Near-IR Imaging Polarimetry toward a Bright-Rimmed Cloud: Magnetic Field in SFO 74

TL;DR

This study uses near-infrared polarimetry to map magnetic fields in SFO 74, revealing enhanced magnetic strength near the rim likely caused by UV radiation-induced shocks, influencing the cloud's morphology.

Contribution

It provides the first detailed magnetic field map of SFO 74's bright rim and demonstrates magnetic field enhancement due to UV radiation effects.

Findings

Magnetic field runs along the bright rim, differing from ambient field.

Magnetic field strength is higher (~90 μG) near the tip rim than farther inside (~30 μG).

Magnetic pressure is comparable to turbulent pressure near the rim.

Abstract

We have made near-infrared (JHKs) imaging polarimetry of a bright-rimmed cloud (SFO 74). The polarization vector maps clearly show that the magnetic field in the layer just behind the bright rim is running along the rim, quite different from its ambient magnetic field. The direction of the magnetic field just behind the tip rim is almost perpendicular to that of the incident UV radiation, and the magnetic field configuration appears to be symmetric as a whole with respect to the cloud symmetry axis. We estimated the column and number densities in the two regions (just inside and far inside the tip rim), and then derived the magnetic field strength, applying the Chandrasekhar-Fermi method. The estimated magnetic field strength just inside the tip rim, ~90 uG, is stronger than that far inside, ~30 uG. This suggests that the magnetic field strength just inside the tip rim is enhanced by the UV radiation induced shock. The shock increases the density within the top layer around the tip, and thus increases the strength of the magnetic field. The magnetic pressure seems to be comparable to the turbulent one just inside the tip rim, implying a significant contribution of the magnetic field to the total internal pressure. The mass-to-flux ratio was estimated to be close to the critical value just inside the tip rim. We speculate that the flat-topped bright rim of SFO 74 could be formed by the magnetic field effect.