Neutron star accretion in a stellar envelope

Abstract

Steady spherically symmetric accretion onto a neutron star is possible for a mass accretion rate M of 0.001 solar mass/yr or greater; the gravitational energy is lost to neutrinos. In the scenario of neutron star spiral-in through a massive stellar envelope, the neutron star accretion rate derived from the Bondi-Hoyle theory is larger than about 0.2 solar mass/yr. Radiation pressure cannot stop the inflow because the radiation is trapped in the flow and the flow is effectively adiabatic. Angular momentum of the accreting material might impede the flow to the neutron star, but its effects are important on a scale much smaller than the accretion radius and there are plausible mechanisms for transporting out the angular momentum. If the accretion to the neutron star does occur, the spiraling process is accompanied by a substantial increase in the neutron star mass and black hole formation is expected. There are implications for the formation of close neutron star binaries, the formation of stars with neutron star cores, encounters of neutron stars with globular cluster stars, and the formation of black hole binaries.