The Ubiquitous H⁺ ₃

Abstract

Although it took many years from the discovery of the laboratory spectrum of H⁺ ₃ (Oka 1980) and its first astronomical search (Oka 1981) to its detection in interstellar space (Geballe & Oka 1996), subsequent observations have revealed ubiquity of this fundamental molecular ion in a wide variety of astronomical objects. H⁺ ₃ with column densities on the order of ∼ 10¹⁴ cm^-2 has been detected not only in many dense clouds (McCall et al. 1998^a, 1999, Geballe et al. 2002, Kulesa et al. 2002), where its presence had been anticipated from chemical model calculations (Herbst & Klemperer 1973, Watson, 1973), but also in many diffuse clouds (McCall et al. 1998^b, Geballe et al. 1999, McCall et al. 2002) where its abundance was not anticipated and introduced an enigma in the chemistry of the diffuse interstellar medium. Many astronomers, physicists and chemists are currently attempting to understand this problem (Oka 2000, McCall & Oka, 2000).