Rotational Quenching of HD in Collisions with H₂: Resolving Discrepancies for Low-lying Rotational Transitions

The deuterated hydrogen molecule HD has been observed in a variety of cool molecular astrophysical environments. By virtue of its small dipole moment the HD molecule is believed to have played an important role in the cooling of the primordial gas in the formation of the first stars and galaxies. HD has also recently been proposed as a tracer of molecular hydrogen in protoplanetary disk evolution, providing a diagnostic for the total disk mass. Here we report benchmark computations of rotational quenching rate coefficients for HD in collisions with H₂ based on quantum coupled channel methods within the rigid rotor model, and validate them against full-dimensional rovibrational scattering formalism. It is found that the rigid rotor model yields accurate rate coeffiicents for rotational transitions in HD+H₂ collisions at astrophysically relevant kinetic temperatures. Results are reported using the most recent highly accurate interaction potentials for the H₂-H₂ system. We obtain excellent agreement with previous results of Schaefer for the most important Δj = ±1, ±2 transitions in HD induced by ortho- and para-H₂, but find significant differences with recent results of Sultanov et al. that employed the same interaction potential as the one adopted here.