High Spectral and Spatial Resolution Observations of the PDR Emission in the NGC2023 Reflection Nebula with SOFIA and APEX
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Göran Sandell
SOFIA Science Center
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We have mapped the NGC2023 reflection nebula (distance 350 pc) 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 regions, which are ionized by FUV radiation from the B2 star HD 37903. The [CII] emission is dominated by PDR emission from an expanding ellipsoidal CII region surrounded by a thin, hot molecular shell. In the southeast, where the CII region expands into a dense, clumpy, molecular cloud ridge, we see narrow, and strong emission in high J CO lines with a temperature of about 85 K or higher. The [CII] lines are broader and show photo evaporating gas flowing into the CII region. Based on the strength of [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. The [CII] and the high J CO lines show double-peaked line profiles, not because of self-absorption, but because the emission originates from the front and the back side of the expanding CII region. We model the physical conditions of the surrounding molecular cloud and the PDR emission using both RADEX and simple PDR models.

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