## 5. Concluding remarks-
We have investigated the process of para to ortho conversion, driven by H-H _{2}nuclear spin-changing reactions, in stationary shocks of both C- and J-type, paying particular attention to the chemistry, ion-neutral coupling within the shock wave, and collision-induced transitions in H_{2}. -
Chemical conversion of oxygen into water is the main source of atomic H in C-type shocks with 20 km s ^{-1}. At higher shock speeds, dissociation by ion impact further increases the atomic fraction, but by no more than a factor of about 3. In J-type shocks, chemical dissociation dominates for 10 km s^{-1}; at higher speeds, collisional dissociation by H and H_{2}becomes effective, increasing the atomic fraction by orders of magnitudes over the chemical value. -
Over a wide range of densities and velocities, the degree of para to ortho conversion is found to be determined mainly by the maximum temperature attained by the neutrals in the shock, . In C-type shocks, where the flow time is long, significant conversion starts at 700 K and is complete for 1300 K, even though the fraction of atomic hydrogen remains low. In J-type shocks, where the time available for conversion is much less, the ortho:para ratio starts to increase only for 1000 K. Complete conversion requires *n*(H)/ , which occurs in our models for K, i.e. for 15 km s^{-1}. -
Our results for C-type shocks differ significantly from those of Timmermann (1998). We find much lower abundances of H ^{+}and and a much lower fraction of atomic H, even when we adopt his (larger) rate coefficient for dissociation of H_{2}by ion impact. The net effect is that Timmermann's calculations overestimate the final ortho:para ratio. -
The ortho:para ratio deduced from observations of levels above is always lower than the ratio of the total ortho-H _{2}and para-H_{2}column densities through the shock wave, which is itself lower than the maximum local value of the ortho:para ratio reached in the postshock gas. In addition, the observed ortho:para ratio depends on the lines that are used. For the same object, the pure rotational lines observed with ISOCAM should yield a higher ortho:para ratio than rovibrational lines. -
We have examined constraints on the initial ortho:para ratio and shock speeds that can be derived from the observed H _{2}excitation temperatures and ortho:para ratio, using both pure rotational lines and rovibrational lines. For C-type shocks, there may be a strong dependence on the assumed . Adopting a lower yields a lower initial ortho:para ratio and a higher . For J-type shocks, the H_{2}excitation temperatures typically observed in protostellar outflows correspond to low velocity shocks ( 10 km s^{-1}) in which the observed ortho:para ratio is still close to its initial value, which is then better constrained. -
As an illustration, we have considered observations of H _{2}in the knots E,K of the Herbig-Haro object HH 54 (Neufeld et al. 1998; Gredel 1994). The pure rotational lines imply an initial ortho:para ratio 1.0. In the case of a C-type shock with = cm^{-3}, the initial ratio could be as low as 0.01 (the LTE value at 25 K). As is often the case in protostellar flows, the rovibrational lines require the presence of a hotter component within the observing beam, distinct from that producing the pure rotational lines. The need for a distinct component arises independently in order to account for the higher ortho:para H_{2}ratio deduced empirically from the rovibrational lines, as compared to the pure rotational lines. Assuming a uniform initial ortho:para ratio in the preshock gas, we show that either a C-type bow shock or a shock wave which has not reached steady-state (and possesses both C- and J-type characteristics) is a plausible model. Observations of the ortho:para H_{2}ratio in various sets of lines seem to provide a useful way of determining the shock structures in protostellar outflows.
© European Southern Observatory (ESO) 2000 Online publication: April 17, 2000 |