The Magnetic Keys to Massive Star Formation: The Western <i>η</i> Carinae Giant Molecular Cloud

Abstract We present SOFIA/HAWC+ continuum polarization data on the magnetic fields ( B fields) threading 17 pc-scale massive molecular clumps at the western end of the η Carinae giant molecular cloud (Region 9 of the Galactic Census of High- and Medium-mass Protostars, or CHaMP, representing all stages of star formation from pre-stellar to dispersing via feedback), revealing important details about the field morphology and role in the gas structures of this clump sample. We performed Davis–Chandrasekhar–Fermi (DCF) and histogram of relative orientation (HRO) analyses tracing column densities 25.0 &lt; log( N /m −2 ) &lt; 27.2. With HRO, B fields change from mostly parallel to column density structures to mostly perpendicular at a threshold N crit = (3.7 ± 0.6) × 10 26 m −2 , indicating that gravitational forces exceed magnetic forces above this value. The same analysis in 10 individual clumps gives similar results, with the same clear trend in alignments and a threshold N crit = (1.9 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) × 10 26 m −2 . In the other seven clumps, the alignment trend with N is much flatter or even reversed, inconsistent with the usual HRO pattern. Instead, these clumps’ fields reflect external environmental forces from the nearby H ii region NGC 3324. DCF analysis reveals field strengths somewhat higher than typical of nearby clouds, with the B–n data lying mostly above the R. M. Crutcher relation. The mass-to-flux ratio λ across all clumps has a Gaussian distribution, log λ DCF = –0.75 ± 0.45; only small areas are dominated by gravity. However, a significant trend of rising log λ with falling T dust parallels R. L. Pitts et al.’s result: T dust falls as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>N</mml:mi> <mml:msub> <mml:mrow/> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msub> </mml:math> rises toward clump centres. In this massive clump sample, B fields provide enough support against gravity to explain their overall low star formation rate.