Self-Similar Spherical Accretion via External Radiation Drag

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  • Self Similar Spherical Accretion via Ex

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Spherically symmetric non-steady accretion via external radiation drag in the optically thin regime is investigated using self-similar transformations in the gravitational potential by a point-mass M. We assume that the external drag force is proportional to the velocity vector v as -betav, where beta is spatially constant and is a function of time. In a regime where the effect of gas pressure is not taken into account, we find self-similar solutions which connect the asymptotic solutions such that v = sqrt(2GM/r), rho chi r^(-3/2)t near to the center and v chi -GMt/r^2, rho = const. far from the center. The freefall region spreads with time as r_t chi t^(2/3). The gas is assumed to be at rest and the density is constant far from the center. It is found that: (1) the drag slows an infalling velocity far from the center, (2) the drag flattens the density distribution near to the center, and (3) the drag decreases the mass-accretion rate. Selecting the radiation drag parameter in order for the ionization degree to be omicron(1) at an epoch z =400-200, it is found that this radiation drag reduces the mass accretion rate by a factor of 2. Such spherical accretion via an external radiation drag proportional to the velocities may be realized when the systems are embedded in radiation fields. The present self-similar solution may be applicable for gas accretion into a point-mass potential, which is, for instance, produced by a massive black hole formed during an early epoch.

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