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