Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...459..322c&link_type=abstract
Astrophysical Journal v.459, p.322
Astronomy and Astrophysics
Astronomy
48
Accretion, Accretion Disks, Equation Of State, Stars: Neutron, Stars: Rotation
Scientific paper
Steady neutron star accretion with neutrino losses can potentially occur in a dense environment such as the interior of a supernova or inside a stellar envelope during a spiral-in phase. With spherical accretion, there is a critical accretion rate, M , below which steady flow is not possible because of radiative diffusion effects. When rotation of the gas is allowed, the centrifugal support lowers the flow pressure and keeps gas from directly reaching the neutron star surface. The value of A cn is correspondingly raised, either because of radiative diffusion or because the shocked envelope around the neutron star reaches the characteristic accretion radius. Even if a disk does form and viscous angular momentum transport occurs, the value of alpha in a steady alpha-disk model must be small (< 2 x 10-6) in order to allow a balance of neutrino cooling and viscous heating. Alternatively, the possibility of an advection-dominated disk relaxes the need for neutrino cooling except close to the neutron star; however, the pressure profile in such a disk increases the required shock radius and thus increases the value of Mcn. For SN 1987A, these considerations reinforce the arguments that either a black hole is accreting dense gas or there has been mass loss from the envelope around a central neutron star. Neutrino cooling may aid the formation of a dense disk around a newly formed neutron star. For spiral-in of a neutron star through a massive star envelope, rotation may prevent rapid accretion for even a fairly compact envelope. The radius of the shocked region becomes greater than the radius of the sonic point in the accretion flow; the sonic radius depends on the relative importance of gas and radiation pressure in the stellar envelope. However, neutron star spiral-in to a stellar core is likely to be accompanied by strong accretion, making it difficult to form stars with neutron star cores in this way.
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