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Looking at the bright side of the Ophiuchi dark cloud *
Far infrared spectrophotometric observations of the Oph cloud with the ISO -LWS
R. Liseau 1,
G.J. White 2,
B. Larsson 1,
S. Sidher 3,
G. Olofsson 1,
A. Kaas 1,
L. Nordh 1,
E. Caux 4,
D. Lorenzetti 5,
S. Molinari 6,
B. Nisini 7 and
F. Sibille 8
Received 7 May 1998 / Accepted 4 December 1998
We present far infrared (45-195 µm) spectrophotometric observations with the ISO -LWS of the active star forming main cloud (L 1688). The [C II ] 158 µm and [O I ] 63 µm lines were detected at each of the 33 positions observed, whereas the [O I ] 145 µm line was clearly seen toward twelve.
The principal observational result is that the [C II ] 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 . The observed [C II ] 158 µm/[O I ] 63 µm flux ratios are larger than unity everywhere. A comparison of the [C II ] 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 [C II ] 158 µm photons ( 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 , 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 ( 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 PDR s to derive the particle density, n(H), and the temperature structures, for and , in the surface layers of the . is relatively low, 60 K, but higher than ( 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 [O I ] 63 µm and [C II ] 158 µm line intensities and the observed absence of any molecular line emission. The models do fail, however, to reproduce the observed small [O I ] 63 µm/[O I ] 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 main cloud (2 pc2) is 2 500 , which implies that the star formation efficiency to date is 4%, significantly lower than previous estimates.
Key words: stars: formation ISM: structure ISM: general ISM: atoms ISM: clouds ISM: individual objects: ae Oph cloud
* Based on observations with Iso, an Esa project with instruments funded by Esa Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) and with the participation of Isas and Nasa.
Send offprint requests to: R. Liseau
Online publication: March 10, 1999