870 µm observations of evolved stars with LABOCA
Journal article, 2010
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
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 (∝r−2.2 typically).
Stars: AGB and post-AGB