly testable by future large-scale surveys of
deeply embedded disks. We define a dimensionless accretion-rotation parameter
which, in conjunction with the disk's temperature, controls the disk evolution.
We track the dominant mode of angular momentum transport, and demonstrate that
for stars with final masses greater than roughly one solar mass, gravitational
instabilities are the most important mechanism as most of the mass accumulates.
We predict that binary formation through disk fission, fragmentation of the
disk into small objects, and spiral arm strength all increase in importance to
higher stellar masses.Comment: 17 pages, 9 figures, accepted for publication in ApJ. Model updated
to better reflect simulations in the literature; discussion of key
assumptions and strategy clarifie
R-modes in Neutron Stars with Crusts: Turbulent Saturation, Spin-down, and Crust Melting
Rossby waves (r-modes) have been suggested as a means to regulate the spin
periods of young or accreting neutron stars, and also to produce observable
gravitational wave radiation. R-modes involve primarily transverse,
incompressive motions of the star's fluid core. However, neutron stars gain
crusts early in their lives: therefore, r-modes also imply shear in the fluid
beneath the crust. We examine the criterion for this shear layer to become
turbulent, and derive the rate of dissipation in the turbulent regime. Unlike
dissipation from a viscous boundary layer, turbulent energy loss is nonlinear
in mode energy and can therefore cause the mode to saturate at amplitudes
typically much less than unity. This energy loss also reappears as heat below
the crust. We study the possibility of crust melting as well as its
implications for the spin evolution of low-mass X-ray binaries. Lastly, we
identify some universal features of the spin evolution that may have
observational consequences.Comment: 12 pages, 4 figures, submitted to Ap
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