Molecular Hydrogen Formation in the Interstellar Medium

By S. Cazaux, A. Tielens. 2002

184 citations recorded by the source corpus.

We have developed a model for molecular hydrogen formation under astrophysically relevant conditions. This model takes fully into account the presence of both physisorbed and chemisorbed sites on the surface, allows quantum mechanical diffusion as well as thermal hopping for absorbed H atoms, and has been benchmarked versus recent laboratory experiments on H2 formation on silicate surfaces. The results show that H2 formation on grain surfaces is efficient in the interstellar medium up to some 300 K. At low temperatures (≤100 K), H2 formation is governed by the reaction of a physisorbed H with a chemisorbed H. At higher temperatures, H2 formation proceeds through a reaction between two chemisorbed H atoms. We present simple analytical expressions for H2 formation that can be adopted to a wide variety of surfaces once their surface characteristics have been determined experimentally.

Subfields: Astronomy and Astrophysics, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/94f2321ba5106c18dad08452fd3fd343cf2ccc9f

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Citations in the corpus

The big problems in star formation: The star formation rate, stellar clustering, and the initial mass function (2014, methodology): “The quantities H2 and S (T ) can be calculated from models or measured from laboratory experiments (e.g. Hollenbach & McKee 1979; Cazaux & Tielens 2002, 2004; Cazaux et al. 2005), while Σgr can be constrained approximately from the level of dust extinction in the UV (Draine 2003, 2011, and…”
H I Narrow Self-Absorption in Dark Clouds: Correlations with Molecular Gas and Implications for Cloud Evolution and Star Formation (2002, background): “A recent analysis including both chemisorption and physisorption on grain surfaces indicates that H2 formation can be efficient over a wide range of temperatures (Cazaux & Tielens 2002), specifically including the 8–20 K range of interest for dark clouds.”
The UMIST database for astrochemistry 2012 (2012, unknown): “Canosa et al. (1997), Canosa et al. (2007), Carty et al. (2001), Cazaux & Tielens (2002), Chastaing et al. (2000), Chastaing et al. (1998), Chastaing et al. (1999), Chastaing et al. (1999), Cohen & Westberg (1991), Collings et al. (2004), Cordiner & Millar (2009), Dalgarno et al. (1990), Danielsson…”
H2 Formation on Grain Surfaces (2004, methodology): “Finally, we apply the model to the problem of H2 formation in the ISM (Cazaux & Tielens 2002).”
Some empirical estimates of the H 2 formation rate in photon-dominated regions (2003, background): “In the following discussion, we describe these processes mainly focusing on the version of the Langmuir-Hinshelwood mechanism whereby one of the adsorbed H-atoms is originally in a chemisorbed site (Hollenbach & Salpeter 1971; Cazaux & Tielens 2002, 2003).”
Molecular Cloud Formation behind Shock Waves (2004, background): “The calculations of Cazaux & Tielens (2004; see also Cazaux & Tielens 2002) indicate a nearunity efficiency of formation of H2, provided the dust temperature does not exceed ∼ 20 K.”
H$_2$ formation on interstellar dust grains: the viewpoints of theory, experiments, models and observations (2017, background): “e main H2 formation processes in diuse to dense interstellar medium. and Monte Carlo simulations for the formation of H 2 on such surfaces, where both physisorption and chemisorption are considered (Cazaux and Tielens, 2002, 2004; Cazaux and Spaans, 2009), or where the roughness of the surface is ta”
Ultraviolet Survey of CO and H2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships (2008, methodology): “H2 is known to form primarily through catalysis on grain surfaces, and we compute the rate of H2 formation using the temperature- and material-dependent rates given in Cazaux & Tielens (2002).”
Radiation thermo-chemical models of protoplanetary disks I. Hydrostatic disk structure and inner rim (2009, unknown): “The formation of H2 on grain surfaces H+H+grain→ H2+grain is taken into account according to (Cazaux & Tielens, 2002) RH2 = 1 2 vthH(Tg) nd 4π〈a 2〉αH ε(Td) (55) with latest updates for the temperature-dependent effici ncy ε(Td) from S. Cazaux (2008, priv.comm.).vthH = (kTg/(2πmH)) 1/2 is the…”
Our astrochemical heritage (2012, background): “First of all, hydrogen atoms can quickly scan their surfaces, meet and form volatile H2 molecules at rates large enough to defeat H2 photodissociation due to the interstellar radiation field ([212]; [351]; [88]; [119]).”
ISO Spectroscopy of Gas and Dust: From Molecular Clouds to Protoplanetary Disks (2004, background): “Quantitative estimates of the rates of both processes have been made (Cazaux & Tielens 2002, Habart et al. 2004).”
The VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman α systems: physical conditions in the neutral gas (2005, background): “The formation of H2 is expected on the surface of dust grains, if the gas is cool, dense and mostly neutral, and from the formation of H− ions if the gas is warm and dust-free (see e.g. Jenkins & Peimbert 1997; Cazaux & Tielens 2002).”
Gas in the Terrestrial Planet Region of Disks: CO Fundamental Emission from T Tauri Stars (2003, background): “The subsequent formation of CO in warm disk atmospheres (>a few 100 K) may be somewhat difficult because the familiar grain-catalyzed formation process for H2 (a necessary chemical precursor for CO) is inefficient at warm temperatures (e.g., Cazaux & Tielens 2002).”
Water and astrobiology (2007, background): “The crucial role of grains and grain mantles in producing H2 and simple hydrides such as water, ammonia, and methane has been recognized explicitly (Cazaux and Tielens, 2002).”
Diagnostics of irradiated gas in galaxy nuclei. I. A far-ultraviolet and X-ray dominated region code (2005, background): “Recently, Cazaux & Tielens (2002, 2004) developed a model for the formation of hydrogen under astrophysically relevant conditions.”
Fragmentation of star-forming clouds enriched with the first dust (2006, background): “In the above equation, βHP is the evaporation rate of physisorbed H and αPC is the mobility to go from a physisorbed to a chemisorbed site, and their ratio can be approximated as (Cazaux & Tielens 2002) βHP αPC = 1 4 ( 1 + √ EHC − ES EHP − ES )2 exp ( − ES kTgr ) .”
The Very Large Telescope Ultraviolet and Visible Echelle Spectrograph survey for molecular hydrogen in high‐redshift damped Lyman α systems (2003, unknown): “Formation of H2 is expected on the surface of dust grains if the gas is cool, dense and mostly neutral, and from the formation of negative hydrogen if the gas is warm and dust-free (see e.g. Jenkins & Peimbert 1997; Cazaux & Tielens 2002).”