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Register a new userH2 formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen
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Molecular hydrogen is the most abundant molecule in the Universe. It is thought that a large portion of H2 forms by association of hydrogen atoms to polycyclic aromatic hydrocarbons (PAHs). We model the influence of PAHs on total H2 formation rates in photodissociation regions (PDRs) and assess the effect of these formation rates on the total cloud structure. We set up a chemical kinetic model at steady state in a PDR environment and included adiative transfer to calculate the chemistry at different depths in the PDR. This model includes known dust grain chemistry for the formation of H2 and a H2 formation mechanism on PAHs. Since H2 formation on PAHs is impeded by thermal barriers, this pathway is only efficient at higher temperatures (T > 200 K). At these temperatures the conventional route of H2 formation via H atoms physisorbed on dust grains is no longer feasible, so the PAH mechanism enlarges the region where H2 formation is possible. We find that PAHs have a significant influence on the structure of PDRs. The extinction at which the transition from atomic to molecular hydrogen occurs strongly depends on the presence of PAHs, especially for PDRs with a strong external radiation field. A sharp spatial transition between fully dehydrogenated PAHs on the outside of the cloud and normally hydrogenated PAHs on the inside is found. As a proof of concept, we use coronene to show that H2 forms very efficiently on PAHs, and that this process can reproduce the high H2 formation rates derived in several PDRs.
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We present a comparative study of the near-infrared (NIR) H$_2$ line emission from five regions near hot young stars: Sharpless 140, NGC 2023, IC 63, the Horsehead Nebula, and the Orion Bar. This emission originates in photodissociation or photon-dominated regions (PDRs), interfaces between photoionized and molecular gas near hot (O) stars or reflection nebulae illuminated by somewhat cooler (B) stars. In these environments, the dominant excitation mechanism for NIR emission lines originating from excited rotational-vibrational (rovibrational) levels of the ground electronic state is radiative or UV excitation (fluorescence), wherein absorption of far-UV photons pumps H$_2$ molecules into excited electronic states from which they decay into the upper levels of the NIR lines. Our sources span a range of UV radiation fields ($G_0 = 10^2$-$10^5$) and gas densities ($n_H = 10^4$-$10^6$ cm$^{-3}$), enabling examination of how these properties affect the emergent spectrum. We obtained high-resolution ($R approx 45,000$) spectra spanning $1.45$-$2.45$~$mu$m on the 2.7m Harlan J. Smith Telescope at McDonald Observatory with the Immersion Grating INfrared Spectrometer (IGRINS), detecting up to over 170 transitions per source from excited vibrational states ($v = 1$-$14$). The populations of individual rovibrational levels derived from these data clearly confirm UV excitation. Among the five PDRs in our survey, the Orion Bar shows the greatest deviation of the populations and spectrum from pure UV excitation, while Sharpless 140 shows the least deviation. However, we find that all five PDRs exhibit at least some modification of the level populations relative to their values under pure UV excitation, a result we attribute to collisional effects.
We present experimental data on H2 formation processes on gas-phase polycyclic aromatic hydrocarbon (PAH) cations. This process was studied by exposing coronene radical cations, confined in a radio-frequency ion trap, to gas phase H atoms. Sequential attachment of up to 23 hydrogen atoms has been observed. Exposure to atomic D instead of H allows one to distinguish attachment from competing abstraction reactions, as the latter now leave a unique fingerprint in the measured mass spectra. Modeling of the experimental results using realistic cross sections and barriers for attachment and abstraction yield a 1:2 ratio of abstraction to attachment cross sections. The strong contribution of abstraction indicates that H2 formation on interstellar PAH cations is an order of magnitude more relevant than previously thought.
We derive total (atomic + molecular) hydrogen densities in giant molecular clouds (GMCs) in the nearby spiral galaxy M33 using a method that views the atomic hydrogen near regions of recent star formation as the product of photodissociation. Far-UV photons emanating from a nearby OB association produce a layer of atomic hydrogen on the surfaces of nearby GMCs. Our approach provides an estimate of the total hydrogen density in these GMCs from observations of the excess far-UV emission that reaches the GMC from the OB association, and the excess 21-cm radio HI emission produced after these far-UV photons convert H2 into HI on the GMC surface. The method provides an alternative approach to the use of CO emission as a tracer of H2 in GMCs, and is especially sensitive to a range of density well below the critical density for CO(1-0) emission. We describe our PDR method in more detail and apply it using GALEX far-UV and VLA 21-cm radio data to obtain volume densities in a selection of GMCs in the nearby spiral galaxy M33. We have also examined the sensitivity of the method to the linear resolution of the observations used; the results obtained at 20 pc are similar to those for the larger set of data at 80 pc resolution. The cloud densities we derive range from 1 to 500 cm-3, with no clear dependence on galactocentric radius; these results are generally similar to those obtained earlier in M81, M83, and M101 using the same method.
We present a simple analytic procedure for generating atomic-to-molecular (HI-to-H$_2$) density profiles for optically thick clouds illuminated by far-ultraviolet radiation. Our procedure is based on the analytic theory for the structure of one-dimensional HI/H$_2$ photon-dominated regions, presented by Sternberg et al. (2014). Depth-dependent HI and H$_2$ density fractions may be computed for arbitrary gas density, far-ultraviolet field intensity, and the metallicity dependent H$_2$ formation rate coefficient, and dust absorption cross section. We use our procedure to generate a set of HI-to-H$_2$ transition profiles for a wide range of conditions, from the weak- to strong-field limits, and from super-solar down to low metallicities. We show that if presented as functions of dust optical depth the HI and H$_2$ density profiles depend primarily on the Sternberg $alpha G$ parameter (dimensionless) that determines the dust optical depth associated with the total photodissociated HI column. We derive a universal analytic formula for the HI-to-H$_2$ transition points as a function of just $alpha G$. Our formula will be useful for interpreting emission-line observations of HI/H$_2$ interfaces, for estimating star-formation thresholds, and for sub-grid components in hydrodynamics simulations.
Recent studies have confirmed the presence of buckminsterfullerene (C$_{60}$) in different interstellar and circumstellar environments. However, several aspects regarding C$_{60}$ in space are not well understood yet, such as the formation and excitation processes, and the connection between C$_{60}$ and other carbonaceous compounds in the interstellar medium, in particular polycyclic aromatic hydrocarbons (PAHs). In this paper we study several photodissociation regions (PDRs) where C$_{60}$ and PAHs are detected and the local physical conditions are reasonably well constrained, to provide observational insights into these questions. C$_{60}$ is found to emit in PDRs where the dust is cool ($T_d = 20-40$ K) and even in PDRs with cool stars. These results exclude the possibility for C$_{60}$ to be locked in grains at thermal equilibrium in these environments. We observe that PAH and C$_{60}$ emission are spatially uncorrelated and that C$_{60}$ is present in PDRs where the physical conditions (in terms of radiation field and hydrogen density) allow for full dehydrogenation of PAHs, with the exception of Ced 201. We also find trends indicative of an increase in C$_{60}$ abundance within individual PDRs, but these trends are not universal. These results support models where the dehydrogenation of carbonaceous species is the first step towards C$_{60}$ formation. However, this is not the only parameter involved and C$_{60}$ formation is likely affected by shocks and PDR age.
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