1999ApJ...522..338M

H⁺₃ has been detected using infrared absorption spectroscopy along the lines of sight to six infrared sources in dense molecular clouds: AFGL 2136, W33A, Mon R2 IRS 3, AFGL 961E, AFGL 2591, and AFGL 490. Upper limits to the column densities of H⁺₃ are reported for an additional nine sources. The column densities of CO toward Mon R2 IRS 3 and AFGL 961E have been determined from observations of the first-overtone lines of CO. For the six sources toward which H⁺₃ was detected, a simple model of H⁺₃ chemistry has been used together with column densities of H₂ derived from infrared CO measurements to estimate column lengths, mean number densities, and temperatures of molecular clouds. The derived column lengths are on the order of a parsec, the number densities are 10⁴-10⁵ cm^–3, and the temperatures are ∼25-50 K.