2003/11/03 by E. Habart, F. Boulanger, L. Verstraete +3 · 2 citations
Earth and Planetary Sciences · Physics and Astronomy · #Astrophysics and Star Formation Studies #Atmospheric Ozone and Climate #Stellar, planetary, and galactic studies #astro-ph
paper · pdf · doi:10.1051/0004-6361:20031659
published as Astron.Astrophys. 414 (2004) 531-544 · A&A in press, 16 pages, 5 figures
arxiv created 2003/11/03 · openalex publication_date 2004/01/19 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/01
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 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 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.