On the duration of the embedded phase of star formation
Jaeyeon Kim
Wednesday December 2nd, 15:45
Massive stars form in the densest regions of molecular clouds. During the earliest stage of star formation, young stars are still embedded in their natal cloud. After a few Myrs, the clouds are effectively disrupted by stellar feedback in the form of photoionization, winds, supernovae, and radiation pressure, exposing the young stellar population. While the evolutionary timeline between molecular clouds and young exposed stars has recently been measured in nearby galaxies, the duration of the embedded phase of massive star formation is still unknown. We apply a recently developed statistical method to Spitzer 24um, Halpha, and CO emission maps of six nearby galaxies, tracing the embedded star formation, exposed star formation, and molecular clouds, respectively. For the first time, this allows us to measure how long massive stellar populations remain embedded within their natal cloud. We find that the embedded phase of star formation, lasts for 2-7 Myr and constitutes ~30% of the cloud lifetime (the other ~70% being an inert phase). This embedded phase of star formation can be decomposed in a heavily obscured phase (with visible CO and 24um emissions, but no detectable Halpha emission) and a partially exposed phase (with visible CO, 24um, and Halpha emissions). The duration of the heavily obscured phase of star formation is on average 2.9 +/- 0.9 Myr, with little variations between galaxies. The duration of the partially exposed star-forming phase varies from 1 to 5 Myr. At the end of this phase, molecular clouds are dispersed by stellar feedback, leaving 24um emission detectable for another 1-6 Myr. The short duration between the onset of massive star formation and molecular cloud disruption suggests that pre-supernova feedback mechanisms such as photoionization and winds from massive stars play important an important role in disrupting molecular clouds. The duration of each phase does not show significant correlations with galactic environments such as metallicity, molecular gas surface density, and star formation rate surface density.