Ionization Equilibrium Timescales in Collisional Plasmas
Abstract
Astrophysical shocks or bursts from a photoionizing source can disturb the typical collisional plasma found in galactic interstellar media or the intergalactic medium. The spectrum emitted by this plasma contains diagnostics that have been used to determine the time since the disturbing event, although this determination becomes uncertain as the elements in the plasma return to ionization equilibrium. A general solution for the equilibrium timescale for each element arises from the elegant eigenvector method of solution to the problem of a nonequilibrium plasma described by Masai and Hughes & Helfand. In general, the ionization evolution of an element Z in a constant electron temperature plasma is given by a coupled set of Z + 1 firstorder differential equations. However, they can be recast as Z uncoupled firstorder differential equations using an eigenvector basis for the system. The solution is then Z separate exponential functions, with the time constants given by the eigenvalues of the rate matrix. The smallest of these eigenvalues gives the scale of the slowest return to equilibrium independent of the initial conditions, while conversely the largest eigenvalue is the scale of the fastest change in the ion population. These results hold for an ionizing plasma, a recombining plasma, or even a plasma with random initial conditions, and will allow users of these diagnostics to determine directly if their bestfit result significantly limits the timescale since a disturbance or is so close to equilibrium as to include an arbitrarily long time.
 Publication:

The Astrophysical Journal
 Pub Date:
 July 2010
 DOI:
 10.1088/0004637X/718/1/583
 arXiv:
 arXiv:1006.0254
 Bibcode:
 2010ApJ...718..583S
 Keywords:

 atomic data;
 atomic processes;
 plasmas;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 4 pages, 2 figures. Accepted for publication by the Astrophysical Journal