C⁺ distribution around S 1 in ρ Ophiuchi

¹ Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400005, India
e-mail: bhaswati@tifr.res.in
² Institute for Astronomy, University of Hawaii, 640 N. Aohoku Place, Hilo, HI 96720, USA
³ USRA/SOFIA, NASA Ames Research Center, Mail Stop 232-12, Building N232, PO Box 1, Moffett Field, CA 94035-0001, USA
⁴ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received: 5 March 2018
Accepted: 8 April 2018

Abstract

We analyze a [C II] 158 μm map obtained with the L2 GREAT receiver on SOFIA of the reflection nebula illuminated by the early B star S 1 in the ρ Oph A cloud core. This data set has been complemented with maps of CO(3–2), ¹³CO(3–2), and C¹⁸O(3–2), observed as a part of the James Clerk Maxwell Telescope (JCMT) Gould Belt Survey, with archival HCO⁺(4–3) JCMT data, as well as with [O I] 63 and 145 μm imaging with Herschel/PACS. The [C II] emission is completely dominated by the strong emission from the photon dominated region (PDR) in the nebula surrounding S 1 expanding into the dense Oph A molecular cloud west and south of S 1. The [C II] emission is significantly blueshifted relative to the CO spectra and also relative to the systemic velocity, particularly in the northwestern part of the nebula. The [C II] lines are broader toward the center of the S 1 nebula and narrower toward the PDR shell. The [C II] lines are strongly self-absorbed over an extended region in the S 1 PDR. Based on the strength of the [¹³C II] F = 2–1 hyperfine component, [C II] is significantly optically thick over most of the nebula. CO and ¹³CO(3–2) spectra are strongly self-absorbed, while C¹⁸O(3–2) is single peaked and centered in the middle of the self-absorption. We have used a simple two-layer LTE model to characterize the background and foreground cloud contributing to the [C II] emission. From this analysis we estimated the extinction due to the foreground cloud to be ~9.9 mag, which is slightly less than the reddening estimated toward S 1. Since some of the hot gas in the PDR is not traced by low-J CO emission, this result appears quite plausible. Using a plane parallel PDR model with the observed [O I](145)/[C II] brightness ratio and an estimated FUV intensity of 3100–5000 G₀ suggests that the density of the [C II] emitting gas is ~3–4  × 10³ cm⁻³.