Molecular Cloud Structure and the Origin of Stellar Masses

Near-infrared dust extinction mapping is opening a new window on molecular cloud research. Applying a straightforward technique to near-infrared large-scale data of nearby molecular complexes, one can easily construct density maps with dynamic ranges in column density covering 3σ˜ 0.5 mag <AV <50 mag or1021 cm-2 <N <1023cm-2. These maps are unique in capturing the low column density distribution of gas in molecular cloud complexes, where most oft he mass resides, and at the same time allow the identification of dense cores (n˜ 104 cm-3) which are the precursors of stars. For example, the application of this technique to the nearby Pipe Nebula complex revealed the presence of 159 dense cores(the largest sample of such objects in a single complex) whose mass spectrum presents the first robust evidence for a departure from a single power law. The form of this mass function is surprisingly similar in shape to the stellar initial mass function but scaled to a higher mass by a factor of about three. This suggests that the distribution of stellar birth masses (initial mass function) is the direct product of the dense core mass function and a uniform star formation efficiency of30% ± 10%, and that the stellar initial mass function may already be fixed during or before the earliest stages of core evolution.