The majority of the supemassive black holes in galactic centers,
including that in our own Galaxy, are dormant. Our knowledge of the
properties of quiescent accretion and the environments of quiescent
galactic centers is very limited. In this talk, I will discuss 2 examples
in which stars can be used as unique probes of quiescent galactic centers.
The tidal disruption of a star by a supermassive black hole provides
us with a rare glimpse of these otherwise dormant beasts.
It has long been predicted that the disruption will be accompanied
by a thermal `flare', powered by the accretion of bound stellar debris.
Recently, we explored the observational consequences if a fraction of
the accretion power is channeled into an ultra-relativistic outflow.
The high-energy transient Sw 1644+57 provides strong support to the
presence of powerful relativistic jets during tidal
disruption events. I will discuss the rich behavior of Sw 1644+57 in
the radio and X-rays, focusing on the information we gain
on the circum-nuclear medium and the jet physics by modeling this event.
Our Galactic center contains a cluster of stars in the inner 0.1 pc.
This, so called, S cluster is thought to coexist with the quiescent
accretion disk. Because of its early stellar type, the S2 star of the cluster
is expected to posses a fairly powerful wind. We show here that the
ram pressure of the accretion
disk shocks the stellar wind fairly close to the star. The shocked
fluid reaches a temperature of ˜1 keV and cools efficiently through
optically thin, thermal bremsstrahlung emission. The radiation from
the shocked wind peaks around the epoch of the pericentre passage
of the star at a luminosity potentially comparable to the quiescent
emission detected from Sgr A*. Detection of shocked wind radiation
can constrain the density of the accretion disc at a distance of
several thousands of gravitational radii from the black hole.