EPoS Contribution
EPoS Contribution
A new Archetype for Molecular Cloud Dynamical Evolution

Peter Barnes
U Florida, Gainesville, US
The formation, evolution, and lifetime of molecular clouds, and the corresponding timescale for star formation (SF) within them, has been debated for over 40 years. The chief issue has been whether SF is "fast" (meaning clouds form stars in just a few free-fall times, and are promptly disrupted by feedback) or "slow" (meaning that clouds exist for many free-fall times, and persist in one form or another despite SF). We describe a consistent set of results from the CHaMP+ThrUMMS surveys, based on the demographics of molecular clouds, which seems to reconcile these two perspectives. This includes new mass conversion laws and SF tracers that reveal 2-3× more mass available for SF in molecular clouds than previously deduced, a long latency period before SF begins in a given cloud, and a locally semi-random mass accretion/dispersal pattern onto/from molecular clumps, but one which is a globally strong function of mass surface density Σ. Over a long enough time period, mass accretion dominates over dispersal, but the growth is near exponential, meaning that observable SF above a presumed Σ threshold is concentrated in time to the last few Myr. We are beginning to test this new observational archetype with numerical models, including forward-modelling of radiative transfer in CO isotopologues (upon which this interpreation is based). We encourage other groups to explore this physics as well. If our interpretation is confirmed, it would resolve this long-standing debate about molecular cloud lifetimes.
Caption: (Left panel) Sample map for CHaMP clump BYF47 (white ellipse) of accretion and dispersal timscales per pixel, overlaid by black contours of CO column density. The colour scale is adjusted so that zero values are a dusky blue, providing an intuitive scheme to visually distinguish accretive motions (cyan and warmer colours) from dispersive ones (cooler blues). (Right panel) Accretion timescale taccr vs mass surface density Σ across all 303 CHaMP clumps. Overlaid as red and green lines are trends in the mean±1σ in log(taccr) for small bins of log(Σ), while a least-squares fit to the binned values is as labelled. This fit has a reduced χ2 = 0.17 and Pearson's correlation coefficient r2 = 0.993. The dotted horizontal line is set at t = 1 Myr. The fit implies a mass accretion timescale for observable star formation of ~40 Myr, but where the fastest mass accretion occurs at the end of this time. Adapted from Barnes et al 2018, submitted.
Collaborators:
A. Hernandez, U Wisconsin, US
E. Muller, U Macquarie, AU
R. Pitts, U Florida, US
W. Schap, U Florida, US
S. Jeram, U Florida, US
S. Lopez, U Florida, US
D. Barnes, EHS, US
Key publication

Suggested Session: Molecular clouds