Abstract. We present maps at high spatial and spectral resolution in emission lines of CCH, c-C 3 H 2 , C 4 H, 12 CO and C 18 O of the edge of the Horsehead nebula obtained with the IRAM Plateau de Bure Interferometer (PdBI). The edge of the Horsehead nebula is a one-dimensional Photo-Dissociation Region (PDR) viewed almost edge-on. All hydrocarbons are detected at high signal-to-noise ratio in the PDR where intense emission is seen both in the H 2 ro-vibrational lines and in the PAH mid-infrared bands. C 18 O peaks farther away from the cloud edge. Our observations demonstrate that CCH, c-C 3 H 2 and C 4 H are present in UV-irradiated molecular gas, with abundances nearly as high as in dense, well-shielded molecular cores. PDR models i) need a large density gradient at the PDR edge to correctly reproduce the offset between the hydrocarbons and H 2 peaks; and ii) fail to reproduce the hydrocarbon abundances. We propose that a new formation path of carbon chains, in addition to gas phase chemistry, should be considered in PDRs: because of intense UV-irradiation, large aromatic molecules and small carbon grains may fragment and feed the interstellar medium with small carbon clusters and molecules in significant amounts.

Abstract. We use far-UV absorption spectra obtained with FUSE towards three late B stars to study the formation and excitation of H 2 in the diffuse ISM. The data interpretation relies on a model of the chemical and thermal balance in photonilluminated gas. The data constrain well the n R product between gas density and H 2 formation rate on dust grains: n R = 1 to 2.2 × 10 −15 s −1 . For each line of sight the mean effective H 2 density n, assumed uniform, is obtained by the best fit of the model to the observed N(J = 1)/N(J = 0) ratio, since the radiation field is known. Combining n with the n R values, we find similar H 2 formation rates for the three stars of about R = 4 × 10 −17 cm 3 s −1 . Because the target stars do not interact with the absorbing matter we can show that the H 2 excitation in the J > 2 levels cannot be accounted for by the UV pumping of the cold H 2 but implies collisional excitation in regions where the gas is much warmer. The existence of warm H 2 is corroborated by the fact that the star with the largest column density of CH + has the largest amount of warm H 2 .

Abstract. We combine recent ISO observations of the vibrational ground state lines of H 2 towards Photon-Dominated Regions (PDRs) with observations of vibrationally excited states made with ground-based telescopes in order to constrain the formation rate of H 2 on grain surfaces under the physical conditions in the layers responsible for H 2 emission. We briefly review the data available for five nearby PDRs. We use steady state PDR models in order to examine the sensitivity of different H 2 line ratios to the H 2 formation rate R f . We show that the ratio of the 0-0 S(3) to the 1-0 S(1) line increases with R f but that one requires independent estimates of the radiation field incident upon the PDR and the density in order to infer R f from the H 2 line data. We confirm earlier work by Habart et al. (2003) on the Oph W PDR which showed that an H 2 formation rate higher than the standard value of 3 × 10 −17 cm 3 s −1 inferred from UV observations of diffuse clouds is needed to explain the observed H 2 excitation. From comparison of the ISO and ground-based data, we find that moderately excited PDRs such as Oph W, S140 and IC 63 require an H 2 formation rate of about five times the standard value whereas the data for PDRs with a higher incident radiation field such as NGC 2023 and the Orion Bar can be explained with the standard value of R f . We compare also the H 2 1-0 S(1) line intensities with the emission in PAH features and find a rough scaling of the ratio of these quantities with the ratio of local density to radiation field. This suggests but does not prove that formation of H 2 on PAHs is important in PDRs. We also consider some empirical models of the H 2 formation process with the aim of explaining these results. Here we consider both formation on classical grains of size roughly 0.1 µm and on very small (∼10 Å) grains by either direct recombination from the gas phase (Eley-Rideal mechanism) or recombination of physisorbed H atoms with atoms in a chemisorbed site. We conclude that indirect chemisorption where a physisorbed H-atom scans the grain surface before recombining with a chemisorbed H-atom is most promising in PDRs. Moreover small grains which dominate the total grain surface and spend most of their time at relatively low (below 30 K for χ ≤ 3000) temperatures may be the most promising surface for forming H 2 in PDRs.

a b s t r a c tMolecular hydrogen is the most abundant molecule in the universe. It is the first one to form and survive photo-dissociation in tenuous environments. Its formation involves catalytic reactions on the surface of interstellar grains. The micro-physics of the formation process has been investigated intensively in the last 20 years, in parallel of new astrophysical observational and modeling progresses. In the perspectives of the probable revolution brought by the future satellite JWST, this article has been written to present what we think we know about the H 2 formation in a variety of interstellar environments.