Evolutionary growth of molecular clouds as traced by their infrared bright fraction

TL;DR

This study investigates the evolution of molecular clouds using infrared bright fraction as a proxy, revealing continuous mass gain during star formation and identifying two main evolutionary paths influenced by the galactic environment.

Contribution

It introduces the infrared bright fraction as a novel metric for tracking molecular cloud evolution and demonstrates that clouds grow in mass during star formation, challenging static mass assumptions.

Findings

Most clouds gain four times more mass as star formation progresses.

A subset of clouds experiences extreme growth, increasing in mass by a factor of 150.

Mass loss occurs after half the cloud area becomes star-forming, likely due to feedback and galactic shear.

Abstract

Understanding how stars form, evolve and impact molecular clouds is key to understanding why star formation is such an inefficient process globally. In this paper, we use the infrared bright fraction, $f_\text{IRB}$ (the fraction of a given molecular cloud that appears bright against the 8 $\mu$m Milky Way background) as a proxy for time evolution to test how cloud properties change as star formation evolves. We apply this metric to 12,000 high-mass star-forming molecular clouds we identify using the Herschel-Hi-GAL survey between $|l|<70^\circ$ on the Milky Way plane. We find clouds are not static while forming stars. Instead, molecular clouds continuously gain mass while star formation progresses. By performing principal component analysis on the cloud properties, we find that they evolve down two paths distinguished by their mass gain. Most clouds (80%) gain four times more mass as a function of $f_\text{IRB}$. The remaining 20% experience an extreme period of growth, growing in mass by a factor of 150 on average and during this period, they initially gain mass fast enough to outpace their star formation. For all clouds, it is only after half their area becomes star forming that mass loss occurs. We expect stellar feedback and potentially galactic shear is responsible. By analysing cloud positions, we suggest that the rate of mass growth may be linked to the larger galactic environment. Altogether, these results have strong implications on how we assess star forming ability on cloud scales when assuming molecular cloud masses are fixed in time.