Studies of stars with neutron cores and of x-ray binaries displaying quasi-periodic oscillations

Details Studies of stars with neutron cores and of x-ray binaries displaying quasi-periodic oscillations Studies of stars with neutron cores and of x-ray binaries displaying quasi-periodic oscillations

Jan 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993phdt........10b&link_type=abstract

Ph.D. Thesis California Inst. of Tech., Pasadena, CA.

Other

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Red Giant Stars, Stellar Luminosity, Stellar Oscillations, Stellar Structure, Supergiant Stars, X Ray Binaries, A Stars, Accretion Disks, Line Spectra, Neutron Stars, Photosphere, Stellar Mass Accretion, Stellar Models

Scientific paper

This thesis contains the results of two investigations: one into the nature of stars with degenerate neutron cores and the other into the interpretation of the phenomenology of luminous low-mass X-ray binaries (LMXB's) displaying slow quasi-periodic oscillations (QPO's) in their X-ray flux. A star with a degenerate neutron core would be a red giant or supergiant. In this thesis we investigate the structure of such a supergiant, particularly examining the energy production and seeking an identifying observational signature. The star's luminosity comes from the rp-process in a burning region within 100 km of the base of the envelope. The envelope convects the products of rp-burning from abundances burning region throughout the envelope, including the photosphere. After about 105 years, the abundances of Br, Rb, Y, Nb, and Mo at the surface of the star will be more than about 200 times greater than their solar abundances. A suitable observational signature would be the strong enhancement of absorption lines for these elements in the star's spectrum. As many as 10 of the nearest 100 red supergiants (those within 5 kpc) could have neutron cores. The other investigation concerns a model of rapid accretion onto an unmagnetized neutron star with radius somewhat less than 6GM/c2. This model is applied to the phenomenology of a class of LMXB's displaying slow (approximately 6 Hz) QPO's in X-ray flux. These sources are highly luminous and display what appears to be three modes ('branches') of accretion. In this model, at low accretion rates, the neutron star lies within the inner edge of the accretion disk, and matter is dripped onto the neutron star from this edge. As the accretion rate increases, the transition from the 'horizontal branch' to the 'normal branch' occurs when the disk thickens and its inner edge touches the star and forms a boundary layer, changing the structure of the inner disk and the spectral character of the X-rays. This thesis presents an exploration of this model, establishing the plausibility that a neutron star could lie inside a rapidly accreting disk and that a change of mass accretion rate could push the inner radius onto the surface of the star.

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