Accretion of helium and metal-rich gas onto neutron stars and black holes at high luminosities

Abstract

Ultraluminous X-ray sources fed by Wolf-Rayet star winds and X-ray bursters in ultracompact binaries with He or C white dwarfs have accretion disks, whose properties may significantly differ from those of pure H alpha-accretion disks. We have therefore included the dependence on charge number Z and mean molecular weights mu_{e/I} into the Shakura and Sunyaev (1973) scaling relations for the key parameters of the disk. Furthermore, we also consider the case of the pseudo-Newtonian potential of Paczynsky and Wiita (1980). These scaling relations might become useful, e.g., when estimating the illumination efficiency of the external parts of the disk. We also address the changes in the structure of the boundary (spreading) layer on the surface of neutron stars, occurring in the case of H depleted accretion disks.