THE GAS-PHASE ABUNDANCES OF HEAVY-ELEMENTS AND THE DESTRUCTION OF DUST GRAINS IN HERBIG-HARO SHOCK-WAVES

The gas-phase abundance ratios Fe/S and Fe/O have been determined for the Herbig-Haro objects HH 1, HH 7, HH 11, HH 43A, and HH 255 (Burnham 's Nebula) from a fairly large number of forbidden emission lines. For HH 1, the ratio Ni/S has been determined as well. Our objective was t o investigate whether Fe and Ni are depleted by dust formation in the line-emitting regions as is typical for normal (neutral or molecular) interstellar regions, or whether the observed matter has gone through sufficiently fast shock waves so that the dust grains have been destro yed and Fe and Ni have gone back into the gas phase. Using observation ally determined electron temperatures and densities we have solved the statistical equilibrium equations for Fe+, Fe++, S+, O-0, O+, O++, an d Ni+ and calculated the intensities of the respective emissions lines . For Fe+ a large number of new collision strengths has recently becom e available which enabled us to include a much greater number of trans itions in our calculations than has previously been possible. For HH 1 and HH 7 both ratios Fe/S and Fe/O agree very well with the solar abu ndance ratios. For HH 11, the more reliable ratio Fe/S is also very cl ose to the solar value, while the less reliable ratio Fe/O is somewhat higher. For HH 43A both abundance ratios suggest an Fe depletion by a factor 2-3 but the result is considered somewhat uncertain. The resul ts for Burnham's Nebula also indicate a depletion of Fe (by a factor o f similar to 3) and are more reliable. We conclude that this object ha s undergone a dust formation and shock wave history which is quite dif ferent from that of HH 1, HH 7, and HH 11. The Ni/S abundance ratio de termined for HH 1 is a factor similar to 10 larger than the solar valu e. The results for the different observed [Ni II] lines are quite cons istent. This situation has been found in other gaseous nebulae as well and is still enigmatic. (C) 1996 American Astronomical Society.