High spectral and spatial resolution observations of the PDR emission in the NGC 2023 reflection nebula with SOFIA and APEX^{⋆,⋆⋆,⋆⋆⋆,⋆⋆⋆⋆,†}

¹ SOFIA-USRA, NASA Ames Research Center, MS 232-12, Building N232, Rm. 146, PO Box 1, Moffett Field, CA 94035-0001, USA
e-mail: Goran.H.Sandell@nasa.gov
² Tata Institute of Fundamental Research, Homi Bhabha Road, 400005 Mumbai, India
e-mail: bhaswati@tifr.res.in
³ Max Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany

Received: 12 February 2015
Accepted: 12 April 2015

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.