http://arxiv.org/abs/1908.01966
We investigate the evolution of the ortho-to-para ratio of overall (gas + ice) H2 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 H2 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 H2. It is found that the spin conversion efficiency depends on the H2 gas density and the surface temperature. As a general trend, enhanced H2 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 H2 is inefficient (i.e., adsorbed H2 does not desorb and hinders another gaseous H2 to be adsorbed), while at warm temperatures, the residence time of H2 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 H2 spin conversion on grain surfaces in existing astrochemical models.
K. Furuya, Y. Aikawa, T. Hama, et. al.
Wed, 7 Aug 19
44/61
Comments: 18 pages, 9 figures, 1 table, Accepted for publication in ApJ