Some empirical estimates of the H2 formation rate in photon-dominated regions

A&A 414, 531-544 (2004)
DOI: 10.1051/0004-6361:20031659

Some empirical estimates of the H $\mathsf{_2}$ formation rate in photon-dominated regions

E. Habart¹, F. Boulanger², L. Verstraete², C. M. Walmsley¹ and G. Pineau des Forêts²

¹ Osservatorio Astrofisico di Arcetri, INAF, Largo E. Fermi 5, 50125 Firenze, Italy
² Institut d'Astrophysique Spatiale, Université Paris-Sud, 91405 Orsay Cedex, France

(Received 20 December 2002 / Accepted 25 July 2003 )

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 $R_{\rm f}$ . We show that the ratio of the 0-0 S(3) to the 1-0 S(1) line increases with $R_{\rm f}$ but that one requires independent estimates of the radiation field incident upon the PDR and the density in order to infer $R_{\rm f}$ 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 $3 \times 10^{-17}$ 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 $R_{\rm f}$ . 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 $\mu$ m 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 $\chi$ $\le$ 3000) temperatures may be the most promising surface for forming H ₂ in PDRs.

Key words: ISM: clouds -- ISM: dust, extinction -- atomic processes -- molecular processes -- radiative transfer

Offprint request: E. Habart, habart@arcetri.astro.it

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Some empirical estimates of the H formation rate in photon-dominated regions

Some empirical estimates of the H $\mathsf{_2}$ formation rate in photon-dominated regions