High-mass stars form within star clusters from dense , molecular regions , but is the process of cluster formation slow and hydrostatic or quick and dynamic ?
We link the physical properties of high-mass star-forming regions with their evolutionary stage in a systematic way , using Herschel and Spitzer data .
In order to produce a robust estimate of the relative lifetimes of these regions , we compare the fraction of dense , molecular regions above a column density associated with high-mass star formation , N ( H _ { 2 } ) > 0.4-2.5 \times 10 ^ { 22 } cm ^ { -2 } , in the ‘ starless ’ ( no signature of stars \gtrsim 10 M _ { \odot } Â forming ) and star-forming phases in a 2 ^ { \circ } \times 2 ^ { \circ } Â region of the Galactic Plane centered at \ell =30 ^ { \circ } .
Of regions capable of forming high-mass stars on \sim 1 pc scales , the starless ( or embedded beyond detection ) phase occupies about 60-70 % of the dense molecular region lifetime and the star-forming phase occupies about 30-40 % .
These relative lifetimes are robust over a wide range of thresholds .
We outline a method by which relative lifetimes can be anchored to absolute lifetimes from large-scale surveys of methanol masers and UCHII regions .
A simplistic application of this method estimates the absolute lifetime of the starless phase to be 0.2-1.7 Myr ( about 0.6-4.1 fiducial cloud free-fall times ) and the star-forming phase to be 0.1-0.7 Myr ( about 0.4-2.4 free-fall times ) , but these are highly uncertain .
This work uniquely investigates the star-forming nature of high-column density gas pixel-by-pixel and our results demonstrate that the majority of high-column density gas is in a starless or embedded phase .