Using deuterated H 3 + and other molecular species to understand the formation of stars and planets

Abstract

The 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 impossible, but lead to large abundances of H 2 D + and D 2 H + that are very useful probes of the early stages of star and planet formation. Maps of H 2 D + and D 2 H + pure rotational line emission towards star-forming regions show that the strong deuteration of 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, transfers its proton to other neutrals such as CO and N 2 , 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 towards the Orion Bar imply a high electron fraction, and we suggest that observations of OH + and H 2 O + emission may be used to probe the electron density in the nuclei of external galaxies.