The Case for Global Gravitational Contraction in Star-Forming Molecular Clouds

Colloquium: Astronomy Colloquia | March 7 | 4-5 p.m. | 1 LeConte Hall

Speaker/Performer: Enrique Vazquez-Semadeni, UNAM

Sponsor: Department of Astronomy

I discuss the question of the relative contribution of turbulence and gravitational contraction to the nonthermal motions observed in molecular gas, from giant molecular clouds (GMCs) to dense cores. I first discuss both observational and numerical evidence suggesting that the motions consitute gravitational contraction. From energetics alone, observations of apparent virialization may be just as well interpreted as infall. This includes the recent generalization of Larson's relations by Heyer et al. In numerical simulations of cloud formation and evolution, cold clouds are born transonically turbulent, and develop highly supersonic motions as the collapse progresses. The role of the initial turbulence is to provide nonlinear density fluctuations that are later "captured" by gravity during the global collapse. The global contraction starts in the cold atomic gas. Star formation as well as physical conditions leading to molecule formation do not start until several Myr after global gravitational contraction has started, although long before the global contraction ends. Thus, the feedback from the newly formed stars must act against the ongoing contraction. The observed effect is that the feedback does reduce the star formation efficiency (SFE), but does so by eroding the infalling gas, rather than by maintaining the clouds in approximate dynamical equilibrium. The topology of the velocity field in this regime is highly chaotic, rather than monolithic. An evolutionary semi-analytical model based on this phenomenology correctly reproduces the locus of clouds in the Kenniccutt-Schmidt diagram, the evolutionary stages of GMCs, and the observed stellar age distribution in individual clouds, while predicting that present-day low-mass star forming regions may evolve into high-mass regions within a few Myr.

Event Contact: rhelgens@astro.berkeley.edu, 510-642-5275