CN Zeeman and dust polarization in a high-mass cold clump

TL;DR

This study measures magnetic fields in a young high-mass star-forming clump using Zeeman effect and dust polarization, revealing strong magnetic support and highlighting challenges in magnetic field measurement techniques.

Contribution

It provides the first combined line-of-sight and plane-of-sky magnetic field measurements in a high-mass star-forming region, demonstrating the reliability of CN as a dense gas tracer.

Findings

Magnetic field strength is approximately 1640 microG for pressure equilibrium.

Plane-of-sky magnetic field is about 740 microG, consistent with pressure balance.

CN molecules are effective tracers without significant depletion in cold, dense cores.

Abstract

We report on the young massive clump (G35.20w) in W48 that previous molecular line and dust observations have revealed to be in the very early stages of star formation. Based on virial analysis, we find that a strong field of ~1640 microG is required to keep the clump in pressure equilibrium. We performed a deep Zeeman effect measurement of the 113 GHz CN (1-0) line towards this clump with the IRAM 30 m telescope. We combine simultaneous fitting of all CN hyperfines with Monte Carlo simulations for a large range in realization of the magnetic field to obtain a constraint on the line-of-sight field strength of -687 +/- 420 microG. We also analyze archival dust polarization observations towards G35.20w. A strong magnetic field is implied by the remarkably ordered field orientation that is perpendicular to the longest axis of the clump. Based on this, we also estimate the plane-of-sky component of the magnetic field to be ~740 microG. This allows for a unique comparison of the two orthogonal measurements of magnetic field strength of the same region and at similar spatial scales. The expected total field strength shows no significant conflict between the observed field and that required for pressure equilibrium. By producing a probability distribution for a large range in field geometries, we show that plane-of-sky projections are much closer to the true field strengths than line-of-sight projections. This can present a significant challenge for Zeeman measurements of magnetized structures, even with ALMA. We also show that CN molecule does not suffer from depletion on the observed scales in the predominantly cold and highly deuterated core in an early stage of high-mass star formation and is thus a good tracer of the dense gas.