Astron. Astrophys. 336, L17-L20 (1998)

Letter to the Editor

The binary RV Tauri star AC Her and evidence for a long-lived dust-disc ^*

Hans Van Winckel ^{** 1}, Christoffel Waelkens ¹, Laurens B.F.M. Waters ², Frank J. Molster ², Stephane Udry ³ and Eric J. Bakker <sup>4

1</sup> Instituut voor Sterrenkunde, K.U. Leuven, Celestijnenlaan 200B, B-3001 Heverlee, Belgium
² Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ Observatoire de Genève, CH-1290 Sauverny, Switserland
⁴ TNO Physics and Electronics Lab., P.O. Box 96864, 2509 JG The Hague, The Netherlands

Received 16 June 1998 / Accepted 19 June 1998

Abstract

We present in this letter a homogeneous set of CORAVEL radial velocity measurements of the well studied RV Tauri star AC Her showing it to be a binary with a period of 1196 6 days. The photospheric abundances are deduced using high resolution, high signal-to-noise optical spectra and prove that AC Her is another RV Tauri star displaying depletion of refractory elements, strengthening the relation between binarity and the depletion process. Our full ISO-SWS spectrum shows that the unusual broad emission feature at 8-12 µm is due to a high crystallisation fraction of the circumstellar silicates. We review all observational evidence that the circumstellar material is trapped in a long-lived disc in the system similar to what is observed in the Red Rectangle nebula.

Key words: stars: binaries: close stars: evolution stars: individual: AC Her, HD 44179 stars: AGB and post-AGB

* based on observations collected at the European Southern Observatory in Chile, at La Palma Spain, and with the Swiss telescope at OHP, France
** Postdoctoral fellow of the Fund for Scientific Research, Flanders

Send offprint requests to: H. Van Winckel, Leuven

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998

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