Molecular cloud evolution - IV. Magnetic fields, ambipolar diffusion and the star formation efficiency

Abstract

We investigate the formation and evolution of giant molecular clouds (GMCs) by the collision of convergent warm neutral medium (WNM) streams in the interstellar medium, in the presence of magnetic fields and ambipolar diffusion (AD), focusing on the evolution of the star formation rate and efficiency (SFE), as well as of the mass‐to‐magnetic‐flux ratio (M2FR) in the forming clouds. We find that (1) clouds formed by supercritical inflow streams proceed directly to collapse, while clouds formed by subcritical streams first contract and then re‐expand, oscillating on the scale of tens of Myr; (2) our suite of simulations with the initial magnetic field strength of 2, 3 and 4 μ G show that only supercritical or marginal critical streams lead to reasonable star forming rates. This result is not altered by the inclusion of AD; (3) the GMC’s M2FR is a generally increasing function of time, whose growth rate depends on the details of how mass is added to the GMC from the WNM; (4) the M2FR is a highly fluctuating function of position in the clouds. This implies that a significant fraction of a cloud’s mass may remain magnetically supported, while SF occurs in the supercritical regions that are not supported; (5) in our simulations, the SFE approaches stationarity, because mass is added to the GMC at a similar rate to which it converts mass to stars. In such an approximately stationary regime, we find that the SFE provides a proxy of the supercritical mass fraction in the cloud; and (6) the low‐M2FR regions exhibit buoyancy within the gravitationally contracting GMCs, so that the latter naturally segregate into a high‐density, high‐M2FR ‘core’ and a low‐density, low‐M2FR ‘envelope’, without the intervention of AD.

Magnetic field amplification in turbulent astrophysical plasmas

“…The interstellar medium of galaxies and molecular cloud formation The importance of magnetic fields for the formation of molecular clouds has been investigated in several works (Passot, Vazquez-Semadeni & Pouquet 1995;Banerjee et al 2009;Heitsch, Stone & Hartmann 2009;Vázquez-Semadeni et al 2011;Seifried et al 2011). Recently, Körtgen & Banerjee (2015) find suppression of star formation by moderate magnetic fields of the order of 3 µG, emphasising the role that the magnetic field plays in regulating the formation of stars.…”

IMPLEMENTATION OF SINK PARTICLES IN THE ATHENA CODE

“…As the sink particle creation criteria of Krumholz et al (2004) do not specifically limit particle creation to a single collapse center, they adopt the approach of merging the spurious sink particles within a given collapsing region so that one final sink particle is created inside each potential well. The extra criteria-checks for a local potential minimum or a gravitationally bound state -adopted by Banerjee et al (2009), Federrath et al (2010 and Vázquez-Semadeni et al (2011) further limit the initial creation of sink particles, so that only one sink particle is created for each local potential minimum. We shall adopt similar criteria to Federrath et al (2010) to ensure that single sink particles are created inside regions that are collapsing.…”

“…Banerjee et al (2009) andVázquez-Semadeni et al (2011) adopt criteria that first checks whether density exceeds a threshold density and then checks if the zone in question is a local gravitational potential minimum. Bate et al (1995) and Federrath et al (2010) perform a series of checks including a density threshold check, a converging flow check, a local gravitational potential minimum check, and additional checks that evaluate whether a region is strongly self-gravitating.…”

“…Krumholz et al (2004), Wang et al (2010), Dubois et al (2010), Teyssier et al (2011) adopt the same methodology, including the criteria for creation of sink particles and the accretion rate onto sink particles. Vázquez-Semadeni et al (2011) used an early version of the implementation in FLASH described by Federrath et al (2010). In these implementations, Krumholz et al (2004), Federrath et al (2010), Wang et al (2010), Padoan & Nordlund (2011), Vázquez-Semadeni et al (2011) mainly focus on star formation simulations; Dubois et al (2010), Teyssier et al (2011) create sink particles to replace super-massive black holes in cosmological simulations.…”

The formation and destruction of molecular clouds and galactic star formation

“…Inoue & Inutsuka (2012) further investigated the formation of molecular clouds in the magnetized ISM and revealed the formation of a magnetized molecular cloud by the accretion of HI clouds created through thermal instability. Since the mean density of the initial multiphase HI medium is an order of magnitude higher than the typical warm neutral medium (WNM) density, this formation timescale is shorter than that of molecular cloud formation solely through the accumulation of diffuse WNM (see, e.g., Koyama & Inutsuka 2002;Hennebelle et al 2008;Heitsch & Hartmann 2008;Banerjee et al 2009;Vázquez-Semadeni et al 2011, for the cases of WNM flows). The resulting timescale of molecular cloud formation of 10 Myr is consistent with the evolutionary timescale of molecular clouds in the Large Magellanic Cloud (LMC; Kawamura et al 2009).…”