**Previous
abstract** **Next
abstract**

**Session 76 - Cataclysmic Variables and Accretion Disks.**

*Display session, Wednesday, January 15*

*Metropolitan Ballroom, *

## [76.12] Cooling Fronts in Accretion Disks and Constraints on the Disk Viscosity

*E. T. Vishniac, J. C. Wheeler (U. Texas)*
We examine the speed of inward traveling cooling fronts in
accretion disks and the structure of the hot phase of the
disk inside the cooling front. We show that the cooling
front speed is determined by the rarefaction wave that
precedes it and is approximately \alpha_F c_F (H/r)^q,
where \alpha_F is the dimensionless viscosity, c_F
is the sound speed, r is the radial coordinate, H is
the disk thickness, and all quantities are evaluated at the
cooling front. The scaling exponent q lies in the
interval [0,1], depending on the slope of the (T,\Sigma)
relation in the hot state. For a Kramers law opacity and
\alpha\propto (H/r)^n, where n is of order unity, we
find that q\sim 1/2. In addition, we derive a similarity
solution which is exact in the limit of a thin disk with
power law opacities and allows us to predict the coefficient
in the cooling front speed scaling law. Our results support
the numerical work of Cannizzo, Chen, and Livio (1995) and
their conclusion that n\approx 3/2 is necessary to
reproduce the exponential decay of luminosity in black hole
X-ray binary systems. Our results are insensitive to the
structure of the disk outside the radius where rapid cooling
sets in. In particular, the width of the rapid cooling zone
is a consequence of the cooling front speed rather than its
cause. This implies that our conclusions depend only on the
structure of the hot phase of the disk, which is relatively
well understood. We discuss the implications of this result
for theoretical models of disk viscosity.

The author(s) of this abstract have provided an email address for comments about the abstract: ethan@astro.as.utexas.edu

**Program
listing for Wednesday**