Interstellar Solid Hydrogen

TL;DR

This study explores the potential presence of solid molecular hydrogen in interstellar space, focusing on the vibrational signatures of H6+ and its deuterated form, which could explain certain infrared astronomical emission bands.

Contribution

It provides quantum-theoretical calculations of vibrational modes of H6+ and (HD)3+, linking them to observed interstellar infrared emission features and suggesting solid hydrogen's astrophysical abundance.

Findings

Calculated vibrational frequencies of H6+ and (HD)3+ match some astronomical emission bands.

(HD)3+ frequencies are close to strong mid-infrared astronomical bands.

Solid hydrogen may be more abundant in interstellar environments than previously thought.

Abstract

We consider the possibility that solid molecular hydrogen is present in interstellar space. If so cosmic-rays and energetic photons cause ionisation in the solid leading to the formation of H6+. This ion is not produced by gas-phase reactions and its radiative transitions therefore provide a signature of solid H2 in the astrophysical context. The vibrational transitions of H6+ are yet to be observed in the laboratory, but we have characterised them in a quantum-theoretical treatment of the molecule; our calculations include anharmonic corrections, which are large. Here we report on those calculations and compare our results with astronomical data. In addition to the H6+ isotopomer, we focus on the deuterated species (HD)3+ which is expected to dominate at low ionisation rates as a result of isotopic condensation reactions. We can reliably predict the frequencies of the fundamental bands for five modes of vibration. For (HD)3+ all of these are found to lie close to some of the strongest of the pervasive mid-infrared astronomical emission bands, making it difficult to exclude hydrogen precipitates on observational grounds. By the same token these results suggest that (HD)3+ could be the carrier of the observed bands. We consider this possibility within the broader picture of ISM photo-processes and we conclude that solid hydrogen may indeed be abundant in astrophysical environments.