On Polyacetylenes in Space


The first member of this group, not really qualifying for "poly", acetylene, HC2H, itself, was detected in absorption at Kitt Peak toward the circumstellar shell of CW Leo, aka IRC +10216. The unresolved Q-branch of the ν4 + ν5 band was detected at 4091.0 cm–1 by:
S. T. Ridgway, D. N. B. Hall, R. S. Wojslaw, S. G. Kleinmann, and D. A. Weinberger,
Circumstellar Acetylene in the Infrared Spectrum of IRC +10o216
Nature 264, 345–346 (1976).

A brief account on the observation of the ν5 band with an origin at 730.3 cm–1 (or 13.7 μm) was given by:
T. Kostiuk, F. Espenak, D. Deming, M. J. Mumma, D. Zipoy, and J. Keady,
Study of Velocity Structure in IRC +10216 Using Acetylene Line Profiles
Bull. Am. Astron. Soc. 17, 570 (1985).

Hot band transitions associated with ν5 band, i.e. ν4 + ν5 – ν4 and 2ν5 – ν5, were reported by:
J. J. Keady and S. T. Ridgway,
The IRC +10216 Circumstellar Envelope. III – Infrared Molecular Line Profiles
Astrophys. J. 406, 199–214 (1993).

The molecule was also detected toward three IR sources embedded in molecular clouds with the NASA IRTF 3 m telescope. Two or three of the R(5), R(10), or the unresolved Q-branch of the ν5 band were detected toward GL 2591, W3 IRS 5, and OMC-1 IRc2 by
J. H. Lacy, N. J. Evans II, J. M. Achtermann, D. E. Bruce, J. F. Arens, and J. S. Carr,
Discovery of Interstellar Acetylene
Astrophys. J. 383, 674–692 (1991).

The observations were extended to more line of this band, to the same band of H13CCH, and to the ν4 + ν5 band at 7.6 μm toward OMC-1 using the same facility:
N. J. Evans II, J. H. Lacy, and J. S. Carr,
Infrared Molecular Spectroscopy toward the Orion IRc2 and IRc7 Sources – A new Probe of Physical Conditions and Abundances in Molecular Clouds
Astrophys. J. 342, L43–L46 (1989).

Di- and Triacetylene

J. Cernicharo, A. M. Heras, A. G. G. M. Tielens, J. R. Pardo, F. Herpin, M. Guélin, and L. B. F. M. Waters,
Infrared Space Observatory's Discovery of C4H2, C6H2, and Benzene in CRL 618
Astrophys. J. 546, L123–L126 (2001);
report the detection of butadiyne, HC4H, aka diacetylene, and hexatriyne, HC6H, aka as triacetylene, employing mid-infrared absorption spectroscopy. The unresolved Q-branches of the cold ν8 band of butadiyne along with hot bands were detected at 15.9 μm. Close by, at 16.1 μm, the Q-branch of ν11 band of hexatriyne is seen.

A separate report;
J. Cernicharo, A. M. Heras, J. R. Pardo, A. G. G. M. Tielens, M. Guélin, E. Dartois, R. Neri, and L. B. F. M. Waters;
Methylpolyynes and Small Hydrocarbons in CRL 618
Astrophys. J. 546, L127–L130 (2001);
identifies one combination band each for butadiyne and hexatriyne. The spectrum around 8 μm is rather complex as is the model which contains many species. Inspection of laboratory spectroscopic literature is useful. A complex feature at almost exactly 8 μm is largely attributable to the strong, unresolved R-branch of ν6 + ν8 band of butadiyne. Features at 8.08 and 8.14 μm can be attributed to the unresolved R- and P-branches of the respective strong ν8 + ν11 band of hexatriyne, both overlapping the P-branch of the butadiyne band.

The spectral same region was also studied under high resolution by:
J. P. Fonfría, J. Cernicharo, M. J. Richter, J. H. Lacy,
The Abundances of Polyacetylenes towards CRL618
Astrophys. J. 728, Art. No. 43 (2011),
revealing clear evidence for several hot bands associated with these rather strong combination bands.
No evidence was found for the ν10 + ν14 band of octatetrayne because of low abundance and possible also because of the absence of high-resolution IR data for this band. It should be pointed out that there are quite consistent abundance ratios among these polyacetylenes: HC4H is only slightly less abundant than HC2H. There is a pronounced drop to HC6H, and possibly an even steeper one to HC8H.

Earlier, the two fundamental vibrations ν8 of butadiyne at 15.9 μm and of ν11 of hexatriyne at 16.1 μm, see also above, were observed toward a Nova like star by
A. Evans, V. H. Tyne, J. Th. van Loon, B. Smalley, T. R. Geballe, R. D. Gehrz, C. E. Woodward, A. A. Zijlstra, E. Polomski, M. T. Rushton, S. P. S. Eyres, S. G. Starrfield, J. Krautter, and R. M. Wagner,
The Spitzer Infrared Spectrometer View of V4334 Sgr (Sakuraiís Object)
Mon. Not. R. Astron. Soc. 373, L75–L79 (2006).

Contributor(s): H. S. P. Müller; 10, 2003; 01, 2011; 06, 2011