  Some well-studied Herbig Haro objects have associated with them one or more
cold, dense, and quiescent clumps of gas. We propose that such clumps near an
HH object can be used as a general measure of clumpiness in the molecular cloud
that contains that HH object. Our aim is to make a survey of clumps around a
sample of HH objects, and to use the results to make an estimate of the
clumpiness in molecular clouds. All known cold, dense, and quiescent clumps
near HH objects are anomalously strong HCO+ emitters. Our method is, therefore,
to search for strong HCO+ emission as an indicator of a clump near to an HH
object. The searches were made using JCMT and SEST in the HCO+ 3-2 and also
H13CO+ 1-0 lines, with some additional searches for methanol and sulphur
monoxide lines. The sources selected were a sample of 22 HH objects in which no
previous HCO+ emission had been detected. We find that half of the HH objects
have clumps detected in the HCO+ 3-2 line and that all searches in H13CO$+ 1-0
lines show evidence of clumpiness. All condensations have narrow linewidths and
are evidently unaffected dynamically by the HH jet shock. We conclude that the
molecular clouds in which these HH objects are found must be highly
heterogeneous on scales of less than 0.1 pc. An approximate calculation based
on these results suggests that the area filling factor of clumps affected by HH
objects is on the order of 10%. These clumps have gas number densities larger
than 3e4 cm-2.
