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

Abstract

We present images of C110$α$ and H110$α$ radio recombination line (RRL) emission at 4.8 GHz and images of H166$α$, C166$α$ and X166$α$ 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$α$ 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$α$ line emission is at the far side. The LSR velocity of C166$α$ 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 $μ$G and that in the far side PDR to be 220 $μ$G. Our field estimates compare well with the values obtained from OH Zeeman observations toward NGC 2024.

To appear in ApJ, 25 pages, 5 figures, 2 tables