H2 Ortho-Para Spin Conversion on Inhomogeneous Grain Surfaces

We investigate the evolution of the ortho-to-para ratio of overall (gas + ice) 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 gase...

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Bibliographic Details
Published in:The Astrophysical journal 2019-09, Vol.882 (2)
Main Authors: Furuya, Kenji, Aikawa, Yuri, Hama, Tetsuya, Watanabe, Naoki
Format: Article
Language:English
Subjects:
astrochemistry
Astrophysics
Binding sites
Conversion coating
Efficiency
Gas density
Ice
ISM: molecules
Low temperature
molecular processes
Nuclear spin
Planet formation
Planets
Potential energy
Star formation
Surface temperature
Temperature
Temperature dependence
Vapor phases
Water ice
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Summary:We investigate the evolution of the ortho-to-para ratio of overall (gas + ice) 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 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 . It is found that the spin conversion efficiency depends on the gas density and the surface temperature. As a general trend, enhanced 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 is inefficient (i.e., adsorbed does not desorb and hinders another gaseous to be adsorbed), while at warm temperatures, the residence time of 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 spin conversion on grain surfaces in existing gas-ice astrochemical models.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ab3790