H${}_2$ ortho-para spin conversion on inhomogeneous grain surfaces

Abstract

We investigate the evolution of the ortho-to-para ratio of overall (gas + ice) H${}_2$ via the nuclear spin conversion on grain surfaces coated with water ice under physical conditions that are relevant to star- and planet-forming regions. We utilize the rate equation model that considers adsorption of gaseous H${}_2$ on grain surfaces which have a variety of binding sites with a different potential energy depth, thermal hopping, desorption, and the nuclear spin conversion of adsorbed H${}_2$. It is found that the spin conversion efficiency depends on the H${}_2$ gas density and the surface temperature. As a general trend, enhanced H${}_2$ gas density reduces the efficiency, while the temperature dependence is not monotonic; there is a critical surface temperature at which the efficiency is the maximum. At low temperatures, the exchange of gaseous and icy H${}_2$ is inefficient (i.e., adsorbed H${}_2$ does not desorb and hinders another gaseous H${}_2$ to be adsorbed), while at warm temperatures, the residence time of H${}_2$ on surfaces is too short for the spin conversion. Additionally, the spin conversion becomes more efficient with lowering the activation barriers for thermal hopping. We discuss whether the spin conversion on surfaces can dominate over that in the gas-phase in star- and planet-forming regions. Finally, we establish a simple but accurate way to implement the H${}_2$ spin conversion on grain surfaces in existing astrochemical models.

18 pages, 9 figures, 1 table, Accepted for publication in ApJ

References in corpus (9)

• Simulation of the Formation and Morphology of Ice Mantles on Interstellar Grains
• The importance of the ortho:para H2 ratio for the deuteration of molecules during pre-protostellar collapse
• Multilayer formation and evaporation of deuterated ices in prestellar and protostellar cores
• Spatial mapping of ices in the Oph-F core: A direct measurement of CO depletion and the formation of CO2
• CO depletion: a microscopic perspective
• H${}_2$ ortho-to-para conversion on grains: A route to fast deuterium fractionation in dense cloud cores?
• Interactions of Atomic and Molecular Hydrogen with a Diamond-like Carbon Surface: H2 Formation and Desorption
• Interaction of Atomic and Molecular Hydrogen with Tholin Surfaces at Low Temperatures
• Isotopic fractionation in interstellar molecules

Cited by in corpus (6)

• Molecules with ALMA at Planet-forming Scales (MAPS). X. Studying deuteration at high angular resolution toward protoplanetary disks
• Adsorption of H${}_2$ on Amorphous Solid Water Studied with Molecular Dynamics Simulations
• Rapid ortho-to-para nuclear spin conversion of H2 on a silicate dust surface
• Measurements of Ortho-to-para Nuclear Spin Conversion of H2 on Low-temperature Carbonaceous Grain Analogues: Diamond-like Carbon and Graphite
• C-type shock modelling -- the effect of new H${}_2$-H collisional rate coefficients
• Piecing together formic acid isomerism in dark clouds. Detection of cis-formic acid in TMC-1 and astrochemical modeling