VLA and GBT Observations of Orion B (NGC 2024, W12) : Photo-dissociation Region Properties and Magnetic field

TL;DR

This study uses radio recombination line observations from VLA and GBT to analyze the properties and magnetic fields of photo-dissociation regions in the Orion B star-forming region, revealing distinct near and far side PDR characteristics.

Contribution

It provides detailed modeling of PDR properties and magnetic fields in NGC 2024 using multi-frequency radio observations, highlighting the location and physical conditions of the PDRs.

Findings

Far side PDR has higher temperature and magnetic field strength.

Near side PDR is at the far side of the ext{HII} region, with specific physical parameters.

Magnetic field estimates agree with OH Zeeman measurements.

Abstract

We present images of C110$\alpha$ and H110$\alpha$ radio recombination line (RRL) emission at 4.8 GHz and images of H166$\alpha$, C166$\alpha$ and X166$\alpha$ RRL emission at 1.4 GHz, observed toward the starforming region NGC 2024. The 1.4 GHz image with angular resolution $\sim$ 70\arcsec\ is obtained using VLA data. The 4.8 GHz image with angular resolution $\sim$ 17\arcsec\ is obtained by combining VLA and GBT data. The similarity of the LSR velocity (10.3 \kms\) of the C110$\alpha$ line to that of lines observed from molecular material located at the far side of the \HII\ region suggests that the photo dissociation region (PDR) responsible for C110$\alpha$ line emission is at the far side. The LSR velocity of C166$\alpha$ is 8.8 \kms. This velocity is comparable with the velocity of molecular absorption lines observed from the foreground gas, suggesting that the PDR is at the near side of the \HII\ region. Non-LTE models for carbon line forming regions are presented. Typical properties of the foreground PDR are $T_{PDR} \sim 100$ K, $n_e^{PDR} \sim 5$ \cmthree, $n_H \sim 1.7 \times 10^4$ \cmthree, path length $l \sim 0.06$ pc and those of the far side PDR are $T_{PDR} \sim$ 200 K, $n_e^{PDR} \sim$ 50 \cmthree, $n_H \sim 1.7 \times 10^5$ \cmthree, $l \sim$ 0.03 pc. Our modeling indicates that the far side PDR is located within the \HII\ region. We estimate magnetic field strength in the foreground PDR to be 60 $\mu$G and that in the far side PDR to be 220 $\mu$G. Our field estimates compare well with the values obtained from OH Zeeman observations toward NGC 2024.