# The Lifetimes of Phases in High-Mass Star-Forming Regions

## TL;DR

This study estimates the relative and absolute lifetimes of starless and star-forming phases in high-mass star-forming regions using Herschel and Spitzer data, revealing most high-density gas is in a starless or embedded stage.

## Contribution

It introduces a systematic method to determine the relative and absolute lifetimes of phases in high-mass star formation based on large-scale survey data.

## Key findings

- Starless phase occupies 60-70% of the dense region lifetime.
- Star-forming phase occupies 30-40% of the dense region lifetime.
- Estimated absolute lifetimes are 0.2-1.7 Myr for starless and 0.1-0.7 Myr for star-forming phases.

## Abstract

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(H2) > 0.4-2.5 x 10^22 cm^-2, in the 'starless (no signature of stars > 10 Msun forming) and star-forming phases in a 2x2 degree region of the Galactic Plane centered at l=30deg. Of regions capable of forming high-mass stars on ~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 lifetimes 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.