Molecular Hydrogen Formation on Porous Dust Grains
نویسندگان
چکیده
Recent laboratory experiments on interstellar dust analogues have shown that H2 formation on dust grain surfaces is efficient in a range of grain temperatures below 20 K. These results indicate that surface processes may account for the observed H2 abundance in cold diffuse and dense clouds. However, high abundances of H2 have also been observed in warmer clouds, including photon-dominated regions (PDRs), where grain temperatures may reach 50 K, making the surface processes extremely inefficient. It was suggested that this apparent discrepancy can be resolved by chemisorption sites. However, recent experiments indicate that chemisorption processes may not be efficient at PDR temperatures. Here we consider the effect of grain porosity on H2 formation. It is found that porosity extends the efficiency of the recombination process to higher temperatures. This is because H atoms that desorb from the internal surfaces of the pores may re-adsorb many times and thus stay longer on the surface. However, this porosity-driven extension may enable efficient H2 formation in PDRs only if porosity also contributes to significant cooling of the grains, compared to non-porous grains.
منابع مشابه
Experimental evidence for water formation on interstellar dust grains by hydrogen and oxygen atoms
Context. The synthesis of water is one necessary step in the origin and development of life. It is believed that pristine water is formed and grows on the surface of icy dust grains in dark interstellar clouds. Until now, there has been no experimental evidence whether this scenario is feasible or not on an astrophysically relevant template and by hydrogen and oxygen atom reactions. Aims. We pr...
متن کاملMolecular Hydrogen Formation on Dust Grains in the High Redshift Universe
Received ; accepted – 2 – ABSTRACT We study the formation of molecular hydrogen on dust grain surfaces and apply our results to the high redshift universe. We find that a range of physical parameters, in particular dust temperature and gas temperature, but not so much dust surface composition, influence the formation rate of H 2. The H 2 formation rate is found to be suppressed above gas kineti...
متن کاملLaboratory evidence for the non-detection of excited nascent H2 in dark clouds
There has always been a great deal of interest in the formation of H2 as well as in the binding energy released upon its formation on the surface of dust grains. The present work aims at collecting experimental evidence for how the bond energy budget of H2 is distributed between the reaction site and the internal energy of the molecule. So far, the non-detection of excited nascent H2 in dense q...
متن کاملThe formation of H2 and HD with the master equation approach
The formation of H2 and HD molecules on interstellar dust grains is studied using rate equation and master equation models. Rate equations are used in the analysis of laboratory experiments which examine the formation of molecular hydrogen on astrophysically relevant surfaces. However, under interstellar conditions, rate equations are not suitable for the calculation of reaction rates on dust-g...
متن کاملStxm Imaging of Organic Coatings on Grains in Primitive Interplanetary Dust: Implications for Formation of Pre-biotic Organic Matter and Grain Sticking in the Early Solar System
Chondritic, porous interplanetary dust particles (CP IDPs) are the most primitive samples of extraterrestrial material available for laboratory analysis [1]. These CP IDPs are unequilibrated aggregates of mostly submicron, anhydrous grains of a diverse variety, including olivine, pyroxene, glass, and sulfide. They contain organic matter that was not produced by parent body aqueous processing [2...
متن کامل