High spectral and spatial resolution observations of the PDR emission in the NGC 2023 reflection nebula with SOFIA and APEX

We have mapped the NGC 2023 reflection nebula in [C ii] and CO(11−10) with the heterodyne receiver GREAT on SOFIA and obtained slightly smaller maps in 13CO(3−2), CO(3−2), CO(4−3), CO(6−5), and CO(7−6) with APEX in Chile. We use these data to probe the morphology, kinematics, and physical conditions of the C ii region, which is ionized by FUV radiation from the B2 star HD 37903. The [C ii] emission traces an ellipsoidal shell-like region at a position angle of ∼−50◦, and is surrounded by a hot molecular shell. In the southeast, where the C ii region expands into a dense, clumpy molecular cloud ridge, we see narrow and strong line emission from high-J CO lines, which comes from a thin, hot molecular shell surrounding the [C ii] emission. The [C ii] lines are broader and show photo evaporating gas flowing into the C ii region. Based on the strength of the [13C ii] F = 2−1 line, the [C ii] line appears to be somewhat optically thick over most of the nebula with an optical depth of a few. We model the physical conditions of the surrounding molecular cloud and the PDR emission using both RADEX and simple PDR models. The temperature of the CO emitting PDR shell is ∼90−120 K, with densities of 105−106 cm−3, as deduced from RADEX modeling. Our PDR modeling indicates that the PDR layer where [C ii] emission dominates has somewhat lower densities, 104 to a few times 105 cm−3.