Some empirical estimates of the H2 formation rate in photon-dominated regions
Osservatorio Astrofisico di Arcetri, INAF, Largo E. Fermi 5, 50125 Firenze, Italy
2 Institut d'Astrophysique Spatiale, Université Paris-Sud, 91405 Orsay Cedex, France
Corresponding author: E. Habart, firstname.lastname@example.org
Accepted: 25 July 2003
We combine recent ISO observations of the vibrational ground state lines of H2 towards Photon-Dominated Regions (PDRs) with observations of vibrationally excited states made with ground–based telescopes in order to constrain the formation rate of H2 on grain surfaces under the physical conditions in the layers responsible for H2 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 H2 line ratios to the H2 formation rate Rf. We show that the ratio of the 0–0 S(3) to the 1–0 S(1) line increases with Rf but that one requires independent estimates of the radiation field incident upon the PDR and the density in order to infer Rf from the H2 line data. We confirm earlier work by [CITE] on the Oph W PDR which showed that an H2 formation rate higher than the standard value of cm3 s-1 inferred from UV observations of diffuse clouds is needed to explain the observed H2 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 H2 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 Rf. We compare also the H2 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 H2 on PAHs is important in PDRs. We also consider some empirical models of the H2 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 H2 in PDRs.
Key words: ISM: clouds / ISM: dust, extinction / atomic processes / molecular processes / radiative transfer
© ESO, 2004