Interest in emission by molecular hydrogen and, more specifically, in the ortho:para-H2 ratio has been stimulated by observations of the pure rotational lines of H2 made with the ISO satellite. Ortho:para ratios 1 have been determined from ISO observations of these lines towards several regions associated with outflows from young stellar objects, such as HH 54 (Neufeld et al. 1998; Cabrit et al. 1999), in which shocks are believed to be propagating.
The evolution of the ortho:para-H2 ratio in interstellar gas was studied by Osterbrock (1962), Dalgarno et al. (1973), DeCampli et al. (1978), Clavel et al. (1978), Flower & Watt (1984), Pineau des Forêts et al. (1991), and recently by Timmermann (1998). It is known that this ratio can be modified by reactive collisions with protons or proton-transferring ions, such as . Whilst reactions with protons do not exhibit a significant activation energy (Gerlich 1990), the barrier to hydrogen atom reactions is approximately 5000 K (Siegbahn & Liu 1978). It follows that reactive collisions with hydrogen atoms are significant only in hot gas, notably in gas which has been heated by the passage of a shock wave. Furthermore, as shown by Flower & Watt (1984), the timescale for the ortho:para ratio to subsequently equilibrate in the cold gas can exceed yr. In regions of low-mass star formation, gas which has undergone shock heating in an episodic event is unlikely to return to equilibrium between such events, given their estimated frequency, of the order of yr-1 (see, for example, Devine et al. 1997).
Timmermann (1998) studied the variation of the ortho:para-H2 ratio through C-type shock waves propagating in predominantly molecular gas. Shock speeds 10 30 km s-1 and preshock densities cm-3 were considered. Crucial to a correct evaluation of the ortho:para ratio are the fraction of atomic hydrogen, the temperature of the neutrals, the degree of ionization, and the ion-neutral drift speed. An adequate description of the cooling of the gas, particularly by H2, and of its chemistry are indispensable to the reliable prediction of these parameters. In the present paper, we re-examine the problem of ortho:para conversion in C-type shocks. Our models include a more accurate treatment of the above phenomena and make use of recent calculations of rate coefficients for the collisional excitation of H2. As will be seen below, the results of our calculations differ substantially from those of Timmermann (1998) in a number of significant respects. For the purposes of comparison, we have also investigated H2 emission and ortho:para conversion in non-dissociative J-type shocks.
In Sect. 2, the theory relating to the models that we have used is summarized. Our results, for both C- and J-type shocks, are described and discussed in Sect. 3. In Sect. 4, we discuss diagnostics available for constraining the initial ortho:para ratio in shocks, and we consider briefly the interpretation of ISO observations of HH 54, postponing a more extensive treatment of this and other sources to a subsequent paper.
© European Southern Observatory (ESO) 2000
Online publication: April 17, 2000