Molecular Cloud Evolution IV: Magnetic Fields, Ambipolar Diffusion, and the Star Formation Efficiency

TL;DR

This study models the formation and evolution of giant molecular clouds considering magnetic fields and ambipolar diffusion, revealing how these factors influence star formation rates, efficiencies, and magnetic flux ratios.

Contribution

It provides new insights into how magnetic criticality and ambipolar diffusion affect GMC collapse, star formation efficiency, and magnetic flux distribution during cloud evolution.

Findings

Supercritical inflow streams lead directly to collapse.

Star formation efficiency approaches a stationary state.

Magnetic flux ratios fluctuate and segregate within clouds.

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 (SFR) 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 initial magnetic field strength of 2, 3, and 4 $\mu\G$ show that only supercritical or marginal critical streams lead to reasonable star forming rates. 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. 5) In our simulations, the SFE approaches stationarity, because mass is added to the GMC at a similar rate at which it converts mass to stars. In such an approximately stationary regime, the SFE provides a proxy of the supercritical mass fraction in the cloud. 6) We observe the occurrence of buoyancy of the low-M2FR regions 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. (Abridged)