Some empirical estimates of the H₂formation rate in photon-dominated regions

Abstract

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.

H2formation via the UV photo-processing of a-C:H nano-particles

“…These ideas appear to be in general agreement with the conclusions of Habart et al (2004Habart et al ( , 2011, who find enhanced H 2 formation and higher than expected column densities of rotationally excited H 2 in moderately-excited PDRs.…”

Incorporation of stochastic chemistry on dust grains in the Meudon PDR code using moment equations

“…Other formation mechanisms may be at work, such as those suggested by Cazaux & Tielens (2004) and Habart et al (2004) in their interpretation of observations of warm molecular hydrogen.…”

The warm gas atmosphere of the HD 100546 disk seen byHerschel

“…We then compare the result to our "standard" H 2 formation rate using the S × η reported by Cuppen et al (2010), in order to capture the theoretical work of Cazaux & Tielens (2004), Cuppen & Herbst (2005) as well as the observational evidence from Habart et al (2004). Figure 14 shows the line fluxes calculated with the models assuming different formation rates.…”