Supercritical Accretion Disks with Superwinds

Abstract

We construct a model for supercritical accretion disks with winds, where the mass-accretion rate highly exceeds the Eddington rate and a significant fraction of the accreting gas is expelled as a superwind, under a self-similar treatment. The mass-accretion rate $ \dot{M}$ decreases with radius $ r$ as $ \dot{M} \propto r^{s+1/2}$, where $ s$ ($ \geq-1/2$) is some arbitrary constant. The surface density $ \Sigma$ and the disk central temperature $ T_\mathrm{c}$ vary as $ \Sigma \propto r^s$ and $ T_\mathrm{c} \propto r^{s/4-1/2}$, respectively. In spite of such a modification, we found that the scaleheight $ H$ and the surface temperature $ T$ of such a superdisk with a superwind is very similar to those of a superdisk without winds; $ H \propto r$ and $ T \propto r^{-1/2}$. Furthermore, the optical depth of the wind becomes remarkably smaller than that in the spherical case, because mass loss takes place at various radii on the disk, and the gas distribution in the wind is not concentrated, but diluted in space. Hence, the observational appearances of both superdisks are quite similar, except for the existence of a strong wind, unless the wind mass-loss rate highly exceeds the critical rate. We then apply a superdisk with a superwind to supersoft X-ray sources. In our picture, although the accretion rate in these objects is of the order of $10^{-5} \,{{{M}_{\odot}}} \,\mathrm{yr}^{-1}$ at the outer edge of the disk and a sufficiently thick superdisk may be established, most of the gas is expelled by the superwind, and the accretion rate at the center becomes of the order of $10^{-7} \hbox{--} 10^{-6} \,{{{M}_{\odot}}} \mathrm{yr}^{-1}$.