H₃⁺ in dense and diffuse clouds

Interstellar H₃⁺ has been detected in dense as well as diffuse clouds using three 3.7 μm infrared spectral lines of the ν₂ fundamental band. Column densities of H₃⁺ from (1.7-5.5) × 10¹⁴ cm^-2 have been measured in dense clouds in absorption against the infrared continua of the deeply embedded young stellar objects GL2136, W33A, MonR2 1RS 3, GL961E, and GL2591. Strong and broad H₃⁺ absorptions have been detected in dense and diffuse clouds towards GC 1RS 3 and GCS3-2 in the region of the galactic center. A large column density of H₃⁺, comparable to that of a dense cloud, has been detected towards the visible star Cygnus OB2 No. 12, which has a line of sight that crosses mostly diffuse clouds. The H₃⁺ chemistry of dense and diffuse clouds are discussed using a very simple model. Some future projects and problems are discussed.