Unfolding the Laws of Star Formation: The Density Distribution of Molecular Clouds

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Science  11 Apr 2014:
Vol. 344, Issue 6180, pp. 183-185
DOI: 10.1126/science.1248724

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Mapping Stardust

A galaxy's structure throughout time depends largely on its ability to convert the raw material of molecular clouds into stars. One of the most influential properties in determining star formation rates is the distribution of densities among individual molecular clouds, which can be described by a probability density function of volume densities. Kainulainen et al. (p. 183) devised a method to quantify these distributions from empirical dust extinction maps of nearby clouds. The threshold for star formation in these observationally based calculations was significantly lower than theoretical predictions.


The formation of stars shapes the structure and evolution of entire galaxies. The rate and efficiency of this process are affected substantially by the density structure of the individual molecular clouds in which stars form. The most fundamental measure of this structure is the probability density function of volume densities (ρ-PDF), which determines the star formation rates predicted with analytical models. This function has remained unconstrained by observations. We have developed an approach to quantify ρ-PDFs and establish their relation to star formation. The ρ-PDFs instigate a density threshold of star formation and allow us to quantify the star formation efficiency above it. The ρ-PDFs provide new constraints for star formation theories and correctly predict several key properties of the star-forming interstellar medium.

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