10:15 AM - 10:30 AM
[PCG23-06] Modeling of the H2 ortho-para spin conversion on grain surfaces in star- and planet-forming regions
Keywords:H2 molecule, Ortho-to-para ratio, Star- and planet-forming regions
Hydrogen is the most abundant element in the universe. In star- and planet-forming regions, hydrogen is primary present in H2, which has two nuclear spin configurations, ortho and para. As the internal energy difference between ortho-H2 and para-H2 (170 K) is much higher than the typical temperature of star-forming regions (around 10 K), the H2 ortho-to-para ratio (OPR) can affect the chemical evolution, including deuterium fractionation, significantly.
H2 molecules form on grain surfaces with the statistical OPR of three. The ortho-para spin conversion proceeds through proton exchange reactions with H+ and/or with H3+ in the gas phase. Laboratory experiments have found that the H2 spin conversion can also occur on bare grains and on amorphous water ice in laboratory timescales (around a few hours). Given this very short timescale, it has been thought that the spin conversion on surfaces affects the H2 OPR evolution in star- and planet-forming regions. However, its efficiency in the astronomical conditions remains unclear; almost all H2 is present in the gas phase rather than on grain surfaces, and thus the spin conversion timescale of overall H2 (i.e., gas + solid) via the spin conversion on surfaces depends on how efficiently gaseous and solid H2 interact.
We investigate the efficiency of the H2 spin conversion on grain surfaces 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 different potential energy depth, thermal hopping and desorption of adsorbed H2, and the spin conversion. We find that the conversion efficiency depends on H2 gas density and surface temperature. As a general trend, increased H2 gas density reduces the conversion efficiency, while the temperature dependence is not monotonic; there is a critical temperature at which the efficiency is maximum. We will discuss whether the spin conversion on surfaces can dominates over that in the gas-phase in star- and planet-forming regions.
H2 molecules form on grain surfaces with the statistical OPR of three. The ortho-para spin conversion proceeds through proton exchange reactions with H+ and/or with H3+ in the gas phase. Laboratory experiments have found that the H2 spin conversion can also occur on bare grains and on amorphous water ice in laboratory timescales (around a few hours). Given this very short timescale, it has been thought that the spin conversion on surfaces affects the H2 OPR evolution in star- and planet-forming regions. However, its efficiency in the astronomical conditions remains unclear; almost all H2 is present in the gas phase rather than on grain surfaces, and thus the spin conversion timescale of overall H2 (i.e., gas + solid) via the spin conversion on surfaces depends on how efficiently gaseous and solid H2 interact.
We investigate the efficiency of the H2 spin conversion on grain surfaces 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 different potential energy depth, thermal hopping and desorption of adsorbed H2, and the spin conversion. We find that the conversion efficiency depends on H2 gas density and surface temperature. As a general trend, increased H2 gas density reduces the conversion efficiency, while the temperature dependence is not monotonic; there is a critical temperature at which the efficiency is maximum. We will discuss whether the spin conversion on surfaces can dominates over that in the gas-phase in star- and planet-forming regions.