Resolving the compact dusty discs around binary post-AGB stars using N-band interferometry

¹ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium e-mail: Pieter.Deroo@ster.kuleuven.be
² Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
³ Leiden Observatory, PB 9513, Leiden 2300 RA, The Netherlands
⁴ European Southern Observatory, Karl-Scharzschild-Strasse 2, 85748 Garching, Germany
⁵ Observatoire de la Côte d'Azur, CNRS-UMR 6203, Avenue Copernic, 06130 Grasse, France

Received: 4 October 2005
Accepted: 5 January 2006

Abstract

We present the first mid-IR long baseline interferometric observations of the circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD 52961, were observed using the VLTI/MIDI instrument during Science Demonstration Time. Both objects are known binaries for which a stable circumbinary disc is proposed to explain the SED characteristics. This is corroborated by our N-band spectrum showing a crystallinity fraction of more than 50% for both objects, pointing to a stable environment where dust processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved in the interferometric observations providing an upper limit of 11 mas (or 18 AU at the distance of this object) on the diameter of the dust emission. This confirms the very compact nature of its circumstellar environment. The dust emission around HD 52961 originates from a very small but resolved region, estimated to be ~35 mas at 8 μm and ~55 mas at 13 μm. These results confirm the disc interpretation of the SED of both stars. In HD 52961, the dust is not homogeneous in its chemical composition: the crystallinity is clearly concentrated in the hotter inner region. Whether this is a result of the formation process of the disc, or due to annealing during the long storage time in the disc is not clear.