15min:

BENJAMIN J. MCCALL, Department of Chemistry, The University of Chicago, Chicago, IL 60637; THOMAS R. GEBALLE, Joint Astronomy Centre, Hilo, HI 96720; KENNETH H. HINKLE, National Optical Astronomy Observatories, Tucson, AZ 85726; TAKESHI OKA, Department of Astronomy and Astrophysics, The University of Chicago, Chicago, IL 60637.

H⁺₃, the cornerstone molecule of interstellar chemistry, was first detected in interstellar space in 1996 \!\! in the dense molecular clouds GL2136 and W33A. We have now extended this ion's diagnostic powers to the diffuse interstellar medium with the detection of H⁺₃ along the line of sight to the visible star Cygnus OB2 No. 12 \!\!. Three rovibrational transitions in the 3.7 m region were observed using the CGS4 infrared spectrometer at the United Kingdom Infrared Telescope and the high-resolution Phoenix spectrometer at the Kitt Peak National Observatory.

We have developed a simple chemical model of interstellar chemistry which describes the abundance of H⁺₃ in both diffuse and dense clouds. The application of this model to Cygnus OB2 No. 12 shows that this line of sight has H⁺₃ number density [H⁺₃] 4 × 10^-7 cm^-3, effective path length L 300 pc, and hydrogen number density [H] + 2[H₂] 20 cm^-3. The H⁺₃ absorptions also provide an estimate of the effective kinetic temperature of the medium T 27 K.