The gas-phase iron abundance in Herbig-Haro objects

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 'Burnham's Nebula' (HH 255). It is the purpose of this study to decide whether a sizeable fraction of the Fe in these HH objects is still bound in dust grains or whether the observed matter has gone through sufficiently fast shock waves so that the dust grains have been essentially destroyed and most of the iron has gone back into the gas phase. We have determined the abundance ratios using statistical equilibrium calculations for the ions Fe(+), S(+), and O(+). (These are the most abundant ions of the elements in question.) Abundance determinations have been made using homogeneous models of the HH objects for which electron temperatures and densities have been determined observationally from forbidden line ratios. The results show that the Fe/S ratio in the objects HH 1, HH 7, HH 11, and HH 43A agrees very well with the Population I abundance ratio. Only Burnham's Nebula (HH255) shows an Fe/S ratio which is about three times lower indicating a shock-wave history which is quite different from that of the other HH objects.