Photoionization and high-density gas

We present results of calculations using the XSTAR version 2 computer code. This code is loosely based on the XSTAR version 1 code, which has been available for public use for some time. However, it represents an improvement and update in several major respects, including atomic data, code structure, user interface, and improved physical description of ionization/excitation. In particular, it now is applicable to high-density situations in which significant excited atomic level populations are likely to occur. We describe the computational techniques and assumptions and present sample runs with particular emphasis on high-density situations.