The evolutionary status of young stellar mass loss driving sources as derived from IRAS observations

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Abstract
A statistical analysis of the IRAS data for young stellar objects associated with mass outflows (Herbig–Haro objects and molecular outflows) is presented. Systematic differences are found between the observed far-infrared behaviour and that predicted by the model by Adams & Shu describing the main accretion phase during the formation of low-mass stars. We suggest that the low-mass outflow sources are at a later evolutionary stage and not truly protostellar. The IRAS colours of the young objects cannot in general be used to infer the presence of circumstellar discs, which could be responsible for the collimation of the associated molecular outflows, although the data are not in conflict with this idea. The IRAS data permit the derivation of relatively accurate bolometric luminosities, which span over six orders of magnitude for the driving sources of the molecular outflows. From the observed luminosity function the mass function is derived, which is not significantly different from the initial mass function for the stars in the solar neighbourhood, suggesting that heavy mass loss has been an important phase during the formation of stars at all times in the history of the Galaxy.

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