Abstract
Molecular hydrogen is the most abundant molecule in the universe. It is the first one to form and survive photo-dissociation in tenuous environments. Its formation involves catalytic reactions on the surface of interstellar grains. The micro-physics of the formation process has been investigated intensively in the last 20 years, in parallel of new astrophysical observational and modeling progresses. In the perspectives of the probable revolution brought by the future satellite JWST, this article has been written to present what we think we know about the H2 formation in a variety of interstellar environments.
Highlights
Molecular hydrogen is, by a few orders of magnitude, the most abundant molecule in the Universe
This paper presents, in a unified account, the current viewpoint regarding the formation of molecular hydrogen on interstellar dust grains from the perspective of observers, modelers and chemical physicists
We propose 8 prototypical classes of surface, each class, often representative of a large variety of substrates: (1) amorphous and (2) crystalline silicates, (3) amorphous carbon, (4) graphite, (5) polycyclic aromatic hydrocarbon (PAH) and (6) PAH cations, and two morphologies of ice, one geometrically flat such as (7) polycrystalline ice, and one with a high degree of molecular disorder, such as (8) porous amorphous ice (ASW) grown at low ( < 30 K) temperature
Summary
By a few orders of magnitude, the most abundant molecule in the Universe. The first detection of this molecule in the interstellar medium (ISM) was obtained via a rocket flight in 1970 (Carruthers, 1970), three decades after the first interstellar detection of CH, CH+ and CN (see Snow and McCall, 2006, and references therein). In diffuse molecular clouds, which are regions characterized by molecular fractions fH2 = 2nH2 /nH > 0.1 (nH2 being the number density of H2 molecules and nH the total proton number density), the first molecule to form is H2 (Snow and McCall, 2006). V. Wakelam et al / Molecular Astrophysics 9 (2017) 1–36
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