Kavli Affiliate: Lile Wang
| First 5 Authors: Xiaolong Yang, Xiaolong Yang, , ,
| Summary:
Molecular hydrogen (H$_2$) is one of the key chemical species that controls
and shapes a wide spectrum of astrophysical processes ranging from galaxy
evolution to planet formation. Although the catalyzation on dust grain surfaces
is considered as the dominant formation channel of H$_2$ in the interstellar
medium (ISM), which could nonetheless suffer from the Boltzmann factor
suppression at low temperatures. Here we demonstrate that quantum tunneling can
dominate the H$_2$ formation process, effectively resolving the long-standing
efficiency problem across a wide range of temperatures. By employing the path
integral method in hybrid Monte Carlo simulations to account for nuclear
quantum effects (NQEs), we quantitatively identify that the tunneling of
hydrogen atoms maintains relatively stable efficiencies even at temperatures
below 50 K on both graphitic and silicate grain surfaces. The potential
barriers associated with chemisorption/desorption and two-H association, rather
than diffusion and hopping, are the dominant factors governing the actual
reaction efficiency at low temperatures. These findings provide a solid
physical foundation for molecule formation, which historically relied on ad-hoc
formation rate multipliers to explain observed rates. The quantitative rates
also offer new methodologies for observational constraints on H$_2$ formation
and destruction, thereby enabling more accurate astrophysical models and
interpretations on interstellar molecular materials.
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