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

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 H 2 levels that is in thermal equilibrium (static limit) and a fixed ortho-H 2 /para-H 2 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 H 2 observations of NGC 2024. Ortho-para interconversions occur mainly through collisions of H 2 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-H 2 into ortho-H 2 during the passage of the shock. However, proton exchange reactions are the principal process for shock speeds u s ≲ 15 km s -1 . At higher shock speeds, the OPR attains its equilibrium quickly through reactive H-H 2 collisions, owing to higher kinetic temperatures and atomic hydrogen abundances reached in the shocked gas. Ortho-para interconversions induced through H 2 interaction with dust grains and due to H 2 formation on dust were found to be too slow in LVSs. A grid of models is calculated for preshock densities of n H = 10 2 , 10 4 , and 10 6 cm -3 and shock speeds from u s = 10-30 km s -1 . It is demonstrated that the OPR attains 3 in regions of shocked gas only if u s ≳ 20 km s -1 for preshock densities ≲10 3 cm -3 , while u s ≳ 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 H 2 line intensities assuming an OPR initial of unity and 3. They are compared with those computed with models in which the level population is in thermal equilibrium.