The Importance of the Magnetic Field from an SMA-CSO-Combined Sample of Star-Forming Regions

TL;DR

This study analyzes polarization data from 50 star-forming regions to quantify magnetic field importance, revealing a correlation between magnetic field misalignment and the magnetic tension-to-gravity force ratio, which helps assess cloud stability.

Contribution

It introduces a sample-based scaling relation between magnetic field misalignment and the force ratio, enabling estimation of magnetic influence solely from polarization measurements.

Findings

Magnetic field tends to be aligned or perpendicular to source axes.

Distribution of misalignment angles varies between SMA and CSO samples.

A scaling relation links misalignment to the magnetic tension-to-gravity force ratio.

Abstract

Submillimeter dust polarization measurements of a sample of 50 star-forming regions, observed with the SMA and the CSO covering pc-scale clouds to mpc-scale cores, are analyzed in order to quantify the magnetic field importance. The magnetic field misalignment $\delta$ -- the local angle between magnetic field and dust emission gradient -- is found to be a prime observable, revealing distinct distributions for sources where the magnetic field is preferentially aligned with or perpendicular to the source minor axis. Source-averaged misalignment angles $\langle|\delta|\rangle$ fall into systematically different ranges, reflecting the different source-magnetic field configurations. Possible bimodal $\langle|\delta|\rangle$-distributions are found for the separate SMA and CSO samples. Combining both samples broadens the distribution with a wide maximum peak at small $\langle|\delta|\rangle$-values. Assuming the 50 sources to be representative, the prevailing source-magnetic field configuration is one that statistically prefers small magnetic field misalignments $|\delta|$. When interpreting $|\delta|$ together with an MHD force equation, as developed in the framework of the polarization-intensity gradient method, a sample-based scaling fits the magnetic field tension-to-gravity force ratio $\langle\Sigma_B\rangle$ versus $\langle|\delta|\rangle$ with $\langle\Sigma_B\rangle = 0.116 \cdot \exp(0.047\cdot \langle|\delta|\rangle)\pm 0.20$ (mean error), providing a way to estimate the relative importance of the magnetic field, only based on measurable field misalignments $|\delta|$. The force ratio $\Sigma_B$ discriminates systems that are collapsible on average ($\langle \Sigma_B\rangle <1$) from other molecular clouds where the magnetic field still provides enough resistance against gravitational collapse ($\langle \Sigma_B\rangle >1$) (abridged).