2015A&A...578A..41S

We have mapped the NGC 2023 reflection nebula in [CII] 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 CII region, which is ionized by FUV radiation from the B2 star HD37903. The [CII] 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 CII 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 [CII] emission. The [CII] lines are broader and show photo evaporating gas flowing into the CII region. Based on the strength of the [¹³CII] F=2-1 line, the [CII] 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 ∼90-120K, with densities of 10⁵-10⁶cm^–3, as deduced from RADEX modeling. Our PDR modeling indicates that the PDR layer where [CII] emission dominates has somewhat lower densities, 10⁴ to a few times 10⁵cm^–3.