Contribution Invited Talk
Interstellar H$_3^+$: From the Laboratory to the Galactic Center
H$_3^+$ was discovered by J.J.Thomson in 1911 . A fundamental exotic molecule, H$_3^+$ intrigued many physicists and chemists. Early studies involving big names such as J. Stark, N. Bohr, A. Dempster, T.R. Hogness, H.D. Smyth, C.A. Coulson, J. Hirschfelder, H. Eyring, G. Herzberg and others are summarized in a recent review . Its abundance in interstellar space was first suggested by 3 plasma physicists .
The science of H$_3^+$ has acquired a new dimension when in 1973 Herbst and Klemperer  and Watson  proposed the ion-neutral reactions as fundamental molecule formation mechanism in which H$_3^+$ plays the central role of the universal proton donor (acid). A search for the laboratory infrared spectrum was initiated in 1975 and succeeded in 1980 . Searches for interstellar H$_3^+$ was immediately started .
The first non-terrestrial H$_3^+$ signal arrived in 1989 as a serendipitous discovery of strong emission from Jupiter  and later from Uranus and Saturn . After 16 years of gradual improvement of infrared spectrometers, interstellar H$_3^+$ was detected in 1996 in two dense molecular clouds . Once detected, H$_3^+$ was found everywhere. To our surprise similar column densities of H$_3^+$ was detected in diffuse clouds where reddening is only 1/10 of dense clouds indicating that the H$_3^+$/H2 ratio is 10 times higher . This has led to a conclusion that cosmic ray ionization rate is 10 times higher in diffuse clouds than in dense clouds .
Unexpectedly, much deeper H$_3^+$ absorption with high velocity dispersion were found toward the Galactic center (GC)  indicating high concentration of H$_3^+$ in the Central Molecular Zone. With the discovery of abundant J=K=3 metastable H3+ , H$_3^+$ has emerged as a powerful probe to study the GC [15,16]. A summary of observations in the last 10 years which have radically changed the concept of the gas in the central 300 pc of the GC will be discussed.
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