H₃⁺

Spectra of three interstellar clouds showing absorption lines due to the R(1,1)^u and R(1,0) transitions of H₃⁺. Arrows indicate the expected positions of the absorption features from previous measurements of the interstellar gas velocity.

Observations

H₃⁺ is extremely important to the chemistry of interstellar clouds. This is because H₃⁺ willingly donates its extra proton to a variety of collision partners, thus laying the foundation for a large network of ion-molecule reactions.

The reactions surrounding the formation and destruction of H₃⁺ are relatively simple. It is generally assumed that a cosmic ray first ionizes H₂, which then collides with another H₂ molecule to form H₃⁺ and H. In diffuse clouds H₃⁺ is destroyed via electron recombination, while in dense clouds destruction primarily occurs by charge tranfer with CO. This overall simplicity allows observations of H₃⁺ to be used in estimating the ionization rate of H₂ due to cosmic rays.

The cosmic-ray ionization rate in the ISM has previously been estimated using various molecular tracers such as OH, HD, HCO⁺, and H₃O⁺. While these studies have found an average value of about 10^-17 s^-1, individual results have varied over a few orders of magnitude. Using the United Kingdom Infrared Telescope, we have observed H₃⁺ absorption in several diffuse molecular cloud sight lines and inferred an average ionization rate of 4x10^-16 s^-1. Because the chemistry behind H₃⁺ is so much simpler than for the other tracers, we believe that this is the most accurate determination of the cosmic-ray ionization rate to date.

Three theoretical distributions of the differential cosmic-ray flux at low energies. These cosmic-ray spectra were used with various cross sections to predict the ionization rate of atomic hydrogen, light element isotope production rates, and gamma-ray line fluxes. Click for full size.

Theoretical Calculations

In addition to ionizing H and H₂, cosmic rays can also spall atomic nuclei and excite atomic nuclear states. Spallation of C, N, and O will produce the light element isotopes ⁶Li, ⁷Li, ⁹Be, ¹⁰B, and ¹¹B, while nuclear excitation of ¹²C and ¹⁶O will emit 4.4 MeV and 6.1 MeV gamma rays, respectively. Exactly how much these processes occur depends on the energy spectrum of cosmic rays in the 1 MeV - 10 GeV range. Unfortunately, cosmic rays in this energy range are unobservable at the Earth due to the solar wind. As a result, we have tested multiple low energy spectra in an attempt to reproduce light element, gamma ray, and ionization observations. From our study, it seems that the power law distribution of cosmic rays at low energies must be steeper than that observed at high energies. One possible explanation for such a discrepancy is that each population is accelerated by a different mechanism, although this is contrary to the current belief that all Galactic cosmic rays are accelerated in supernova remnants. We continue to study this problem in an attempt to better constrain the cosmic-ray spectrum at low energies.

Spectra covering various H₃⁺ transitions in 6 sight lines near the supernova remnant IC 443. Click for full size.

H₃⁺ Near Supernova Remnants

We recently searched for H₃⁺ absorption in sight lines which pass through molecular material in close proximity to the supernova remnant IC 443. If supernova remnants do accelerate a large number of low-energy cosmic rays, then the ionization rate in such a region should be rather high. Spectra were obtained using the Keck and Subaru Telescopes, and are shown for the six target sight lines in this study. The sight lines toward HD 254577 and ALS 8828 show large column densities of H₃⁺, and indicate ionization rates of about 2x10^-15 s^-1, higher than previously found toward any diffuse cloud. This suggests that supernova remnants do in fact accelerate a large population of low-energy cosmic rays. However, it is still unclear whether or not such a population can propagate far enough from remnants to account for the high ionization rate inferred toward several Galactic diffuse cloud sight lines. Future studies of H₃⁺ in molecular material near supernova remnants should help to further our understanding on the subject.