2003A&A...397..623H

We present imaging and spectroscopic observations of dust and gas (H₂) emission, obtained with ISO, from the western edge of the ρ Ophiuchi molecular cloud illuminated by the B2 star HD147889 (χ∼400). This photodissociation region (PDR) is one of the nearest PDRs to the Sun (d=135±15pc from the stellar parallax) and the layer of UV light penetration and of H₂ 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 H₂ are observed to coincide spatially. The spectroscopic data, obtained with ISO-SWS, allows us to estimate the gas temperature to be 300-345K in the H₂ emitting layer, in which the ortho-to-para H₂ ratio is about 1 or significantly smaller than the equilibrium ratio (∼3 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≃5x10^–5), we find that the H₂ formation rate R_f must be high in warm gas (∼6 times the rate derived by Jura, 1975ApJ...197..575J), in order to account for the observed H₂ emission. This result and the spatial coincidence between the PAHs and H₂ emission suggest that H₂ forms efficiently by chemisorption on the PAHs surface. If the latter interpretation is correct, the enhancement in R_f may also result from an increased PAH abundance: assuming that R_f scales with the PAH abundance, the observed H₂ excitation is well explained with R_f≃1x10^–16cm³/s at T_gas=330K (∼3 times the rate derived by Jura, 1975ApJ...197..575J) and [C/H]_PAH≃7.5x10^–5.