Hoyle–Lyttleton Accretion onto Superdisks

Abstract

We examine Hoyle–Lyttleton accretion onto superdisks, where the mass-accretion rate greatly exceeds the critical one and the luminosity is of the order of the Eddington one. Hoyle–Lyttleton accretion onto a luminous accretion disk is drastically changed from the spherical case, due to the non-spherical nature of disk radiation fields. In edge-on accretion, for example, mass accretion does not cease at $ \Gamma=1$, but continues in the regime of $ \Gamma \geq 1$, where $ \Gamma$ is the disk luminosity normalized by the Eddington one. We found that Hoyle-Lyttleton accretion onto superdisks is not a simple extension of that onto standard accretion disks, where the mass-accretion rate gradually decreases with increasing $ \Gamma$. Instead, in the case of a superdisk, the mass-accretion rate increases with increasing $ \Gamma$. This is due to a geometrical shadowing effect. That is, in contrast to the infinitesimally-thin standard disk, the superdisk has finite thickness. As $ \Gamma$ increases, the relative thickness increases and the shadowing region in the equatorial direction increases. We also briefly discuss the canonical luminosity in the steady state and the system evolution.