Accretion Disk Evolution with Wind Infall. I. General Solution and Application to Sagittarius A*

The evolution of an accretion disk can be influenced significantly by thedeposition of mass and angular momentum by an infalling Bondi-Hoyle wind. Sucha mass influx impacts the long-term behavior of the disk by providingadditional sources of viscosity and heating. In this paper, we derive and solvethe disk equations when these effects are taken into account. We present asurvey of models with various wind configurations and demonstrate that the diskspectrum may then differ substantially from that of a standard alpha-disk. Inparticular, it is likely that a wind-fed disk has a significant infrared bumpdue to the deposition of energy in its outer region. We apply some of theresults of our calculations to the Galactic Center black hole candidate Sgr A*and show that if a fossil disk is present in this source, it must have a verylow viscosity parameter (alpha<10^-4) and the Bondi-Hoyle wind must beaccreting with a very high specific angular momentum to prevent it fromcircularizing in the inner disk region where its impact would be mostnoticeable.