STEADY SPHERICAL HYPERCRITICAL ACCRETION ONTO NEUTRON-STARS

We have examined the structure of hypercritical accretion flows onto neutron stars, considering steady state, spherically symmetric flows with accretion rates in the range 10(-4) less-than-or-similar-to M less-than-or-similar-to 10(4) M. yr-1. In this regime, the gravitational accretion energy is carried away by neutrinos. Our models include pair production, radiation diffusion, and general relativistic effects. Pairs are the dominant pressure component near the neutron star, enhancing the pressure and density in that region by a factor of approximately 11/4. At accretion rates above approximately 10(2) M. yr-1, pair pressure dominates throughout the accretion envelope, increasing the shock radius by as much as a factor of 2. Relativistic effects increase the pressure and density gradients in the envelope and decrease the envelope radii by as much as a factor of 2 compared to the Newtonian envelopes. Radiation diffusion becomes important when the accretion rate falls below approximately 10(-3) M. yr-1 because the shock has moved out far enough to approach the outer edge of the photon trapping region. We find that the luminosity exceeds 50% of the Eddington luminosity once M less-than-or-similar-to 4 x 10(-4) M. yr-1. The characteristic radiation temperature is relatively low, lying in the range 4 x 10(3) K to 5 x 10(4) K, because the effective photosphere is orders of magnitude larger than the neutron star radius. At the lower accretion rates, free fall toward the neutron star surface ceases and an extended, quasistatic radiation pressure supported envelope is present which is likely to be dynamically unstable. Applying these results to the case of SN 1987A, we find that the luminosity at infinity from accretion onto a central neutron star should reach 50% of the Eddington luminosity within approximately 6 months of the explosion, approaching the Eddington luminosity within a year. We discuss possible alternatives which may explain the much lower luminosity actually observed.