Discovery of a TiO emission band in the infrared spectrum of the S star NP Aurigae

K. Smolders^1⋆, T. Verhoelst^1,2, P. Neyskens^3⋆⋆, J. A. D. L. Blommaert¹, L. Decin¹, H. Van Winckel¹, S. Van Eck³, G. C. Sloan⁴, J. Cami^5,6, S. Hony⁷, P. De Cat⁸, J. Menu¹ and J. Vos¹

¹ Instituut voor Sterrenkunde (IvS), Katholieke Universiteit Leuven, Celestijnenlaan 200 D, 3001 Leuven, Belgium
e-mail: kristof.smolders@ster.kuleuven.be
² Belgian Institute for Space Aeronomy (BIRA-IASB), Ringlaan-3-Avenue Circulaire, 1180 Brussels, Belgium
³ Institut d’Astronomie et d’Astrophysique (IAA), Université Libre de Bruxelles, CP 226, Boulevard du Triomphe, 1050 Bruxelles, Belgium
⁴ Cornell University, Astronomy Department, Ithaca, NY 14853, USA
⁵ Department of Physics and Astronomy, University of Western Ontario, London, Ontario N6A 3K7, Canada
⁶ SETI Institute, 189 Bernardo Ave, Suite 100, Mountain View, CA 94043, USA
⁷ Service d’Astrophysique, CEA Saclay, Bât. 709, Orme des Merisiers, 91191 Gif-sur-Yvette, France
⁸ Koninklijke Sterrenwacht van België, Ringlaan 3, 1180 Brussel, Belgium

Received: 2 May 2012
Accepted: 4 June 2012

Abstract

We report on the discovery of an infrared emission band in the Spitzer spectrum of the S-type AGB star NP Aurigae that is caused by TiO molecules in the circumstellar environment. We modeled the observed emission to derive the temperature of the TiO molecules (≈600 K), an upper limit on the column density (≈10^17.25 cm^-2) and a lower limit on the spatial extent of the layer that contains these molecules. (≈4.6 R_⋆). This is the first time that this TiO emission band is observed. A search for similar emission features in the sample of S-type stars yielded two additional candidates. However, owing to the additional dust emission, the identification is less stringent. By comparing the stellar characteristics of NP Aur to those of the other stars in our sample, we find that all stars with TiO emission show large-amplitude pulsations, s-process enrichment, and a low C/O ratio. These characteristics might be necessary requirements for a star to show TiO in emission, but they are not sufficient.