Steeper stellar cusps in galactic centers from binary disruption

Abstract

The relaxed distribution of stars around a massive black hole is known to follow a cusp profile $\rho(r)\propto r^{-\alpha}$ with characteristic slope $\alpha=7/4$. This follows from energy conservation and a scattering rate as given by two body encounters. However, we show that injection of stars close to the black hole, i.e. a source term in the standard cusp picture, modifies this profile. In the steady-state configuration, the cusp develops a central region with typical slope $\alpha=9/4$ in which stars diffuse outward. Binary disruption by the intense tidal field of the massive black hole is among the phenomena that take place in the Galactic Center. In such disruption, one of the binary members remains bound to the black hole, thus providing a source term of stars close to the black hole. Assuming a binary fraction of $0.1$ and an orbital circularization efficiency of $0.35$, we show that this source is strong enough to modify the cusp profile within $\approx 0.07$ pc in the Galactic Center. If the binary fraction at the influence radius is of order unity and the orbits of all the captured stars are efficiently circularized, the steeper cusp extends almost as far as the radius of influence of the black hole.