Some empirical estimates of the H 2 formation rate in photon-dominated regions

By É. Habart, F. Boulanger, L. Verstraete, C. Walmsley, G. Forêts. 2003

146 citations recorded by the source corpus.

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 [CITE] 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 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 μ m and on very small (~10 A) 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.

Subfields: Astronomy and Astrophysics, Atmospheric Science

View Paper: https://www.semanticscholar.org/paper/016ef06f677cdb4c9ce5a6e511e7744c977b9e38

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Citations in the corpus

Are PAHs precursors of small hydrocarbons in photo-dissociation regions? The Horsehead case (2005, methodology): “Model E and F uses a density profile provided by Habart et al. (2004, 2005) to fit the 2.12µm–H2 emission.”
[Si II], [Fe II], [C II], and H2 Emission from Massive Star-forming Regions (2006, background): “However, the 30% that is absorbed in the diffuse ISM is sufficient to produce roughly 90% of the observed [C ii] emission.”
H$_2$ formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations (2017, background): “As the large ranges of UV fields and gas densities result in arge variations of the gas conditions and dust properties (which an affect the possible H 2 formation mechanisms at work) we eparate low-illumination PDRs ( G 0 < 10 0 0) from high illumination DRs ( G 0 > 10 0 0).”
ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks (2004, background): “Quantitative estimates of the rates of both processes have been made (Cazaux & Tielens 2002, Habart et al. 2004).”
Our astrochemical heritage (2012, background): “Moreover, PAHs boost the formation of H2 molecules (Habart et al. 2004a), thus the atomic-to-molecular transition in the upper disk atmospheres.”
The warm gas atmosphere of the HD 100546 disk seen by Herschel - Evidence of a gas-rich, carbon-poor atmosphere? (2012, methodology): “We then compare the result to our “standard” H2 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).”