EPoS Contribution
EPoS Contribution
An Evolutionary Model for the Star Formation Efficiency in Gravitationally Collapsing Molecular Clouds.

Manuel Zamora-Aviles
CRyA-UNAM, Morelia, Mexico
We present an idealized, semi-empirical model for the star formation efficiency (SFE) in a molecular cloud (MC) formed by the collision of warm neutral medium cylindrical streams. The cloud thus formed continuously accretes mass from the surrounding diffuse gas and develops turbulence through the combined action of various instabilities. However, the sole role of turbulence is to generate density fluctuations on top of the cloud's mean density, which act as seeds for subsequent gravitational collapse. No turbulent support is assumed, and magnetic fields are neglected, so the cloud is assumed to begin contracting gravitationally as soon as it reaches its own Jeans mass. The high-density tail ($n>n_{\rm SF}$, where $\nsf$ is a free parameter) of the density distribution is assumed to produce instantaneous star formation, at a rate given by the mass at $n>n_{\rm SF}$ divided by the free-fall time at $\nsf$, while the bulk of the cloud continues to collapse. During the collapse, the mean density of the cloud increases, and thus the star formation rate accelerates over time. From the total stellar mass, we compute the massive star fraction, which we assume feeds back on the cloud through ionizing radiation. The evolution is terminated when the entire bulk of the cloud is ionized, or when the remaining dense gas becomes gravitationally unbound. We present two fiducial cases, one for an isolated cloud and one for a giant molecular cloud (GMC), which we compare against various observational results. The total evolutionary cycle of the model clouds lasts a few tens of Myr, depending on their mass. The low- and high-mass fiducial models achieve time-averaged SFEs of 0.5 and 3\%, respectively, and terminal SFEs of 39 and 15\%, respectively. The GMC model adheres very well to the evolutionary scenario recently inferred by \citet{Kawamura+09} for GMCs in the LMC. The individual cloud model's evolutionary track in the Kennicutt-Schmidt diagram passes through the locus of typical low- to intermediate-mass clouds like those studied by \citet{Evans+09} at intermediate stages, and moves towards the locus of massive SF clouds like OMC-1 as it reaches the final stages of evolution, suggesting that some low-mass star-forming clouds may evolve into high-mass ones over the course of roughly 10 Myr. The stellar age histograms for the isolated cloud fiducial model a few Myr before the cloud's destruction agree very well with those presented for various associations at various evolutionary stages by \citet{PS00}, while at the same time the model explains why fully-formed, yet non-star-forming, MCs are not observed. Our model suggests that the scenario in which molecular clouds are in a global state of gravitational contraction and their SFE is regulated by stellar feedback is entirely feasible and in agreement with key observed properties of molecular clouds, such as their masses, sizes, lifetimes, and star formation efficiencies and populations.
Enrique Vazquez-Semadeni, Pedro Colin (CRyA-UNAM)