1999A&A...344..342L

We present far infrared (45-195µm) spectrophotometric observations with the Iso-Lws of the active star forming ρ Oph main cloud (L1688). The [CII]158µm and [OI]63µm lines were detected at each of the 33 positions observed, whereas the [OI]145µm line was clearly seen toward twelve. The principal observational result is that the [CII]158µm line fluxes exhibit a clear correlation with projected distance from the dominant stellar source in the field (HD147889). We interpret this in terms of Pdr-type emission from the surface layers of the ρ Oph cloud. The observed [CII]158µm/[OI]63µm flux ratios are larger than unity everywhere. A comparison of the [CII]158µm line emission and the Fir dust continuum fluxes yields estimates of the efficiency at which the gas in the cloud converts stellar to [CII]158µm photons (χ_CIIγ0.5%). We first develop an empirical model, which provides us with a three dimensional view of the far and bright side of the dark ρ Oph 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 T_gas and T_dust, in the surface layers of the ρ Oph cloud. T_gas is relatively low, ∼60K, but higher than T_dust (∼30K), and densities are generally found within the interval (1-3)10⁴cm^–3. These Pdr models are moderately successful in explaining the Lws observations. They correctly predict the [OI]63µm and [CII]158µm 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]145µm 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 ρ Oph main cloud (2pc²) is ∼2500M_☉, which implies that the star formation efficiency to date is ≲4%, significantly lower than previous estimates.