===== On the Detection of Polyacetylenes in Extragalactic Sources =====

==== Acetylene ====

An early account on Extragalactic acetylene was presented by\\
J. T. van Loon, A. A. Zijlstra, and M. A. T. Groenewegen,\\
**[[https://ui.adsabs.harvard.edu/abs/1999A%26A...346..805V|Luminous Carbon Stars in the Magellanic Clouds]]**\\
//Astron. Astrophys.// **346**, (1999) 805.\\
Telescopes of ESO and CTIO were used to carry out observations in the L-band (at 3 μm). Broad, strong absorption was found between 3.0 and 3.3 μm which was attributed to HCN and HCCH. As usual also in the later accounts, no spectroscopic information is given and no reference to laboratory spectroscopy. The band is caused by the strong CH stretching mode of both molecules, and the contributions of the molecules can not be disentangled.

**[[https://doi.org/10.1086/345680|Very Large Telescope Spectra of Carbon Stars in the Large Magellanic Cloud and Their Metallicity Dependence]]**\\
//Astrophys. J.// **580**, (2002) L133,\\
by\\
M. Matsuura, A. A. Zijlstra, J. T. van Loon, I. Yamamura, A. J. Markwick, P. M. Woods, and L. B. F. M. Waters\\
did not only reveal the 3.1 μm band, but also a much weaker one at 3.8 μm which is caused by //ν//<sub>2</sub> + //ν//<sub>5</sub> of acetylene, the combination of the CC stretch with the asymmetric bending mode. The longer wavelength band is comparatively stronger than in spectra of Galactic carbon stars. This, in turn, is interpreted in terms of, on average, larger C/O ratio in carbon stars of the LMC.

Further observations at these and even longer wavelengths of 7.5 μm (//ν//<sub>4</sub> + //ν//<sub>5</sub>) and 13 μm (//ν//<sub>5</sub>) have been carried out by several groups with the //Spitzer// Space Telescope.

==== Di- and Triacetylene ====

J. Bernard-Salas, E. Peeters, G. C. Sloan, J. Cami, S. Guiles, and J. R. Houck\\
obtained\\
**[[https://doi.org/10.1086/509777|The //Spitzer// IRS spectrum of SMP LMC 11]]**\\
//Astrophys. J.// **652**, (2006) L29.\\
The spectrum resembles that of the protoplanetary nebula V353 Aur, which is somewhat better known as CRL (or AFGL) 618. Butadiyne, HC<sub>4</sub>H, aka diacetylene, and hexatriyne, HC<sub>6</sub>H, aka as triacetylene, were iditified by their unresolved //Q//-branches of the //ν//<sub>8</sub> band of butadiyne at 15.9 μm and the //ν//<sub>11</sub> band of hexatriyne at 16.1 μm, respectively, as in the Galactic source.


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Contributor(s): H. S. P. Müller; 5, 2012

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