Accretion Disks Driven by External Radiation Drag around Central Luminous Sources

Accretion disks/disk accretions (beta-disks) driven by the external radiation drag exerted by a central luminous source are presented under the steady and subrelativistic approximations. In a cold regime, where the gravity of the central object, the radiation force, and the radiation drag are included, but the pressure-gradient force is neglected, we find steady solutions such that the infalling velocity V_r is inversely proportional to radius r far from the center and becomes constant near to the center, while the rotation velocity v_phi is Keplerian far from the center and drops exponentially near to the center. In a warm regime, where the effect of the gas pressure is also taken into account, we find steady transonic solutions such that a flow accreting subsonically and rotating with the Keplerian velocity far from the center becomes, after passing a sonic point, an almost radially accreting supersonic flow with no angular momentum. Due to the effect of external radiation drag, the angular momentum of the gas is removed. In particular, it is quickly lost inside the characteristic radius r_0, which is expressed as r_0 = Gamma^2/(1-Gamma)r_g,where Gamma is the central luminosity normalized by the Eddington luminosity and r_g is the Schwarzschild radius of the central object. As a result, the nearly Keplerhin rotating disk outside r_0 turns to a nearly radial flow inside r_0. Furthermore, in the vicinity of the central object the infall velocity attains a terminal value, at which the effective gravity is balanced by radiation drag. The terminal speed v_infty is found to be v_infty = -(1-Gamma)/(2Gamma)c. Such accretion disks, where the angular momentum is removed via the external drag of radiation fields from the central source, are possible in several astrophysical contexts. For example, in the case of an X-ray burster the radiation density at the burst phases is very high in the inner region of the accretion disk, and therefore, the gasaccretion processes are remarkably enhanced due to the external radiation drag. Also, in a protoplanetary disk the external radiation drag may play an important role if a sufficient amount of dust exists in the disk