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

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 $⟨|\delta |⟩$ fall into systematically different ranges, reflecting the different source-magnetic field configurations. Possible bimodal $⟨|\delta |⟩$-distributions are found for the separate SMA and CSO samples. Combining both samples broadens the distribution with a wide maximum peak at small $⟨|\delta |⟩$-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 $⟨{\Sigma }_B⟩$ versus $⟨|\delta |⟩$ with $⟨{\Sigma }_B⟩=0.116\cdot \exp (0.047\cdot ⟨|\delta |⟩)\pm 0.20$ (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 ($⟨{\Sigma }_B⟩<1$) from other molecular clouds where the magnetic field still provides enough resistance against gravitational collapse ($⟨{\Sigma }_B⟩>1$) (abridged).

arXiv comment43 pages, 11 figures, 2 tables; accepted for publication in ApJ

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