Very Large Array and Green Bank Telescope Observations of Orion B (NGC 2024, W12): Photodissociation Region Properties and Magnetic Field

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 star-forming region NGC 2024. The 1.4 GHz image with angular resolution similar to 70 " is obtained using Very Large Array (VLA) data. The 4.8 GHz image with angular resolution similar to 17 " is obtained by combining VLA and Green Bank Telescope data in order to add the short and zero spacing data in the uv plane. These images reveal that the spatial distributions of C110 alpha line emission is confined to the southern rim of the H II region close to the ionization front whereas the C166 alpha line emission is extended in the north-south direction across the H II region. The LSR velocity of the C110 alpha line is 10.3 km s(-1) similar to that of lines observed from molecular material located at the far side of the H II region. This similarity suggests that the photodissociation region (PDR) responsible for C110 alpha line emission is at the far side of the H II region. The LSR velocity of C166 alpha is 8.8 km s(-1). 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 H II region. Non-LTE models for carbon line-forming regions are presented. Typical properties of the foreground PDR are T-PDR similar to 100 K, n(e)(PDR) similar to 5 cm(-3), n(H) similar to 1.7 x 10(4) cm(-3), and path length l similar to 0.06 pc, and those of the far side PDR are T-PDR similar to 200 K, n(e)(PDR) similar to 50 cm(-3), n(H) similar to 1.7 x 10(5) cm(-3), and l similar to 0.03 pc. Our modeling indicates that the far side PDR is located within the H II region. We estimate the 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. The H166 alpha spectrum shows narrow (1.7 km s(-1)) and broad (33 km s(-1)) line features. The narrow line has spatial distribution and central velocity (similar to 9 km s(-1)) similar to that of the foreground carbon line emission, suggesting that they are associated. Modeling the narrow H166 alpha emission provides physical properties TPDR similar to 50 K, n(e)(PDR) similar to 4 cm(-3), and l similar to 0.01 pc and implies an ionization fraction of similar to 10(-4). The broad H166 alpha line originates from the H II region. The X166 alpha line has a different spatial distribution compared to other RRLs observed toward NGC 2024 and is probably associated with cold dust clouds. Based on the expected low depletion of sulfur in such clouds and the -8.1 km s(-1) velocity separation between the X166 alpha and C166 alpha lines, we interpret that the X166 alpha transition arises from sulfur.