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

¹ Department of Physics & Astronomy, University College London, Gower Street, London WC1E 6BT, UK e-mail: wwhyatt@star.ucl.ac.uk
² Institut de Ciències de l'Espai (CSIC-IEEC), Campus UAB, Facultat de Ciències, Torre C5 - parell 2, 08193 Bellaterra, Catalunya, Spain
³ Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Martí i Franquès 1, 08028 Barcelona, Catalunya, Spain

Received: 25 July 2009
Accepted: 6 November 2009

Abstract

Context. Some well-studied Herbig Haro objects have one or more cold, dense and quiescent clumps of gas associated with them. 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.

Aims. Our aim is to survey clumps around a sample of HH objects and to use the results to estimate the clumpiness in molecular clouds.

Methods. 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 an HH object. The searches were made using JCMT (for Northern hemisphere objects) and SEST (for Southern hemisphere objects) in the HCO⁺ (3–2) and also (for SEST observations) H¹³CO⁺ (1–0) lines, with some additional searches for methanol and sulphur monoxide lines. The selected sources were a sample of 22 HH objects in which no previous HCO⁺ emission had been detected.

Results. 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.

Conclusions. 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 of ~10%. These clumps have gas number densities of  cm^-2.