Star Formation in Transient Molecular Clouds

1 THE STAR FORMING ENVIRONMENT The formation of stars in giant molecular clouds appears to be a very inefficient process. Global estimates of the star formation efficiency, based on the mass in molecular clouds and the Galactic star formation rate(Scalo 1986; Evans 1999), are of order a few percent while small-scale estimates for stellar clusters approach 50 % (Lada 1992; Lada & Lada 2003). The dispersion in estimates can, to some extent, be decreased under the assumption that molecular clouds are long-lived entities that exist for many dynamical times, although this necessitates a supporting mechanism to balance gravity (McKee et al.1993). As star formation from molecular gas involves the gravitational contraction through many orders in magnitude in size, it is commonly assumed that the largest scale objects associated with star formation, molecular clouds, are themselves bound. In this case, the problem is how does a large bound region (10 4 − 10 6 M⊙) only permit a small fraction of its mass to undergo gravitational collapse. Various mechanisms, such as magnetic fields and feedback from young stars, have been evoked over the years in order to support molecular clouds and explain the low efficiency of star formation