Iron Opacity and the Pulsar of SN 1987A

Abstract

Neutron stars formed in Type II supernovae are likely to be initially obscured by late-time fallback. Although much of the late-time fallback is quickly accreted via neutrino cooling, some material remains on the neutron star, forming an atmosphere that slowly accretes through photon emission. In this paper, we derive structure equations of the fallback atmosphere and present results of one-dimensional simulations of that fallback. The atmosphere remaining after neutrino cooling (L_ν) becomes unimportant (L_νL_{Edd, e⁻}, the Compton Eddington limit) is only a fraction of the total mass accreted (10^-8 M_acc=10⁻⁹ M_☉). Recombined iron dominates the opacity in the outer regions, leading to an opacity 10³-10⁴ times higher than that of electron scattering alone. The resultant photon emission of the remnant atmosphere is limited to 10⁻³L_{Edd, e⁻}. The late-time evolution of this system leads to the formation of a photon-driven wind from the accretion of the inner portion of the atmosphere, leaving, for most cases, a bare neutron star on timescales shorter than 1 yr. The degenerate remnant of 1987A may not be a black hole. Instead, the fallback material may have already accreted or blown off in the accretion-driven wind. If the neutron star has either a low magnetic field or a low rotational spin frequency, we would not expect to see the neutron star remnant of 1987A.