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. Columndensities of H3+ from (1.7–5.5)×1014 cm-2 have been measured in denseclouds in absorption against the infrared continua of the deeply embedded young stellar objects GL2136, W33A, MonR2 IRS 3, GL961E, and GL2591. Strong and broad H3+ absorptions have been detected in dense and diffuse clouds towards GC IRS 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.