Takeshi Oka
Department of Astronomy and Astrophysics, and Department of Chemistry,
The Enrico Fermi Institute, University of Chicago, Chicago, IL, USA, 60637

With three protons and two electrons, protonated molecular hydrogen, H3+, is the simplest polyatomic molecule discovered by J. J. Thomson in 1911. It is the most abundant ion in hydrogen dominated laboratory plasmas and in interstellar space.
In 1973, Herbst and Klemperer and Watson introduced ion-neutral reactions as the production mechanism of interstellar molecules. H3+ plays the central role in this scheme as the universal proton donor (acid) initiating chain-reactions. Since the timely production of molecules is essential for cooling the gravitationally condensing gas, H3+ is an important ingredient for star formation.
The project of finding interstellar H3+ began in 1975. The necessary spectrum was discovered in 1980.1 Search for interstellar H3+ was immediately started but it took many years to detect its weak absorptions in 1996, toward two young stars.2
Once discovered, H3+ has been detected everywhere. It has been observed not only in dense clouds where its abundance was predicted, but also in diffuse clouds where it was not anticipated. Surprisingly, our observations showed that the abundance of H3+ relative to H2 is 10 times higher in diffuse clouds than in dense clouds.3 Unexpectedly, H3+ has emerged as a powerful tool to probe the diffuse interstellar medium.
A remarkable outcome of this development has been the recent study of the Galactic center. The discovery in 20024 of H3+ in the J=K=3 metastable rotational level was crucial in this development. Using the R(3,3)l metastable line, we have revealed a vast amount of high temperature (~ 250 K) and low density (~ 100 cm-3) gas that pervades the space between dense clouds.5 This is a new category of gas in addition to the previous known three categories of gas, that is, (i) the dense and mostly cold molecular gas observed by radio emission of CO, NH3, CS, HCN etc., (ii) the hot ionized gas (104 – 106 K) observed by radio recombination lines and radio wave scattering, and (iii) the ultra hot plasma gas (107 – 108 K) reported by X-ray observers.
The unique characteristics of H3+ as an astronomical probe, based on its simple chemistry and spectroscopy, have been revealed and fully used. H3+ acts as a dosimeter to measure the ionization rate ? by cosmic rays, X-ray and EUV radiation, a depth meter to measure the cloud dimension L, and a thermometer and a densitometer to measure T and n.6 Our on-going work toward a dozen bright stars will be discussed.7

1 T. Oka, Phys. Rev. Lett. 45, 531 (1980)
2 T. R. Geballe and T. Oka, Nature 384, 334 (1996)
3 B. J. McCall, K. H. Hinkle, T. R. Geballe, G. H. Moriarty-Schieven, N. J. Evans II, K. Kawaguchi, S. Takano, V. V. Smith, and T. Oka, ApJ, 567, 391 (2002)
4 M. Goto, B. J. McCall, T. R. Geballe, T. Usuda, N. Kobayashi, H. Terada, and T. Oka, Publ. Astron. Soc. Japan, 54, 951 (2002)
5 T. Oka, T. R. Geballe, M. Goto, T. Usuda, and B. J. McCall, ApJ, 632, 882 (2005)
6 T. Oka, Proc. Natl. Acad. Sci, USA, 103, 12235 (2006)
7 M. Goto, et al. ApJ, (2008) submitted.