ABSTRACTS OF CONTRIBUTED PAPERS

摘要

We calculate the abundance of molecular hydrogen (H2) for a wide range of interstellar conditions, in-cluding the following cases: i) constant density and temperature, it) constant density and variable tem-perature (isochoric case), and Hi) constant pres-sure (isobaric case). Starting with a diffuse atomic medium, the time evolution of the H2 abundance can be followed with dnn2/dt = Rj — Rph,d — Rd, where Rj is the formation rate of H2 on interstel-lar grains, Rph,d is the photodissociation rate, and Rd is the collisional dissociation rate. The forma-tion and. collisional dissociation rates depend on the gas temperature and density, and a self-consistent treatment of the problem should include the effects of H2 cooling. Photodissociation of interstellar H2 is initiated by absorption of discrete lines of the Ly-man and Werner series, and within the clouds the general interstellar radiation field is quickly atten-uated at those wavelengths, As the optical depth increases, the photodissociation rate decreases and becomes negligible at large optical depths (rv > 1). Also, in the absence of local energy sources, at large optical depths the collisional dissociation rate be-comes negligible (i.e., Rd tends toward zero at tem-peratures below T ~ 2000° K). For all three cases considered, we find that the time required to con-vert atomic to molecular hydrogen is longer than the timescale for a number of cloud formation mecha-nisms. For the isochoric case we find that the molec-ular formation timescale can be written as a power-law of the total density, f/or = Aa{n/V cm"3)? yr, with Ao = 1.06 X 1010 and a = -1.02. Similarly,