Astronomy and Astrophysics – Astrophysics
Scientific paper
Apr 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999a%26a...344..342l&link_type=abstract
Astronomy and Astrophysics, v.344, p.342-354 (1999)
Astronomy and Astrophysics
Astrophysics
63
Stars: Formation, Ism: Structure, Ism: General, Ism: Atoms, Ism: Clouds, Ism: Individual Objects: Rho Oph Cloud
Scientific paper
We present far infrared (45-195 mu m) spectrophotometric observations with the Iso-Lws of the active star forming rho Oph main cloud (L 1688). The [C ii] 158 mu m and [O i] 63 mu m lines were detected at each of the 33 positions observed, whereas the [O i] 145 mu m line was clearly seen toward twelve. The principal observational result is that the [C ii] 158 mu m 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 rho Ophc. The observed [C ii] 158 mu m/[O i] 63 mu m flux ratios are larger than unity everywhere. A comparison of the [C ii] 158 mu m 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 mu m photons (chi_ {_C II},>_{ ~ },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 rho Ophc, 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 (G_0 ~ , \powten{1} - \powten{2}) 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 T_gas and T_dust, in the surface layers of the rho Ophc. T_gas is relatively low, ~ ,60 K, but higher than T_dust ( ~ ,30 K), and densities are generally found within the interval (1-3) \powten{4} cm(-3) . These Pdr models are moderately successful in explaining the Lws observations. They correctly predict the [O i] 63 mu m and [C ii] 158 mu m line intensities and the observed absence of any molecular line emission. The models do fail, however, to reproduce the observed small [O i] 63 mu m/[O i] 145 mu m 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 rho Oph main cloud (2 pc(2) ) is ~ ,2 500 Msun, which implies that the star formation efficiency to date is <_{ ~ },4%, significantly lower than previous estimates.
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.
Caux Emmanuel
Kaas Anlaug Amanda
Larsson Bengt
Liseau Rene
Lorenzetti Dario
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