Molecular Cloud Formation behind Shock Waves

Abstract

Motivated by our previous paper, in which we argued for the formation of molecular clouds from large-scale flows in the diffuse Galactic interstellar medium, we examine the formation of molecular gas behind shocks in atomic gas using a one-dimensional chemical/dynamical model. In our analysis we place particular emphasis on constraints placed on the dynamical evolution by the chemistry. The most important result of this study is to stress the importance of shielding the molecular gas from the destructive effects of UV radiation. For shock ram pressures comparable to or exceeding typical local interstellar medium pressures, self-shielding controls the formation time of molecular hydrogen, but CO formation requires shielding of the interstellar radiation field by dust grains. We find that for typical parameters the molecular hydrogen fractional abundance can become significant well before CO forms. The timescale for (CO) molecular cloud formation is not set by the H 2 formation rate on grains, but rather by the timescale for accumulating a sufficient column density or extinction, A V ≳ 0.7. The local ratio of atomic to molecular gas (4 : 1), coupled with short estimates for the lifetimes of molecular clouds (3-5 Myr), suggests that the timescales for accumulating molecular clouds from atomic material typically must be no longer than about 12-20 Myr. Based on the shielding requirement, this implies that the typical product of preshock density and velocity must be nv ≳ 20 cm -3 km s -1 . In turn, depending on the shock velocity, this implies shock ram pressures that are a few times the typical estimated local turbulent gas pressure and comparable to the total pressures (gas plus magnetic plus cosmic rays). Coupled with the rapid formation of CO once shielding is sufficient, flow-driven formation of molecular clouds in the local interstellar medium can occur sufficiently rapidly to account for observations. We also provide detailed predictions of atomic and molecular emission and absorption that track the formation of a molecular cloud from a purely atomic medium, with a view toward helping to verify cloud formation by shock waves. However, our predictions suggest that the detection of the pre-CO stages will be challenging. Finally, we provide an analytic solution for time-dependent H 2 formation that may be of use in numerical hydrodynamic calculations.