Photometric distances to dark clouds: cometary globule CG 12

A method for determining distances to dark clouds and Bok globules based on broad-band optical and near-infrared photometry is presented. In this method, intrinsic colour indices of stars projected towards the direction of a cloud are computed by dereddening the observed colour indices using various trial values of extinction A(V) and a standard extinction law. The computed intrinsic colour indices for a star are then compared with the intrinsic colour indices of normal main-sequence stars and a spectral type is assigned to the star for which the computed colour indices best match the standard intrinsic colour indices. Distances (d) to the stars are determined using the A(V) and absolute magnitudes (M-V) corresponding to the spectral types thus obtained. A plot of A(V) against d undergoes a sharp rise at a distance corresponding to the distance to the cloud. Using this method, we have determined a distance of 550 pc to the cometary globule CG 12. The distance of 550 pc and a Galactic latitude of b = 21degrees imply that CG 12 is at a height of similar to200 pc above the galactic mid-plane. The star formation efficiency in this cloud is found to be relatively high (greater than or similar to16 per cent, to as large as similar to33 per cent). The existence of an HI shell centred at l = 315degrees, b = 30 with CG 12 near its boundary and its tail pointing away from the centre of the shell supports the suggestion of a supernova explosion, near the centre of the HI shell, being responsible for the cometary morphology and the triggering of star formation. Thus CG 12 is a rare example of triggered high-mass star formation at relatively large galactic height.