On the duration of the embedded phase of star formation
  Feedback from massive stars plays a key role in molecular cloud evolution.
After the onset of star formation, the young stellar population is exposed by
photoionization, winds, supernovae, and radiation pressure from massive stars.
Recent observations of nearby galaxies have provided the evolutionary timeline
between molecular clouds and exposed young stars, but the duration of the
embedded phase of massive star formation is still ill-constrained. We measure
how long massive stellar populations remain embedded within their natal cloud,
by applying a statistical method to six nearby galaxies at 20-100 pc
resolution, using CO, Spitzer 24$\rm\,\mu m$, and H$\alpha$ emission as tracers
of molecular clouds, embedded star formation, and exposed star formation,
respectively. We find that the embedded phase (with CO and 24$\rm\,\mu m$
emission) lasts for $2{-}7$ Myr and constitutes $17{-}47\%$ of the cloud
lifetime. During approximately the first half of this phase, the region is
invisible in H$\alpha$, making it heavily obscured. For the second half of this
phase, the region also emits in H$\alpha$ and is partially exposed. Once the
cloud has been dispersed by feedback, 24$\rm\,\mu m$ emission no longer traces
ongoing star formation, but remains detectable for another $2{-}9$ Myr through
the emission from ambient CO-dark gas, tracing star formation that recently
ended. The short duration of massive star formation suggests that pre-supernova
feedback (photoionization and winds) is important in disrupting molecular
clouds. The measured timescales do not show significant correlations with
environmental properties (e.g. metallicity). Future JWST observations will
enable these measurements routinely across the nearby galaxy population.
