The physical conditions within dense cold clouds in cooling flows - II

This is a progress report on our numerical simulations of conditions in the cold cores of cooling flow condensations. The physical conditions in any non-equilibrium plasma are the result of a host of microphysical processes, many involving reactions that are research areas in themselves. We review the dominant physical processes in our previously published simulations, to clarify those issues that have caused confusion in the literature. We show that conditions in the core of an X-ray-illuminated cloud are very different from those found in molecular clouds, largely because carbon remains substantially atomic and provides powerful cooling through its far infrared lines. We show how the results of the Opacity Project have had a major impact on our predictions, largely because photoionization cross-sections of atoms and first ions are now calculated to be far larger than estimated previously. Finally, we show that the predicted conditions are strongly affected by complexities such as microturbulence or the presence of small amounts of dust. Large masses of cold dense gas, in addition to the warmer molecular gas detected recently, could be present in cooling flows.