# $H_{3}^{+}$ IN THE DIFFUSE INTERSTELLAR MEDIUM TOWARD HD183143

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 Title: $H_{3}^{+}$ IN THE DIFFUSE INTERSTELLAR MEDIUM TOWARD HD183143 Creators: McCall, Benjamin J.; Geballe, T. R.; Hinkle, Kenneth H.; Evans, N. J.; Moriarty-Schieven, G. H.; Takano, S.; Oka, T. Issue Date: 2001 Publisher: Ohio State University Abstract: The recent $observation^{a, b}$ of $H_{3}^{+}$ in the diffuse interstellar medium toward Cygnus OB2 No.12 has introduced an enigma related to the chemistry of such medium in addition to the long standing mystery of the diffuse interstellar bands (DIBs) and the unexplained abundance of $CH^{+}$. In order to accept the large observed $H_{3}^{+}$ column density of $3.8 \times 10^{14} cm^{-2}$ based on the accepted experimental value of the dissociative recombination rate constant, an extraordinarily long pathlength needs to be assumed. Black $proposed^{c}$ that the direction of Cygnus OB2 No. 12 is exceptional in that the extraordinary brightness of the star (more than a million times brighter than the Sun) will ionize the surrounding gas through X-rays. We have continued to search for $H_{3}^{+}$ in the diffuse medium toward closer and less reddened stars and detected a high column density $(\sim 2 \times 10^{14} cm^{-2})$ of $H_{3}^{+}$ toward HD183143, the well known star for its strength of the DIBs and used by Herbig as the representative star in his discussion of the DIBs. The existence of the large column density toward this more ordinary star demonstrates the ubiquity and more general abundance of $H_{3}^{+}$ in the diffuse interstellar medium. This direction is characteristic in that, in spite of its large extinction $(A_{v} \sim 4)$ and strong DIBs, heavy diatomic species such as CO and $C_{2}$ are not detectable. Our observations of $H_{3}^{+}$ and other molecules toward this direction and the chemistry of the medium as well as our detection of $H_{3}^{+}$ in other diffuse clouds will be discussed. Description: $^{a}$B. J. McCall, T. R. Geballe, K.H. Hinkle and T. Oka Science 279, 1910 1998. $^{b}$T. R. Geballe, B. J. McCall, K. H. Hinkle and T. Oka ApJ.522, 338 1999. $^{c}$J. H. Black Phil. Trans. R. Soc. Lond. A 358, 2515 2000. Author Institution: Department of Chemistry, The University of Chicago; Department of Chemistry and Department of Astronomy and Astrophysics, Gemini Observatory; Department of Chemistry and Department of Astronomy and Astrophysics, National Optical Astronomy Observatories; Department of Chemistry and Department of Astronomy and Astrophysics, University of Texas; Astronomy Department, University of Texas; Joint Astronomy Centre, Nobeyama Radio Observatory; Joint Astronomy Centre, The University of Chicago URI: http://hdl.handle.net/1811/20106 Other Identifiers: 2001-RC-01