Ortho‐H2/Para‐H2Ratio in Low‐Velocity Shocks

Abstract

New detailed MHD-shock model computations are carried out for C-type low-velocity shocks (LVSs) that propagate through the molecular interstellar medium. Unlike many other C-shock models that adopt a population of H2 levels that is in thermal equilibrium (static limit) and a fixed ortho-H2/para-H2 ratio (OPR), the present one assumes a level population that is, along with the OPR, in a stationary state. This modification is necessary because the OPR of preshock gas is probably smaller than 3, as has been verified in H2 observations of NGC 2024. Ortho-para interconversions occur mainly through collisions of H2 with H+, H+3, and H. The densities of H+ and H+3 are generally low in C-shocks. These species convert only a small fraction of para-H2 into ortho-H2 during the passage of the shock. However, proton exchange reactions are the principal process for shock speeds us 15 km s-1. At higher shock speeds, the OPR attains its equilibrium quickly through reactive H-H2 collisions, owing to higher kinetic temperatures and atomic hydrogen abundances reached in the shocked gas. Ortho-para interconversions induced through H2 interaction with dust grains and due to H2 formation on dust were found to be too slow in LVSs. A grid of models is calculated for preshock densities of nH = 102, 104, and 106 cm-3 and shock speeds from us = 10-30 km s-1. It is demonstrated that the OPR attains 3 in regions of shocked gas only if us 20 km s-1 for preshock densities 103 cm-3, while us 25 km s-1 is required for higher densities. The OPR remains less than 3 for shocks with smaller speeds and may not even deviate much from its initial value at the lowest shock speeds examined. We present H2 line intensities assuming an OPRinitial of unity and 3. They are compared with those computed with models in which the level population is in thermal equilibrium.