870 μm observations of evolved stars with LABOCA ^*,**

¹ Instituut voor Sterrenkunde, Katholieke Universiteit Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium e-mail: Djazia.Ladjal@ster.kuleuven.be
² Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
³ Royal Observatory of Belgium, Ringlaan 3, 1180 Brussels, Belgium
⁴ University College London, Gower Street, London, WC1E 6BT, UK

Received: 19 August 2009
Accepted: 4 January 2010

Abstract

Context. During their evolution, asymptotic giant branch (AGB) stars experience a high mass loss which leads to the formation of a circumstellar envelope (CSE) of dust and gas. The mass loss process is the most important phenomenon during this evolutionary stage. In order to understand it, it is important to study the physical parameters of the CSE. The emission of the CSE in the (sub)millimetre range is dominated by the dust continuum. This means that (sub)millimetre observations are a key tool in tracing the dust and improving our knowledge of the mass loss process.

Aims. The aim of this study is to use new submillimetre observations of a sample of evolved stars to constrain the CSE physical parameters.

Methods. We used aperture photometry to determine the fluxes at 870 μm and to investigate the extended emission observed with the new APEX bolometer LABoCa. We computed the spectral energy distribution (SEDs) with the 1D radiative transfer code DUSTY, which we compared to literature data. Grain properties were calculated with both the spherical grains distribution and the continuous distribution of ellipsoids (CDE), and a comparison between the two is drawn. Synthetic surface brightness maps were derived from the modelling and were compared to the LABoCa brightness maps.

Results. A sample of nine evolved stars with different chemistry was observed with LABoCa. We detected extended emission around four stars. Physical parameters of the circumstellar envelope were derived from SED modelling, like the dust chemical composition, the dust condensation temperature and the total mass of the envelope. It proved to be difficult to fit the SED and the intensity profile simultaneously however. The use of the CDE leads to “broad” SEDs when compared to spherical grains, and this results in steep density distributions ( typically).