H2 infrared line emission across the bright side of the ρ Ophiuchi main cloud

We present imaging and spectroscopic observations of dust and gas (H-2) emission, obtained with ISO, from the western edge of the rho Ophiuchi molecular cloud illuminated by the B2 star HD147889 (chi similar to 400). This photodissociation region (PDR) is one of the nearest PDRs to the Sun (d = 135 +/- 15 pc from the stellar parallax) and the layer of UV light penetration and of H-2 emission is spatially resolved. It is therefore an ideal target to test the prediction of models on the integrated fluxes but also on the spatial distribution. The emission from dust heated by the external UV radiation, from collisionally excited and fluorescent H-2 are observed to coincide spatially. The spectroscopic data, obtained with ISO-SWS, allows us to estimate the gas temperature to be 300-345 K in the H-2 emitting layer, in which the ortho-to-para H2 ratio is about I or significantly smaller than the equilibrium ratio (similar to3 at that temperature). We interpret this data with an equilibrium PDR model. In this low excitation PDR, the gas heat budget is dominated by the contribution of the photoelectric heating from very small grains and polycyclic aromatic hydrocarbons (PAHs). With the standard PAH abundance ([C/H](PAH) similar or equal to5 x 10(-5)), we find that the H-2 formation rate R-f must be high in warm gas (similar to6 times the rate derived by Jura, 1975), in order to account for the observed H-2 emission. This result and the spatial coincidence between the PAHs and H-2 emission suggest that H-2 forms efficiently by chemisorption on the PAHs surface. If the latter interpretation is correct, the enhancement in Rf may also result from an increased PAH abundance: assuming that R-f scales with the PAH abundance, the observed H-2 excitation is well explained with R-f similar or equal to 1 x 10(-16) cm(3) s(-1) at T-gas = 330 K (similar to3 times the rate derived by Jura 1975) and [C/H](PAH) similar or equal to7.5 x 10(-5).