Growing molecular clouds in a turbulent ISM

We investigate the lifetimes, accretion rates, and physical properties of a population of dense clouds with masses between 10^2 - 10^6 Msolar, formed in magnetohydrodynamical simulations of a box with 1 kpc^2 footprint and +-20 kpc height, containing a self-gravitating, stratified, random and clustered supernova (SN) driven turbulent interstellar medium (ISM). We compare our cloud population with that of the Galactic Ring Survey (GRS) cloud catalog. Prior to the self-gravitating period of the simulations, clouds are very long lived, ~100Myr, and have low velocity dispersions, sigma < 1 km/s, not correlated with cloud mass or size, disagreeing with the size- and mass-velocity dispersion Larson relations in both normalization and slope. Self-gravity is then turned on for about ~5 Myr after a multiphase-turbulent ISM has been stablished. During this time the dense clouds, with average densities of n > 50 cm^-3, begin collapsing and fragmenting, increasing their internal velocity dispersions and approaching the Larson's mass-velocity dispersion relation. On the other hand, the low density clouds remain practically unchanged, probably because self-gravity has not yet acted for as long as their large free fall timescales. We demonstrate that the combination of SN turbulence and self-gravity can reproduce basic observed characteristics of molecular clouds, providing us with a useful model to study the environmental effects in the formation and collapse of giant molecular clouds in the galaxy.

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