2021A&A...648A..40M

The early B star S 1 in the ρ Ophiuchus cloud excites an HII region and illuminates a large egg-shaped photon-dominated (PDR) cavity. The PDR is restricted to the west and southwest by the dense molecular ρ Oph A ridge, expanding more freely into the diffuse low-density cloud to the northeast. We analyzed new SOFIA GREAT, GMRT, and APEX data together with archival data from Herschel/PACS and JCMT/HARP to study the properties of the photo-irradiated ionized and neutral gas in this region. The tracers include [CII] at 158µm, [OI] at 63 and 145µm, the J=6-5 transitions of CO and ¹³CO, HCO⁺ (4-3), the radio continuum at 610 and 1420MHz, and HI at 21cm. The PDR emission is strongly redshifted to the southeast of the nebula, and primarily blueshifted on the northwestern side. The [CII] and [OI]63 spectra are strongly self-absorbed over most of the PDR. By using the optically thin counterparts, [¹³CII] and [OI]145 respectively, we conclude that the self-absorption is dominated by the warm (>80 K) foreground PDR gas and not by the surrounding cold molecular cloud. We estimate the column densities of C⁺ and O⁰ of the PDR to be ∼3x10¹⁸ and ∼2x10¹⁹cm^–2, respectively. Comparison of stellar far-ultraviolet flux and reprocessed infrared radiation suggest enhanced clumpiness of the gas to the northwest. Analysis of the emission from the PDR gas suggests the presence of at least three density components consisting of high-density (10⁶cm^–3) clumps, medium-density (10⁴cm^–3) and diffuse (10³cm^–3) interclump medium. The medium-density component primarily contributes to the thermal pressure of the PDR gas, which is in pressure equilibrium with the molecular cloud to the west. Emission velocities in the region suggest that the PDR is tilted and somewhat warped, with the southeastern side of the cavity being denser at the front and the northwestern side being denser at the rear.