2003A&A...407..551W

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 ¹²C/¹³C ratios. Peak shifts to longer wavelengths due to the substitution of ¹²C by ¹³C are clearly observed in several absorption bands. The shifts are almost linearly proportional to the ¹³C fraction. New features associated with ¹³C 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 (Δλ∼ 0.23-0.26µm per ¹³C fraction) is detected in the 6.2µm band, which is attributed to a carbon-carbon vibration. A peak shift (Δλ∼ 0.16-0.18µm per ¹³C fraction) in an out-of-plane bending mode of aromatic C-H at 11.4µm is also observed, while only a small shift (Δλ<0.015µm per ¹³C fraction) is detected in the 3.3µm 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 6.2µm band, should be detectable in celestial objects with low ¹²C/¹³C 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 7.7µm 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 ¹²C/¹³C, 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.