2018A&A...616A..31M

We analyze a [CII] 158µm 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 [OI] 63 and 145 µm imaging with Herschel/PACS. The [CII] 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 [CII] 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 [CII] lines are broader toward the center of the S 1 nebula and narrower toward the PDR shell. The [CII] lines are strongly self-absorbed over an extended region in the S 1 PDR. Based on the strength of the [¹³CII] F=2-1 hyperfine component, [CII] 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 [CII] emission. From this analysis we estimated the extinction due to the foreground cloud to be ∼9.9mag, 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 [OI](145)/[CII] brightness ratio and an estimated FUV intensity of 3100-5000G₀ suggests that the density of the [CII] emitting gas is ∼3-4x10³cm^–3.