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= 21° imply that CG 12 is at a height of ~200 pc above the galactic mid-plane. The star formation efficiency in this cloud is found to be relatively high (>~16 per cent, to as large as ~33 per cent). The existence of an HI shell centred at l= 315°, 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.