The distribution of ionized, atomic and PDR gas around S1 in Rho Ophiuchus

Abstract

The early B star S 1 in the ρ Ophiuchus cloud excites an H ii region and illuminates a large egg-shaped photodissociation (PDR) cavity. The PDR is restricted to the west and south-west by the dense molecular ρ Oph A ridge, expanding more freely into the diffuse low density cloud to the north-east. We analyze new SOFIA GREAT, GMRT and APEX data together with archival data from Herschel/PACS, JCMT/HARPS to study the properties of the photo-irradiated ionized and neutral gas in this region. The tracers include [C ii] at 158 µm, [O i] at 63 and 145 µm, J=6-5 transitions of CO and 13 CO, HCO + (4-3), radio continuum at 610 and 1420 MHz and H i at 21 cm. The PDR emission is strongly red-shifted to the south-east of the nebula, and primarily blue-shifted on the north western side. The [C ii] and and [O i]63 spectra are strongly self-absorbed over most of the PDR. By using the optically thin counterparts, [ 13 C ii] and [O i]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 O 0 of the PDR to be ∼ 3 × 10 18 and ∼ 2 × 10 19 cm −2 , respectively. Comparison of stellar far-ultraviolet flux and reprocessed infrared radiation suggest enhanced clumpiness of the gas to the north-west. Analysis of the emission from the PDR gas suggests the presence of at least three density components consisting of high density (10 6 cm −3 ) clumps, medium density (10 4 cm −3 ) and diffuse (10 3 cm −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 south-eastern side of the cavity being denser on the front and the north-western side being denser on the rear.