Rotational Excitation of HD by Hydrogen Revisited

Abstract

The HD molecules are key species for the cooling of pristine gas at temperatures below 100 K. They are also known to be key tracers of H2 in protoplanetary disks and thus, they can be used as a measure of protoplanetary disks mass. Accurate modeling of the cooling mechanism and of HD abundance in astrophysical media requires a proper modeling for its excitation by both radiative and collisional processes. Here, we report quantum time-independent calculations of collisional rate coefficients for the rotational excitation of HD by H for temperatures ranging from 10 to 1000 K. The reactive and hydrogen exchange channels are taken into account in the scattering calculations. New exact quantum results are compared to previous calculations performed neglecting reactive and exchange channels. We found that for temperatures higher than ∼300 K, the impact of these channels on the rate coefficients cannot be neglected. Such results suggest that the new HD–H collisional data have to be used for properly modeling HD cooling function and HD abundance in all the astrophysical environments where HD plays a role, e.g. in photon-dominated regions, protoplanetary disks, early Universe chemistry, and primordial star forming regions.

Quantum mechanical study of the high-temperature H+ + HD → D+ + H2 reaction for the primordial universe chemistry

“…Therefore, it is crucial to investigate the chemical reactions involving HD, especially with neutral and ionised atoms, in order to characterise the cooling dynamics. If the reaction with the most abundant species H has been widely studied (see for instance (Flower & Roueff 1999;Ely et al 2016;Desrousseaux et al 2018)), very little is known about its ionic counterpart H + + HD, which is the subject of the present article.…”

HD–H+ collisions: statistical and quantum state-to-state studies

“…Hence, since the Hamiltonian is symmetric with respect to interchange of the two hydrogens, there will be no coupling between odd (ortho) and even (para) rotational states of H 2 , so that the number of coupled channels is reduced. We then obtained inelastic, reactive and exchange integral cross sections following the approach described by Tao and Alexander 43 and recently used to study the rotational excitation of several hydrides in collision with H [44][45][46][47] . Excited vibrational states up to v = 17 and up to j = 6 (E max = 4.4 eV) are included in the basis to ensure convergence of the cross sections.…”

Controlling rotational quenching rates in cold molecular collisions

“…This was confirmed by subsequent theoretical studies which revealed that the angular distribution of the scattered HD in collisions with H 2 was dominated by a single (l = 2) partial-wave shape resonance as around 1 K 19 . Collisions of HD with H 2 are also of current astrophysical interest as 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 [20][21][22] .…”