The structure of stationary photodissociation fronts is revisited. H_2 self- shielding is discussed, including the effects of line overlap. We find that line overlap is important for N(H_2) > 10^{20} cm^{-2}. We compute multiline UV pumping models, and compare these with simple analytic approximations for the effects of self-shielding. 
The overall fluorescent efficiency of the photodissociation front is obtained for different ratios of chi/n_H (where chi characterizes the intensity of the incident UV) and different dust extinction laws. The dust optical depth tau_{pdr} to the point where 50% of the H is molecular is found to be a simple function of a dimensionless quantity phi_0 depending on chi/n_H, the rate coefficient for H_2 formation on grains, and the UV dust opacity. The fluorescent efficiency of the PDR also depends primarily on phi_0 for chi 10^4K, but shows some sensitivity to the v-J distribution of newly-formed H_2. The 1-0S(1)/2-1S(1) and 2-1S(1)/6-4Q(1) intensity ratios, the ortho/para ratio, and the rotational temperature in the $v$=1 and $v$=2 levels are computed as functions of the temperature and density, for different values of chi and n_H. 
We apply our models to the reflection nebula NGC 2023. We are best able to reproduce the observations with models having chi=5000, n_H=10^5 cm^{-3}.

Structure of Stationary Photodissociation Fronts

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 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 the earlier result of Habart et al. (2003) that the H2 formation rate in regions of moderate excitation such as Oph W, S140 and IC 63 is a factor of 5 times larger than the standard rate inferred from UV observations of diffuse clouds. On the other hand, towards regions of higher radiation field such as the Orion Bar and NGC 2023, we derive H2 formation rates consistent with the standard value. We find also a correlation between the H2 1-0 S(1) line and PAH emission suggesting that Rf scales with the PAH abundance. With the aim of explaining these results, we consider some empirical models of the H2 formation process. Here we consider both formation on big (a~0.1 microns) and small (a~10 Angstroms) grains by either direct recombination from the gas phase or recombination of physisorbed H atoms with atoms in a chemisorbed site. We conclude that indirect chemisorption is most promising in PDRs. Moreover small grains which dominate the total grain surface and spend most of their time at relatively low temperatures may be the most promising surface for forming H2 in PDRs.

/pdf/some-empirical-estimates-of-the-h2-formation-rate-in-photon-3nkups0yim.pdf

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

Context. In photo-dissociation regions (PDRs), polycyclic aromatic hydrocarbons (PAHs) may be produced by evaporation of very small grains (VSGs) by the impinging UV radiation field from a nearby star. Aims. We quantitatively investigate the transition zone between evaporating VSGs (eVSGs) and PAHs in several PDRs. Methods. We studied the relative contribution of PAHs and eVSGs to the mid-IR emission in a wide range of excitation conditions. We fitted the observed mid-IR emission of PDRs by using a set of template band emission spectra of PAHs, eVSGs, and gas lines. The fitting tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the community as an IDL routine. From the results of the fit, we derived the fraction of carbon feVSG locked in eVSGs and compared it to the intensity of the local UV radiation field. Results. We show a clear decrease of feVSG with increasing intensity of the local UV radiation field, which supports the scenario of photo-destruction of eVSGs. Conversely, this dependence can be used to quantify the intensity of the UV radiation field for different PDRs, including unresolved ones. Conclusions. PAHTAT can be used to trace the intensity of the local UV radiation field in regions where eVSGs evaporate, which correspond to relatively dense (nH = [100, 10 5 ]c m −3 ) and UV irradiated PDRs (G0 = [100, 5 × 10 4 ]) where H2 emits in rotational

/pdf/evaporating-very-small-grains-as-tracers-of-the-uv-radiation-7lkl31ha7o.pdf

Evaporating very small grains as tracers of the UV radiation field in photo-dissociation regions

Context. In photo-dissociation regions (PDRs), Polycyclic Aromatic Hydrocarbons (PAHs) could be produced by evaporation of Very Small Grains (VSGs) by the impinging UV radiation field from a nearby star. Aims. We investigate quantitatively the transition zone between evaporating Very Small Grains (eVSGs) and PAHs in several PDRs. Methods. We study the relative contribution of PAHs and eVSGs to the mid-IR emission in a wide range of excitation conditions. We fit the observed mid-IR emission of PDRs by using a set of template band emission spectra of PAHs, eVSGs and gas lines. The fitting tool PAHTAT (PAH Toulouse Astronomical Templates) is made available to the community as an IDL routine. From the results of the fit, we derive the fraction of carbon f_eVSG locked in eVSGs and compare it to the intensity of the local UV radiation field. Results. We show a clear decrease of f_eVSG with increasing intensity of the local UV radiation field, which supports the scenario of photo-destruction of eVSGs. Conversely, this dependence can be used to quantify the intensity of the UV radiation field for different PDRs, including non resolved ones. Conclusions. PAHTAT can be used to trace the intensity of the local UV radiation field in regions where eVSGs evaporate, which correspond to relatively dense (nH = [100, 10^5 ] cm-3) and UV irradiated PDRs (G0 = [100, 5x10^4]) where H2 emits in rotational lines.

/pdf/evaporating-very-small-grains-as-tracers-of-the-uv-radiation-3fdnvza4jr.pdf

Evaporating Very Small Grains as tracers of the UV radiation field in Photo-dissociation Regions

We present 5-20 micron spectral maps of the reflection nebula NGC2023 obtained with the Infrared Spectrograph SL and SH modes on board the Spitzer Space Telescope which reveal emission from polycyclic aromatic hydrocarbons (PAHs), C60, and H2 superposed on a dust continuum. We show that several PAH emission bands correlate with each other and exhibit distinct spatial distributions revealing a spatial sequence with distance from the illuminating star. We explore the distinct morphology of the 6.2, 7.7 and 8.6 micron PAH bands and find that at least two spatially distinct components contribute to the 7--9 micron PAH emission in NGC2023. We report that the PAH features behave independently of the underlying plateaus. We present spectra of compact oval PAHs ranging in size from C_66 to C_210, determined computationally using density functional theory, and investigate trends in the band positions and relative intensities as a function of PAH size, charge and geometry. Based on the NASA Ames PAH database, we discuss the 7--9 micron components in terms of band assignments and relative intensities. We assign the plateau emission to very small grains with possible contributions from PAH clusters and identify components in the 7--9 micron emission that likely originates in these structures. Based on the assignments and the observed spatial sequence, we discuss the photochemical evolution of the interstellar PAH family as they are more and more exposed to the radiation field of the central star in the evaporative flows associated with the PDRs in NGC2023.

/pdf/the-pah-emission-characteristics-of-the-reflection-nebula-2df5fix5bc.pdf

The PAH emission characteristics of the reflection nebula NGC2023

Spectra from 1 to 2.5 micrometres, at 230-430 spectral resolution, are presented of the fluorescent molecular hydrogen line emission from two locations in the reflection nebula NGC 2023. Over 100 H2 lines can be identified in the spectra, although blending and poor atmospheric transmission mean that reliable level column densities can only be obtained from 35 lines. This latter group includes lines from v = 1-8 and v = 10, spanning an energy range from 6000 to 45,000 K above the ground state These data may be used to constrain models of photodissociation regions and of fluorescent excitation for molecular hydrogen.

/pdf/near-ir-fluorescent-molecular-hydrogen-emission-from-ngc-1lxbpxz4ye.pdf

Near-IR Fluorescent Molecular Hydrogen Emission from NGC 2023

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Near-IR Fluorescent Molecular Hydrogen Emission from NGC 2023