Accurate rate coefficients for models  of interstellar gas-grain chemistry

Context. The H 2 formation on grains is known to be sensitive to dust temperature, which is also known to fluctuate for small grain sizes due to photon absorption. Aims. We aim at exploring the consequences of simultaneous fluctuations of the dust temperature and the adsorbed H-atom population on the H 2 formation rate under the full range of astrophysically relevant UV intensities and gas conditions. Methods. The master equation approach is generalized to coupled fluctuations in both the grain's temperature and its surface population and solved numerically. The resolution can be simplified in the case of the Eley-Rideal mechanism, allowing a fast computation. For the Langmuir-Hinshelwood mechanism, it remains computationally expensive, and accurate approximations are constructed. Results. We find the Langmuir-Hinshelwood mechanism to become an efficient formation mechanism in unshielded photon dominated region edge conditions when taking those fluctuations into account, despite hot average dust temperatures. It reaches an importance comparable to the Eley-Rideal mechanism. However, we show that a simpler rate equation treatment gives qualitatively correct observable results in full cloud simulations under the most astrophysically relevant conditions. Typical differences are a factor of 2−3 on the intensities of the H 2 v = 0 lines. We also find that rare fluctuations in cloud cores are sufficient to significantly reduce the formation efficiency. Conclusions. Our detailed analysis confirms that the usual approximations used in numerical models are adequate when interpreting observations, but a more sophisticated statistical analysis is required if one is interested in the details of surface processes.

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“…In a simple approximation (see Lohmar & Krug 2006;Lohmar et al 2009, for Katz et al (1999); (2) Perets et al (2007); (3) Hasegawa et al (1992).…”

Section: Surface Chemistry Via Langmuir-hinshelwood Mechanismmentioning

confidence: 99%

Surface chemistry in the interstellar medium

Bron

Bourlot

Petit

2014

A&A

118

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“…The sweeping rate probability was subsequently revisited by Lohmar & Krug (2006) and Lohmar et al (2009) for various surface geometries.The desorption probability is assumed to follow a similar exponential de-…”

Section: H 2 Formation Efficiencymentioning

confidence: 99%

Surface chemistry in the interstellar medium

Bourlot

Petit

Pinto

et al. 2012

A&A

154

183

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Context. It has been found from ISO, Spitzer, and Herschel observations that molecular hydrogen, H 2 , can form on warm grains. Numerical models of interstellar chemistry have failed to reproduce the observed formation rates of H 2 , which remains a difficulty when interpreting observations of photon-dominated regions (PDRs). Aims. We attempt to include as much experimental and theoretical information as possible to describe H 2 formation in astrophysical environments to solve this problem. Methods. We modified our "Meudon PDR code" to include a detailed treatment of H 2 formation mechanisms including: i) the Langmuir-Hinshelwood mechanism taking into account the contribution of the different sizes of dust grains in the diffusion processes; and ii) the Eley-Rideal mechanism. Results. We are able to form H 2 even in regions where the dust temperature is higher than 25 K. We also show that formation by the Eley-Rideal mechanism can be a significant source of gas heating. We derive line intensities for various astrophysical conditions. Conclusions. Our approach results in a higher H 2 formation rate than for the "standard" 3 × 10 −17 n H n(H) cm 3 s −1 expression.

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“…When an H atom (radical) impinges upon a hydrocarbon target, a number of processes may occur (see Cazaux & Tielens 2002; Papoular 2005; Lohmar, Krug & Biham 2009):…”

Section: Elementary Processesmentioning

confidence: 99%

Excitation of unidentified infrared bands by H atom impacts

Papoular¹

2011

Monthly Notices of the Royal Astronomical Society

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A model was developed for the excitation of the unidentified infrared bands (UIBs) by H atom impacts in the interstellar medium. It builds upon the fact that, in the presence of far‐ultraviolet (far‐UV) radiation and hydrocarbon grains, the hydrogen gas will be partially dissociated and the grain surface will be partially hydrogenated and partially covered with free C bonds. Under such a statistical equilibrium, H atoms from the gas will recombine with C atoms at the grain surface at some rate. At each recombination, the H atom deposits an energy of about 5 eV in the grain. Half of this is directly converted into vibrational excitation, always distributed in the same way among the most tightly coupled vibration modes of the grain. Absent frequent grain–grain collisions, the only outlet for this energy is infrared re‐emission, part of it in the UIBs, provided the chemical structure of the grains is adequate, and the other part in the continuum. The partition only depends upon the grain size, all grains being assumed to have the same constitution. Only a fraction, about 0.25, of the grains (among the smallest ones) will contribute significantly to the UIBs. It is shown quantitatively that H atom impacts are generally more efficient excitation agents than UV absorption because of the overwhelming abundance of H relative to UV photons. Only very close to young bright stars is this no longer true because photon flux then largely exceeds H atom flux. Thus, H atom impacts and far‐UV absorption are both necessary to understand the variety of observed UIB spectra. The model translates into a small number of equations enabling a quantitative comparison of its predictions with available astronomical observations, which have become exquisitely rich and accurate in the last two decades.

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Accurate rate coefficients for models of interstellar gas-grain chemistry

Cited by 22 publications

References 24 publications

Surface chemistry in the interstellar medium

Surface chemistry in the interstellar medium

Surface chemistry in the interstellar medium

Excitation of unidentified infrared bands by H atom impacts

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