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M. ADAMKOVICS, Astronomy Department, 601 Campbell Hall, University of California, Berkeley, CA 94720; G. A. BLAKE, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125; B. J. MCCALL, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign, 601 S. Mathews Ave., Urbana, IL 61801.
\hspace0.2in The simplest carbon-chain molecule C2 was discovered in the interstellar medium in 1977. Fourteen years later, J.~P.~Maier and others reported the first detection of C3 in three diffuse cloud sightlines. A followup survey at lower resolution detected C3 in 15 lines of sight with EB-V ranging from 0.33 to 1.12 and noted that the sightline toward HD 204827 showed particularly high C2 and C3 column densities. This result was confirmed by a survey at higher resolution.
\hspace0.2in The next-longest carbon chains, C4 and C5, have not yet been detected in the interstellar medium. A low resolution search was performed by Oka et al.c, and a more sensitive high resolution search has been reported towards
Ophiuchi using the 3789 Å band of C4 and the 5109 Å band of C5. While
Ophiuchi is a very bright star (V=2.56), its C3 column density (1.5 × 1012 cm-2) is considerably lower than that towards HD 204827 (1.15 × 1013 cm-2). Consequently, we decided to attempt a sensitive search for C4 and C5 in this carbon-rich sightline. The relative faintness (V=7.94) of the source was overcome by the large aperture (10-m) of the Keck telescope and a long (two nights) integration. Unfortunately, this work has not yielded a detection. A detailed interpretation of this non-detection in terms of carbon chain chemistry in diffuse clouds is hampered both by uncertainties in the oscillator strengths of the transitions as well as by the comparatively immature state of chemical models of diffuse cloud chemistry (relative to models of dense cloud chemistry). We encourage further theoretical and modelling efforts along these lines.