Star Formation Relations in the Milky Way

TL;DR

This study investigates the relationship between star formation rates and molecular cloud properties in the Milky Way, testing theoretical models and comparing efficiencies across different gas densities.

Contribution

It provides empirical evidence that dense molecular gas better predicts star formation rates and challenges existing theoretical models based on free-fall and crossing times.

Findings

Dense gas mass correlates more strongly with star formation rate.

Neither of the tested theoretical models is supported by the data.

Star formation efficiency varies widely when considering all molecular gas.

Abstract

The relations between star formation and properties of molecular clouds are studied based on a sample of star forming regions in the Galactic Plane. Sources were selected by having radio recombination lines to provide identification of associated molecular clouds and dense clumps. Radio continuum and mid-infrared emission were used to determine star formation rates, while 13CO and submillimeter dust continuum emission were used to obtain masses of molecular and dense gas, respectively. We test whether total molecular gas or dense gas provides the best predictor of star formation rate. We also test two specific theoretical models, one relying on the molecular mass divided by the free-fall time, the other using the free-fall time divided by the crossing time. Neither is supported by the data. The data are also compared to those from nearby star forming regions and extragalactic data. The star formation "efficiency," defined as star formation rate divided by mass, spreads over a large range when the mass refers to molecular gas; the standard deviation of the log of the efficiency decreases by a factor of three when the mass of relatively dense molecular gas is used rather than the mass of all the molecular gas.