Using deuterated H3+ and other molecular species to understand the formation of stars and planets

TL;DR

This paper discusses how deuterated H3+ and related molecules serve as probes for understanding the chemical processes and physical conditions during star and planet formation in interstellar clouds, especially through their emission lines.

Contribution

It highlights the role of deuterated H3+ species in tracing molecular depletion, ion chemistry, and physical conditions in star-forming regions, and suggests new observational methods for external galaxies.

Findings

Deuteration of H3+ indicates molecular depletion in cold cores.

H3+ transfers protons to molecules like CO and N2 in warmer regions.

Emission lines of HF, OH+, and H2O+ can probe electron densities in galaxies.

Abstract

The H3+ ion plays a key role in the chemistry of dense interstellar gas clouds where stars and planets are forming. The low temperatures and high extinctions of such clouds make direct observations of H3+ impossible, but lead to large abundances of H2D+ and D2H+ which are very useful probes of the early stages of star and planet formation. Maps of H2D+ and D2H+ pure rotational line emission toward star-forming regions show that the strong deuteration of H3+ is the result of near-complete molecular depletion of CNO-bearing molecules onto grain surfaces, which quickly disappears as cores warm up after stars have formed. In the warmer parts of interstellar gas clouds, H3+ transfers its proton to other neutrals such as CO and N2, leading to a rich ionic chemistry. The abundances of such species are useful tracers of physical conditions such as the radiation field and the electron fraction. Recent observations of HF line emission toward the Orion Bar imply a high electron fraction, and we suggest that observations of OH+ and H2O+ emission may be used to probe the electron density in the nuclei of external galaxies.