Letter to the Editor

The effects of expansion line opacity in accretion disks

¹ Institut fr Theoretische Astrophysik Tiergartenstrasse 15, 69121 Heidelberg, Germany
² On leave from Department of Physics and Asher Space Research Institute, Technion, 32000 Haifa, Israel e-mail: gioras@physics.technion.ac.il

Accepted: 19 July 2005

Abstract

We estimate the effect of the shear flow in accretion disks on the radiation field. We apply the recent results of Wehrse et al. (2003, A&A, 401, 43) for the expansion line opacity in shear flows. We use an approximation and derive an approximate equation for the radiation field in an accretion disk in the diffusion approximation. We find that in the appropriate limit for the flow in disks: – The effect depends on a product of three factors: the first depends solely atomic physics and hence on the composition of the accreted matter, the second depends on the density while the third depends solely on the shear flow. 
– The radiation field in accretion disks around neutron stars, black holes and white dwarfs is strongly affected by the expansion line opacity. 
– The radiation field is very non isotropic leading to significantly higher specific intensities in the vertical direction, a higher temperature gradient and increased susceptibility to convection and other instabilities.