Appropriate mean opacity for thermalization-radius calculations

Determine which frequency-averaged opacity, specifically the Planck mean or Rosseland mean opacity, is appropriate for calculating the effective optical depth and thermalization radius in marginally optically thin tidal-disruption-event outflows treated with grey radiative transfer.

Background

The paper estimates the thermalization radius of the simulated tidal-disruption-event wind using an effective optical depth within a grey radiative-transfer treatment. Grey calculations require frequency-independent mean opacities, and the authors consider Planck and Rosseland averages as possible choices.

Because the relevant outflow is marginally optically thin, the weighting inherent in the two means can lead to different effective optical depths and therefore different thermalization radii. The authors state that it is unclear which mean opacity is more appropriate, although they argue that their absorption-dominated simulation makes the adopted approximation reasonable for the specific calculation presented.