A pr 1 99 7 The Wardle Instability in Interstellar Shocks : II . Gas Temperature and Line Emission

We have modeled the gas temperature structure in unstable C-type shocks and obtained predictions for the resultant CO and H2 rotational line emissions, using numerical simulations of the Wardle instability that were carried out by Stone (1997) and that have been described in a companion paper. Our model for the thermal balance of the gas includes ion-neutral frictional heating; compressional heating; radiative cooling due to rotational and rovibrational transitions of the molecules CO, H2O and H2; and gas-grain collisional cooling. We obtained results for the gas temperature distribution in – and H2 and CO line emission from – shocks of neutral Alfvenic Mach number 10 and velocity 20 or 40 km s in which the Wardle instability has saturated. Both twoand three-dimensional simulations were carried out for shocks in which the preshock magnetic field is perpendicular to the shock propagation direction; and a 2-D simulation was carried out for the case in which the magnetic field is obliquely oriented with respect to the shock propagation direction. Although the Wardle instability profoundly affects the density structure behind C-type shocks, most of the shock-excited molecular line emission is generated upstream of the region where the strongest effects of the instability are felt. Thus the Wardle instability has a relatively small effect upon the overall gas temperature distribution in – and the emission line spectrum from – C-type shocks, at least for the cases that we have considered. In none of the cases that we have considered thus far did any of the predicted emission line luminosities change by more than a factor 2.5, and in most cases the effects of instability were significantly smaller than that. Slightly larger changes in the line luminosities seem likely for 3-D simulations of oblique shocks, although such simulations have yet to be carried out and lie beyond the scope of this study. Given the typical uncertainties that are always