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

Abstract

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.

The NASA/DLR Stratospheric Observatory for Infrared Astronomy (SOFIA) is jointly operated by the Universities Space Research Association, Inc. (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI) under DLR contract 50 OK 0901 to the University of Stuttgart.The Atacama Pathfinder Experiment (APEX) is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.Appendices are available in electronic form at http://www.aanda.orgData cubes as FITS files are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/578/A41