On the magnetic field through the Upper Centaurs-Lupus Super bubble, in the vicinity of the Southern Coalsack

TL;DR

This study investigates the magnetic field environment of the Southern Coalsack within the Upper Centaurus-Lupus super bubble using Faraday Rotation measurements, revealing a significant contrast between the cloud's internal magnetic field and the surrounding super bubble.

Contribution

It provides the first Faraday Rotation measurements near the Coalsack, contrasting optical polarimetry estimates with magnetic field strength derived from pulsar data, highlighting the influence of the super bubble.

Findings

Measured magnetic field in the super bubble is ~1.1 μG, much weaker than the 64-93 μG estimated inside the cloud.

The magnetic field contrast suggests the super bubble significantly affects the cloud's magnetic environment.

Results imply the super bubble's influence may suppress magnetic field amplification within the cloud.

Abstract

The Southern Coalsack is located in the interior of the Upper Centaurus-Lupus (UCL) super bubble and shows many traits that point to a much more energetic environment than might be expected from a dark, starless molecular cloud. A hot, X-ray emitting, envelope surrounds the cloud, it has a very strong internal magnetic field and its darkest core seems to be on astronomical time scales "just about" to start forming stars. In order to probe the magnetic environment of the cloud and to compare with the optical/near infrared polarimetry-based field estimates for the cloud, we have acquired Faraday Rotation measurements towards the pulsar PSR J1210$-$6550, probing the magnetic field in the vicinity of the cloud, and a comparison target, PSR J1435$-$5954, at a similar line of sight distance but several degrees from the cloud. Both lines of sight hence primarily probe the UCL super bubble. The earlier estimates of the magnetic field inside the Coalsack, using the Chandrasekhar-Fermi method on optical and near-infrared polarimetry, yield B$_\perp$ = 64--93 $\mu$G. However, even though PSR J1210$-$6550 is located only $\sim$30 arc minutes from the (CO) edge of the cloud, the measured field strength is only B$_\parallel$ = 1.1$\pm$0.2 $\mu$G. While thus yielding a very high field contrast to the cloud we argue that this might be understood as due to the effects on the cloud by the super bubble.