Star formation in the Monoceros OB1 dark cloud.

Abstract

A survey of the Monoceros OB1 dark cloud has been made for molecular outflows and young stellar objects. In all, nine molecular outflows and thirty far-infrared sources were identified in a portion of the cloud composed of about 3 x 10⁴ M(⊙)of material. Statistical arguments suggest that 90% of the far-infrared sources actually are young stellar objects embedded in the cloud. If the star formation rate in the Mon OB1 cloud is roughly constant with time then molecular outflows in the cloud should be able to support it against collapse due to gravity. This suggests that the birthrate of outflows in the solar neighborhood is very high. In fact, regardless of considerations of cloud support, the large number of outflows identified in the Mon OB1 cloud and the propensity of the youngest stellar objects in the cloud to be associated with outflows suggest that outflows have a high birthrate in the solar neighborhood and are part of a common stage in early stellar evolution. The young stellar objects identified in the cloud can be fit into a spectral classification system. In fact, in terms of spectral slopes, far-infrared luminosity, and source size the properties of the objects are consistent with expectations if the system represents an evolutionary sequence. It is also found that the outflow phase in early stellar evolution tends to occur at about the time that young stellar objects lose a large fraction of their circumstellar envelopes. As a result it seems likely that outflows play an important role in sweeping out the circumstellar gas around many young stellar objects and may, in fact, play an important part in the evolutionary transition between the protostellar and stellar stages of evolution.