13C isotope effects on infrared bands of quenched carbonaceous composite (QCC)

A&A 407, 551-562 (2003)
DOI: 10.1051/0004-6361:20030881

$\mathsf{^{13}}$ C isotope effects on infrared bands of quenched carbonaceous composite (QCC)

S. Wada¹, T. Onaka², I. Yamamura³, Y. Murata¹ and A. T. Tokunaga⁴

¹ Department of Applied Physics and Chemistry, The University of Electro-Communications, Chofugaoka, Chofu, Tokyo 182-8585, Japan
² Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
³ The Institute of Space and Astronautical Science (ISAS), Yoshino-dai 3-1-1, Sagamihara, Kanagawa 229-8510, Japan
⁴ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Dr., Honolulu, HI 96822, USA

(Received 6 May 2003 / Accepted 2 June 2003)

Abstract
We investigate carbon isotope effects on the infrared bands of a laboratory analogue of carbonaceous dust, the quenched carbonaceous composite (QCC), synthesized from a plasma gas of methane with various $\element[][12]{C}$ / $\element[][13]{C}$ ratios. Peak shifts to longer wavelengths due to the substitution of $\element[][12]{C}$ by $\element[][13]{C}$ are clearly observed in several absorption bands. The shifts are almost linearly proportional to the $\element[][13]{C}$ fraction. New features associated with $\element[][13]{C}$ are not seen, indicating that the infrared bands in the QCC are not very localized vibration modes but come from vibrations associated with rather large carbon structures. An appreciable peak shift ( $\Delta \lambda \sim 0.23{-}0.26~\mu$ m per $\element[][13]{C}$ fraction) is detected in the 6.2 $\mu$ m band, which is attributed to a carbon-carbon vibration. A peak shift ( $\Delta \lambda \sim 0.16{-}0.18~\mu$ m per $\element[][13]{C}$ fraction) in an out-of-plane bending mode of aromatic C-H at 11.4 $\mu$ m is also observed, while only a small shift ( $\Delta \lambda < 0.015~\mu$ m per $\element[][13]{C}$ fraction) is detected in the 3.3 $\mu$ m band, which arises from a C-H stretching mode. The present experiment suggests that peak shifts in the unidentified infrared (UIR) bands, particularly in the 6.2 $\mu$ m band, should be detectable in celestial objects with low $\element[][12]{C}$ / $\element[][13]{C}$ ratios ( <10). The isotopic shifts seen in the QCC are discussed in relation to the variations in the UIR band peaks observed in post-asymptotic giant branch stars and planetary nebulae. The observed peak shift pattern of the UIR bands is qualitatively in agreement with the isotopic shifts in the QCC except for the 7.7 $\mu$ m band complex although the observed shifts in the UIR bands are larger than those inferred from derived isotope ratios for individual objects. The poor quantitative agreement may be attributed partly to large uncertainties in the derived $\element[][12]{C}$ / $\element[][13]{C}$ , to possible spatial variations of the isotope abundance within the object, and to combinations of other effects, such as hetero-atom substitutions. The present investigation suggests that part of the observed variations in the UIR band peaks may come from the isotopic effects.

Key words: infrared: ISM -- dust, extinction -- stars: AGB and post-AGB -- planetary nebulae: general -- ISM: abundances -- ISM: lines and bands

Offprint request: S. Wada, wada@e-one.uec.ac.jp

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