Carbonaceous molecules in the oxygen-rich circumstellar environment of binary post-AGB stars^⋆

C₆₀ fullerenes and polycyclic aromatic hydrocarbons

¹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: clio.gielen@ster.kuleuven.be
² Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
³ Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
⁴ SETI Institute, 189 Bernardo Ave, Suite 100, Mountain View, CA 94043, USA
⁵ Astronomical Institute, University of Utrecht, PO Box 80000, 3508 TA Utrecht, The Netherlands

Received: 27 August 2011
Accepted: 19 October 2011

Abstract

Context. The circumstellar environment of evolved stars is generally rich in molecular gas and dust. Typically, the entire environment is either oxygen-rich or carbon-rich, depending on the evolution of the central star.

Aims. In this paper we discuss three evolved disc sources with evidence of atypical emission lines in their infrared spectra. The stars were taken from a larger sample of post-AGB binaries for which we have Spitzer infrared spectra, characterised by the presence of a stable oxygen-rich circumbinary disc. Our previous studies have shown that the infrared spectra of post-AGB disc sources are dominated by silicate dust emission, often with an extremely high crystallinity fraction. However, the three sources described here are selected because they show a peculiar molecular chemistry.

Methods. Using Spitzer infrared spectroscopy, we study in detail the peculiar mineralogy of the three sample stars. Using the observed emission features, we identify the different observed dust, molecular and gas species.

Results. The infrared spectra show emission features due to various oxygen-rich dust components, as well as CO₂ gas. All three sources show the strong infrared bands generally ascribed to polycyclic aromatic hydrocarbons. Furthermore, two sample sources show C₆₀ fullerene bands.

Conclusions. Even though the majority of post-AGB disc sources are dominated by silicate dust in their circumstellar environment, we do find evidence that, for some sources at least, additional processing must occur to explain the presence of large carbonaceous molecules. There is evidence that some of these sources are still oxygen-rich, which makes the detection of these molecules even more surprising.