# $C^{-}_{7}$ AND THE DIFFUSE INTERSTELLAR BANDS

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 Title: $C^{-}_{7}$ AND THE DIFFUSE INTERSTELLAR BANDS Creators: McCall, Benjamin J.; York, Donald G.; Oka, T. Issue Date: 1999 Publisher: Ohio State University Abstract: The diffuse interstellar bands (DIBs) are a set of absorption features appearing in the visible spectra of stars whose lines of sight travese interstellar dust and gas. The identification of the carriers of these bands has eluded astronomers and laboratory spectroscopists for over half a century. Many promising suggestions have been made, only to be negated on observational or spectroscopic grounds. The hypothesis that the DIBs are caused by long-chain carbon $molecules^{b}$ remains one of the most attractive alternatives. The recent $measurement^{c}$ of the electronic spectrum of $C_{7}$ in the gas phase lends new support to this hypothesis, since the five strongest $A^{2}II_{u} \leftarrow X^{2}II_{g}$ transitions of $C^{-}_{7}$ roughly match diffuse interstellar bands both in wavelength (within $\sim 2.8 {\AA}$) and in relative intensity. The remaining two $A^{2}II_{u} \leftarrow X^{2}II_{g}$ transitions, and five $B^{2}II_{u} \leftarrow X^{2}II_{g}$ transitions (which are broadened by internal conversion), have not yet been observed in astronomical spectra containing the diffuse interstellar bands. In order to test the hypothesis that $C^{-}_{7}$ is a carrier of the diffuse interstellar bands and to search for other carbon molecules, we have obtained medium $(\Delta/\lambda \sim 2000)$ and high $(\Delta/\lambda \sim 40000)$ resolution visible spectra of reddened stars using the Double Imaging Spectrograph (DIS) and the echelle spectrometer at Apache Point Observatory. We will present the results of these observations and critically evaluate the viability of the $C_{7}$ DIB hypothesis. Description: $^{a}$ Supported by the Fannie and John Hertz Foundation $^{b}$ A.E. Douglas, Nature 269, 130, (1977). W.H. Smith, T.P. Snow, and D.G. York, Astrophys. J. 218, 124 (1977) $^{c}$ M. Tulej, D.A. Kirkwood, M. Pachkov, and J.P. Maier, Astrophys. J. 506, L69 (1998) Author Institution: Department of Astronomy and Astrophysics, The University of Chicago URI: http://hdl.handle.net/1811/19357 Other Identifiers: 1999-RF-09