A bathtub model for the star-forming interstellar medium

TL;DR

The paper introduces a bathtub model for the star-forming interstellar medium that emphasizes mass conservation and predicts star formation rates based on gas accretion, explaining observed efficiencies and gas fractions.

Contribution

It presents a novel, simplified model linking gas flow, star formation, and feedback, providing insights into star formation regulation in molecular clouds and galaxies.

Findings

Star formation rate equals gas accretion rate in equilibrium.

Star formation efficiency is approximately 1%.

Dense gas fraction is around 10%, with gas depletion timescales of 0.5-1 billion years.

Abstract

The bathtub model of the star forming interstellar medium is based on the powerful constraint that mass has to be conserved when gas flows through its various thermal and density phases, ending up eventually in a young star or being blown away by stellar feedback. It predicts that the star formation rate of a molecular cloud is not determined by the cloud's mass or its internal collapse timescale, but rather by the accretion rate of new gas. For the most simple case of a constant accretion flow an equilibrium state is reached quickly where the star formation rate equals the accretion rate and where the dense gas mass is constant and independent of time. The mass of the young star cluster, on the other hand, increases linearly with time. The stellar mass fraction therefore represents a sensitive clock to measure the age of the star-forming region. The bathtub model predicts that the efficiency of star formation is small, of order 1%, even in the dense filamentary phases of molecular clouds. It provides a simple explanation for the dense gas fraction of order 10% in molecular clouds and for the large gas depletion timescales of star-forming galaxies of order $5 \times 10^8 - 10^9$ yrs.