Detailed Magnetic Field Measurements in the Southern Coalsack
B. G. Andersson
The halo of the Southern Coalsack dark cloud has been shown to be a near-by
example of an O VI bearing hot-warm gas interface, as seen in sightlines
throughout the Galaxy and in the IGM. Because of its proximity and relative
ease of study, the Coalsack thus provides us with a unique laboratory for
understanding the details of such regions.
I will discuss some aspects of an ongoing multi-wavelength campaign studying
the hot envelpe of the Cloud. I will focus primarily on a determination
of the magnetic field-strength, using the Chandrasekhar-Fermi method, based on
a high sampling-density map of optical polarization of field stars. Accounting
for different line-of-sight components of polarization and the large-scale
structure over the face of the cloud, we derive a plane-of-the-sky B-field
of ~90 microGauss, much higher than would be expected from pressure equilibrium
considerations. I will address some possible consequences of this result,
including the effects on the thermal transport as measured from O VI and
X-ray observations.