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Astron. Astrophys. 356, 1010-1022 (2000)

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5. Concluding remarks

  • We have investigated the process of para to ortho conversion, driven by H-H2 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 H2.

  • Chemical conversion of oxygen into water is the main source of atomic H in C-type shocks with [FORMULA] [FORMULA] 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 [FORMULA] [FORMULA] 10 km s-1; at higher speeds, collisional dissociation by H and H2 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, [FORMULA]. In C-type shocks, where the flow time is long, significant conversion starts at [FORMULA] [FORMULA] 700 K and is complete for [FORMULA] [FORMULA] 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 [FORMULA] [FORMULA] 1000 K. Complete conversion requires n(H)/[FORMULA] [FORMULA], which occurs in our models for [FORMULA] [FORMULA] K, i.e. for [FORMULA] [FORMULA] 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 [FORMULA] and a much lower fraction of atomic H, even when we adopt his (larger) rate coefficient for dissociation of H2 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 [FORMULA] is always lower than the ratio of the total ortho-H2 and para-H2 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 [FORMULA] rovibrational lines.

  • We have examined constraints on the initial ortho:para ratio and shock speeds that can be derived from the observed H2 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 [FORMULA]. Adopting a lower [FORMULA] yields a lower initial ortho:para ratio and a higher [FORMULA]. For J-type shocks, the H2 excitation temperatures typically observed in protostellar outflows correspond to low velocity shocks ([FORMULA] [FORMULA] 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 H2 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 [FORMULA] 1.0. In the case of a C-type shock with [FORMULA] = [FORMULA] 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 H2 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 H2 ratio in various sets of lines seem to provide a useful way of determining the shock structures in protostellar outflows.

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© European Southern Observatory (ESO) 2000

Online publication: April 17, 2000