We present spectroscopic and imaging observations of dust and gas emission from the western edge of the ρ Ophiuchus molecular cloud facing the B2 III/IV star HD 147889. The emissions from dust heated by the external UV radiation, from collisionally excited and fluorescent H2 are resolved and observed to coincide spatially. The spectroscopic data allows to estimate the gas temperature to 350 ± 30 K in the H2 emitting layer. In the framework of a steady state model of the photo-dissociation region, a high formation rate: 210−16cm3s−1 at 350 K, seems to be required to account for this temperature. For smaller formation rates the H2 emitting layer moves into the cloud where the gas is colder due to radiation attenuation.
ISO observations of the dust emission and H2 rotational lines are bringing a new perspective on the structure and physical conditions in regions of H2 photodissociation (PDRs) at the surface of molecular clouds illuminated by hot stars. Spectroscopic observations of bright PDRs such as NGC 2023 have allowed to build detailed excitation diagrams of H2 with numerous lines to test physical models (Draine this conference). In this paper, we present observations of a fainter PDR on the western edge of the ρ Ophiuchus molecular cloud heated by the B2III/IV star HD 147889. This is a nearby PDR (d = 135±15 pc from the star parallax) with an edge-on geometry where the observations allow to spatially resolve the layer of UV light penetration and of H2 photo-dissociation.