1995A&A...301..522L

We have observed the globule CG7S (located inside the HII region S190/IC1848) in order to test the Radiation-Driven Implosion mechanism (RDI) as a model of formation and evolution of Cometary Globules. CG7S is a globule which has not yet fully developed the common "tail-head" structure of Cometary Globules. These observations were made with the IRAM 30-m radio-telescope in the lines of ¹²CO, ¹³CO, C¹⁸O, CS. This globule consists of a dense head containing a small bright dense core "hidden" by a low-density halo, prolonged by some less dense gas and a diffuse tail traced by ¹²CO. The main body of the cloud is pervaded by a general velocity gradient of +3km/s/pc. CG7S exhibits a second (blueshifted) kinematic component, displaced by 3-4km/s, associated with some dense and brighter surface gas of the "front side", expanding towards the observer. We compare the CG7S observations to numerical simulations of a photo-ionised globule. The main features of CG7S are easily reproduced. The observational data are consistent with an interpretation where CG7S is a globule which has already collapsed and is now re-expanding: it undergoes the transient phase, prior to the cometary stage. We find a rather good agreement between the observed and modelled velocity-position maps along the cloud's axis. CG7S is the first object to exhibit so detailed features of the pre-cometary phase. Our observations provide strong support for RDI as a mechanism of formation for Cometary Globules.