Letter to the Editor

When an old star smolders

On the detection of hydrocarbon emission from S-type AGB stars

¹ Instituut voor Sterrenkunde (IvS), Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium e-mail: kristof.smolders@ster.kuleuven.be
² Universiteit van Amsterdam, Sterrenkundig Instituut “Anton Pannekoek”, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ Service d'Astrophysique, CEA Saclay, Bât.709, Orme des Merisiers, 91191 Gif-sur-Yvette, France
⁴ Cornell University, Astronomy Department, Ithaca, NY 14853, USA
⁵ Institut d'Astronomie et d'Astrophysique (IAA), Université Libre de Bruxelles, C.P. 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
⁶ Jodrell Bank Centre for Astrophysics, Alan Turing Building, University of Manchester, Oxford Road, Manchester, M13 9PL, UK

Received: 12 February 2010
Accepted: 9 April 2010

Abstract

Polycyclic aromatic hydrocarbons (PAHs) produce characteristic infrared emission bands that have been observed in a wide range of astrophysical environments, where carbonaceous material is subjected to ultraviolet (UV) radiation. Although PAHs are expected to form in carbon-rich AGB stars, they have up to now only been observed in binary systems where a hot companion provides a hard radiation field. In this letter, we present low-resolution infrared spectra of four S-type AGB stars, selected from a sample of 90 S-type AGB stars observed with the infrared spectrograph aboard the Spitzer satellite. The spectra of these four stars show the typical infrared features of PAH molecules. We confirm the correlation between the temperature of the central star and the centroid wavelength of the 7.9 μm feature, present in a wide variety of stars spanning a temperature range from 3000 to 12 000 K. Three of four sources presented in this paper extend this relation towards lower temperatures. We argue that the mixture of hydrocarbons we see in these S-stars has a rich aliphatic component. The fourth star, BZ CMa, deviates from this correlation. Based on the similarity with the evolved binary TU Tau, we predict that BZ CMa has a hot companion as well.