Other
Scientific paper
Oct 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt........11b&link_type=abstract
Thesis (PhD). MASSACHUSETTS INSTITUTE OF TECHNOLOGY, Source DAI-B 65/04, p. 1907, Oct 2004, pages.
Other
5
Scientific paper
The massive stellar progenitor of a hypernova explosion and an associated gamma-ray burst must satisfy two primary constraints: (1)the outer layers of the stellar core must possess sufficient angular momentum to form a centrifugally supported torus about the collapsed central object (a Kerr black hole); and, (2)the envelope of the star must not be excessively massive or distended, so that the energetic, ultrarelativistic outflow generated by the central engine in the core of the star does not risk being smothered before it can escape from the star and expand outward to produce a gamma-ray burst. We have developed a new one-dimensional stellar evolution code that includes the effects of rotation on equilibrium stellar structure, and calculates the transport of angular momentum through the stellar interior due to convection, dynamical and secular shear instabilities, and gravity (buoyancy) waves. We have used this code to calculate a variety of evolutionary sequences involving the transfer of mass from one component of the binary system to the other. We have also calculated an evolutionary sequence ending in the merger of one component of the system with the core of the other, induced by a prior common-envelope phase. We find that over a wide range of initial binary system parameters, the initially less massive component of the system can accrete a substantial amount of mass and angular momentum from the initially more massive component. The accreted angular momentum is efficiently transported inward from the surface of the accreting star toward its core by a combination of convection and dynamical and secular shear instabilities. If accretion commences while the accretor is still on the main sequence, we find that the inward-progressing wave of angular momentum can penetrate the core of the mass- gaining star, contributing to its store of rotational angular momentum without the need for gravity wave transport of angular momentum across the core-envelope interface. These stars end their evolution (just prior to core carbon ignition) as red supergiants, with cores endowed with sufficient angular momentum to give rise to a hypernova explosion. We also find that a subsequent common-envelope phase with the compact remnant of the primary might result in the ejection of the accretor's red-giant envelope, leaving either a bare helium or carbon-oxygen star. Such a star would be expected to explode in a Type Ib or Ic supernova/hypernova. Accretors which begin to receive mass later, while they are undergoing early post main sequence evolution, also develop rapidly rotating envelopes, but require the action of gravity wave angular momentum transport in order to share the angular momentum acquired at their surfaces with the bulk of their more compact cores. As a result, these stars reach core collapse with more slowly rotating cores and more massive and tightly bound envelopes. We find that cores which redistribute their angular momenta after carbon ignition to come into solid body rotation (rotational angular velocity Ω = constant) just prior to core collapse have the most favorable rotational profiles for generating hypernova events that emit observable gamma-ray bursts. We also find that our stellar merger model ends its evolution with a large reservoir of angular momentum in the layers exterior to the carbon oxygen core. With redistribution to Ω = constant, we expect this core as well to be a viable gamma-ray burst progenitor. (Copies available exclusively from MIT Libraries, Rm. 14- 0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.) (Abstract shortened by UMI.)
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