The formation and destruction of molecular clouds and galactic star formation

Abstract

We describe an overall picture of galactic-scale star formation. Recent high-resolution magneto-hydrodynamical simulations of twofluid dynamics with cooling, heating, and thermal conduction have shown that the formation of molecular clouds requires multiple episodes of supersonic compression. This finding enables us to create a scenario in which molecular clouds form in interacting shells or bubbles on a galactic scale. First, we estimated the ensemble-averaged growth rate of molecular clouds on a timescale longer than a million years. Next, we performed radiation hydrodynamics simulations to evaluate the destruction rate of magnetized molecular clouds by the stellar far-ultraviolet radiation. We also investigated the resulting star formation efficiency within a cloud, which amounts to a low value (a few percent) if we adopt the power-law exponent ∼− 2.5 for the mass distribution of stars in the cloud. We finally describe the time evolution of the mass function of molecular clouds on a long timescale (>1 Myr) and discuss the steady state exponent of the power-law slope in various environments.

Filamentary structure and magnetic field orientation in Musca

“…This then gives rise to a bimodal distribution of filamentary structures as observed in Musca. Recent numerical MHD simulations of molecular cloud formation tend to reproduce this pattern (Chen & Ostriker 2014;Inutsuka et al 2015, for example). The pattern observed in Musca is reminiscent of the situation seen around the Taurus B211/3 filament system, where velocity information is consistent with accretion of low-density striations onto the denser B211/3 filament (Goldsmith et al 2008;Palmeirim et al 2013).…”

“…First, large-scale compression of interstellar material due to, for example, an expanding bubble locally creates a finite-sized, self-gravitating layer with a magnetic field primarily in the plane of the dense layer of compressed gas (Inutsuka et al 2015, for example). Such a dense, self-gravitating layer is expected to fragment into one or several thermally supercritical filaments oriented perpendicular to the mean direction of the magnetic field (cf.…”

Molecular gas in high-mass filament WB673

“…We calculate the column density of CS molecules N CS in LTE using standard approach described by Mangum, Shirley (2015), Eq.80. In the beginning N CS is calculated under assumption that CS (2-1) line is optically thin but then optical depth correction factor, Goldsmith, Langer (1999), is used to make the value of N CS reliable. For diatomic linear molecules:…”

“…Theoretical calculations, e.g. Inutsuka et al (2015), predict formation of filamentary molecular clouds after multiple compression of interstellar gas by supersonic waves. So the periphery of extended bubbles (H regions or supernova remnants) could be reliable places for the formation of the molecular filaments.…”

“…So the filamentary structure could be an interesting example to test theoretical models by e.g. Inutsuka et al (2015). The aim of our study is to look for a dense gas in the inter-clump medium of the filamentary structure and check if it is a real physically connected structure or the clumps are separated from each other but embedded into a common gas envelope.…”