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 ) .
This small number is 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 .
We introduce a “ characteristic number , ” A _ { me } , whose definition is based only on the often observed line fluxes of [ Fe II ] 5159 , [ Fe II ] 7155 , [ Ca II ] 7291 , as well as H \alpha and H \beta .
These fluxes can easily be determined from existing observations .
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 .

The results are the following : 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 ( very modest ) Fe gas phase depletion which , however , 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 show a normal population I abundance ( strictly speaking , an even slightly higher abundance than this ) .
This is completely unexpected .
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 .
We discuss this possible explanation and the question of whether low excitation HH objects have a different “ history ” than usually assumed .