Formation of clouds in a cooling interstellar medium

An interstellar medium cooling from 8200 K after sudden heating (e.g., by soft X-rays or by low- energy cosmic rays) favors the development of thermal . One-dimensional hydrodynamic calculations show that the condensation process will yield interstellar clouds with densities of 100 times the intercloud density, given a small ( 10 percent) initial density perturbation, provided the scale length X of the initial perturbation satisfies X, < Cr,, where C is the sound speed in the gas and r, is the radiative cooling time. The condensations reach maximum density in a few cooling timescales. For an initially uniform medium of density fl = 0.3 hydrogen atoms per cm3 that is 5 percent ionized, a density perturbation of wavelength 3 pc will grow by a factor of 100 in about 10 years, reaching a final ionized fraction x 10- and temperature T < 20' K. The intercloud medium at this time will have cooled to about 2000' K. In the one-dimensional case, the final extent of the cloud will be about 0.01 pc. Cloud dimensions more typical of those observed in the interstellar medium are a likely result of a three-dimensional treatment of this problem. An increase in the critical condensation wavelength , as well as higher temperatures for the time-dependent intercloud medium, will result if cooling agents such as C, Si, Fe, and 0 are more highly ionized than assumed in these calculations. Soft X-rays are a plausible source of such anomalously high ionization.