TIME-DEPENDENT COOLING AND GRAM DESTRUCTION IN HOT DUSTY PLASMAS - A SIMPLIFIED MODEL AND PRINCIPAL RESULTS

We present a simple method of including the principal effects of inter stellar dust in hot gas evolution codes, including a consistent set of elemental abundances, their distribution among gas and grains in diff use regions, the distribution of grain sizes, the sputtering of grains by impact with nuclei, and the cooling rate from gas-grain collisions . When combined with a gas evolution code, the time-dependent evolutio n of gas phase abundances, ion concentrations, and gas and grain cooli ng can be followed. Sample calculations are presented to explore the r elative timescales for grain destruction and radiative cooling, the re lative importance of grain and gas cooling coefficients in evolving ga s, the overall significance of grain inclusion to the thermal history of the gas, and the possibility of comparative dating of hot gas regio ns via their X-ray spectral characteristics. We find that the straight forward comparison between the cooling coefficient of newly heated dus t with that of gas in collisional equilibrium is particularly misleadi ng. The cooling coefficient of newly heated material is overwhelmingly dominated by the nonequilibrium gas cooling, during which the ionizat ion is rapidly rising and dust is being sputtered. The gas cooling coe fficient drops rapidly during this brief period. Dust cooling also dro ps, because of the reduction in grain surface area. At high temperatur e, gas cooling soon falls below that of the grains, but grain cooling continues to fall rapidly, raising the gas cooling via the return of e lements to the gas. For material whose temperature exceeds roughly 4 x 10(6) K, the total radiated energy during these ''ion flash'' and dus t destruction epochs is small compared to the total energy in the syst em. Conversely, for temperatures below about 4 x 10(5) K, both grain d estruction by ion sputtering and grain cooling are small. It is a some what remarkable coincidence: The range of temperature for which the du st destruction and radiative cooling timescales are comparable is also the temperature range for which grain and gas cooling rates are simil ar. We conclude that the inclusion of dust in codes will usually have little overall effect on the thermal and dynamical history of the gas. But there can be a quite significant alteration of the X-ray spectra of recently heated gas, behind ''nonradiative'' shocks, for example. D ust inclusion at least at our level of complexity is required in any m odels purporting to examine spectral details. As an example, the inabi lity of shockwave models to produce the surprising intensity of the [F e X] line in the nonradiative shocks of the Cygnus Loop was once used to argue that the shock was proceeding through a medium infested with microscopic interstellar clouds, evaporating them as it went. It was s uggested that evaporative injection of low ion stages into the hot gas could potentially produce the [Fe X], as iron is being ionized. Dust sputtering should produce a similar effect. There is probably an impor tant spectral line whose intensity and surface brightness distribution map the pattern and rate of dust destruction in the Cygnus Loop and h ot gas elsewhere, but it will not be found in gas-phase-only models. F ortunately, at the level of complexity of our modeling, dust inclusion is straightforward.