Liseau, R.; White, G. J.; Larsson, B.; Sidher, S.; Olofsson, G.; Kaas, A.; Nordh, L.; Caux, E.; Lorenzetti, D.; Molinari, S.; Nisini, B. and Sibille, F.
Looking at the bright side of the ρ Ophiuchi dark cloud. Far infrared spectrophotometric observations of the ρ Oph cloud with the ISO-LWS.
Astronomy and Astrophysics, 344 pp. 342–354.
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We present far infrared (45-195 μm) spectrophotometric observations with the ISO-LWS of the active star forming ρOph main cloud (L 1688). The [CII] 158 μm and [OI] 63 μm lines were detected at each of the 33 positions observed, whereas the [OI] 145 μm line was clearly seen toward twelve. The principal observational result is that the [CII] 158 μm line fluxes exhibit a clear correlation with projected distance from the dominant stellar source in the field (HD 147889). We interpret this in terms of PDR-type emission from the surface layers of the ρOph cloud. The observed [CII] 158 μm/[OI] 63 μm flux ratios are larger than unity everywhere. A comparison of the [CII] 158 μm line emission and the FIR dust continuum fluxes yields estimates of the efficiency at which the gas in the cloud converts stellar to [CII] 158 μm photons (χCII≳0.5%).
We first develop an empirical model, which provides us with a three dimensional view of the far and bright side of the dark ρOph cloud, showing that the cloud surface towards the putative energy source is concave. This model also yields quantitative estimates of the incident flux of ultraviolet radiation (G0 ~ 101 - 102) and of the degree of clumpiness/texture of the cloud surface (filling of the 80" beam ~0.2).
Subsequently, we use theoretical models of PDRS to derive the particle density, n(H), and the temperature structures, for Tgas and Tdust, in the surface layers of the ρOph cloud. Tgas is relatively low, ~60 K, but higher than Tdust ( ~30 K), and densities are generally found within the interval (1-3) 104 cm-3 . These PDR models are moderately successful in explaining the LWS observations. They correctly predict the [OI] 63 μm and [CII] 158 μm line intensities and the observed absence of any molecular line emission. The models do fail, however, to reproduce the observed small [OI] 63 μm/[OI] 145 μm ratios. We examine several possible explanations, but are unable to uniquely identify (or to disentangle) the cause(s) of this discrepancy.
From pressure equilibrium arguments we infer that the total mass of the ρOph main cloud (2pc2) is ~2 500 M⊙, which implies that the star formation efficiency to date is ≲4%, significantly lower than previous estimates.
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