An Approximate Determination of the Gas-Phase Metal Abundance in Herbig-Haro Outflows and their Shocks

(modified for brevity) It is important to determine whether the observed bow shocks in the working surfaces of Herbig-Haro outflows has lead to a destruction of dust grains and consequently to a change in the gas phase metal abundances (say of Fe) in the cooling regions of HH bow shocks. Detailed studies are currently available for only 5 HH outflows (Beck-Winchatz et al.\ 1996), due to the large observational and theoretical effort required to determine metal abundances in HH objects. Information about metal abundances in more HH objects is badly needed. We therefore use a very approximate method by introducing a "characteristic number", A_me, whose definition is based only on often observed line fluxes. We find a good correlation between A_me and the Fe abundance for the 5 well-studied HH objects. We use this correlation to determine approximate values of the gas phase Fe abundance in 13 additional high excitation and in 3 additional low excitation HH objects. Of the 16 high excitation HH objects studied, there are 6 which approximately agree with normal population I abundance (i.e., no depletion due to dust formation). The remaining 10 show some Fe gas phase depletion which never gets larger than by a factor of 2.5. This result is in agreement with our qualitative expectations that fast shocks efficiently destroy dust grains. Of the 5 low excitation HH objects studied, there are 4 which, unexpectedly, show a normal population I abundance. In low excitation objects, one might expect strong gas phase Fe depletion (showing the unchanged molecular cloud composition), unless the matter has previously gone through shocks of much higher shock velocities.