Takeshi Oka
University of Chicago
The Ubiquitous H3+,  from Giant Planets to Star Forming Regions

           Although it took many years from the discovery of the laboratory spectrum1 of H3+ and its first search in interstellar space2 to its detection,3 subsequent observations have shown surprising ubiquity of this molecular ion. H3+ has been detected not only in dense clouds where its presence had been predicted by model calculations, but also in diffuse clouds4 where its abundance was not anticipated. The recent advent of large diameter telescopes equipped with high resolution-sensitivity spectrometer with wide wavelength coverage is causing an avalanche of H3+ observation in a wide variety of astronomical objects.  
            Because of cosmic ray ionization of H2, followed by the efficient reaction H2+ + H2 H3+ + H, H3+ is produced wherever H2 abounds, and acts as the universal proton donor (acid) initiating myriads of chain chemical reactions.  It was first observed towards infrared stars that are deeply embedded in dense molecular clouds with high extinction (Av ~ 100) but later towards brighter stars including bright constellation stars  with low extinction (Av ~ 1).  The recent detection toward z Persei is particularly noteworthy.5 
           The strongest spectrum of H3+ is observed toward bright infrared sources near the Galactic center. The observations samples the local core clouds as well as several clouds in intervening spiral arms and produce a rich spectrum6. Some high temperature clouds show presence of H3+ in a high energy metastable rotational level.  An emission spectrum from a gas giant protoplanet7 and an absorption spectrum in an obscured active galactic nucleus8 have also been reported.
           I shall discuss the observed spectra and related physics of H3+.

1 Observation of the Infrared Spectrum of H3+,  T. Oka, Phys. Rev. Lett. 45, 531 (1980)
2 A Search for Interstellar H3+,  T. Oka, Phil. Trans. R. Soc. Lond. A303, 543 (1981) 
3 Detection of H3+in Interstellar Space,  T. R. Geballe and T. Oka, Nature 384, 334 (1996)
Observation of H3+in the Diffuse Interstellar Medium,  B. J. McCall et al. ApJ 567, 391 (2002)
An enhanced Cosmic-Ray Flux towards z Persei inferred from a Laboratory Study of the H3+-e- Recombination Rate, B. J. McCall et al. Nature 422, 500 (2003)
Aborption Line Survey of H3+ toward the Galactic Center Sources I. GCS 3-2 and GC IRS3,  M. G     M. Goto, et al. PASJ 54, 951 (2002)
CO and H3+ in the Protoplanetary Disk around the Star HD 141569,  S. D. Brittain and T. W. Rettig, Nature   418, 57 (2002)
Detection of H3+ in the Interstellar Medium of IRAS 08572+3915, T. R. Geballe, Astro-ph/0109166 (2001)