2014ApJ...793...83R

We present images of C110α and H110α radio recombination line (RRL) emission at 4.8 GHz and images of H166α, C166α, and X166α RRL emission at 1.4 GHz, observed toward the star-forming region NGC 2024. The 1.4 GHz image with angular resolution ∼70'' is obtained using Very Large Array (VLA) data. The 4.8 GHz image with angular resolution ∼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 C110α line emission is confined to the southern rim of the H II region close to the ionization front whereas the C166α line emission is extended in the north-south direction across the H II region. The LSR velocity of the C110α line is 10.3 km/s 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 C110α line emission is at the far side of the H II region. The LSR velocity of C166α is 8.8 km/s. 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∼ 100 K, n_e^PDR ∼ 5/cm3, n_H∼ 1.7x10⁴/cm3, and path length l ∼ 0.06 pc, and those of the far side PDR are T_PDR∼ 200 K, n_e^PDR ∼ 50/cm3, n_H∼ 1.7x10⁵/cm3, and l ∼ 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 µG and that in the far side PDR to be 220 µG. Our field estimates compare well with the values obtained from OH Zeeman observations toward NGC 2024. The H166α spectrum shows narrow (1.7 km/s) and broad (33 km/s) line features. The narrow line has spatial distribution and central velocity (∼9 km/s) similar to that of the foreground carbon line emission, suggesting that they are associated. Modeling the narrow H166α emission provides physical properties T_PDR∼ 50 K, n_e^PDR ∼ 4/cm3, and l ∼ 0.01 pc and implies an ionization fraction of ∼10^–4. The broad H166α line originates from the H II region. The X166α 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 velocity separation between the X166α and C166α lines, we interpret that the X166α transition arises from sulfur.