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Some well-studied Herbig Haro objects have associated with them one or morecold, dense, and quiescent clumps of gas. We propose that such clumps near anHH object can be used as a general measure of clumpiness in the molecular cloudthat contains that HH object. Our aim is to make a survey of clumps around asample of HH objects, and to use the results to make an estimate of theclumpiness in molecular clouds. All known cold, dense, and quiescent clumpsnear 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 HHobject. The searches were made using JCMT and SEST in the HCO+ 3-2 and alsoH13CO+ 1-0 lines, with some additional searches for methanol and sulphurmonoxide lines. The sources selected were a sample of 22 HH objects in which noprevious HCO+ emission had been detected. We find that half of the HH objectshave clumps detected in the HCO+ 3-2 line and that all searches in H13CO$+ 1-0lines show evidence of clumpiness. All condensations have narrow linewidths andare evidently unaffected dynamically by the HH jet shock. We conclude that themolecular clouds in which these HH objects are found must be highlyheterogeneous on scales of less than 0.1 pc. An approximate calculation basedon these results suggests that the area filling factor of clumps affected by HHobjects is on the order of 10%. These clumps have gas number densities largerthan 3e4 cm-2.

The clumpiness of molecular clouds: HCO+ (3--2) survey near Herbig-Haro objects