Imaging the circumstellar dust around AGB stars with PolCor^⋆

¹ Argelander Institute for Astronomy, University of Bonn, 53121 Bonn, Germany
e-mail: sofia@astro.uni-bonn.de
² European Southern Observatory, Karl Schwarzschild Str. 2, Garching bei München, Germany
³ Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
⁴ Onsala Space Observatory, Dept. of Radio and Space Science, Chalmers University of Technology, 43992 Onsala, Sweden

Received: 21 October 2010
Accepted: 16 May 2011

Abstract

Aims. The aim of this paper is to investigate how the new imaging Polarimeter and Coronograph (PolCor) at the Nordic Optical Telescope^⋆⋆ (NOT) can be used in the study of circumstellar structures around AGB stars. The purpose is to prepare for a study of a larger sample.

Methods. We have observed two types of AGB stars using the PolCor instrument on the NOT: the binary S-type star W Aql and two carbon stars with detached shells, U Cam and DR Ser. The polarized light traces the dust distribution around the stars. From the polarimeter images the polarized intensity, the polarization degree, and the polarization angle over the images are calculated. The location and extent of dust structures are examined in the images. The total dust mass and the dust-to-gas ratios of the detached shells are also calculated.

Results. The images of the circumstellar envelope of W Aql show what seems to be an elongated structure in the south-west direction. The detached shells of U Cam and DR Ser are clearly seen in the images. This is the first time the detached shell around DR Ser has been imaged. The radii (R_sh) and widths (ΔR_sh) of the shells are determined and found to be and 76, and and 12, for U Cam and DR Ser, respectively. This is consistent with previous results. The dust masses of the feature south-west of W Aql, and in the shells of U Cam and DR Ser are also estimated and found to be 1 × 10^-6, 5 × 10^-7, and 2 × 10^-6   M_⊙, respectively.

Conclusions. W Aql is a known binary and the shape of the circumstellar envelope seems to be in line with what could be expected from binary interaction on these scales. For the shells, the results are in agreement with previous investigations. Ages and formation time-scales are also estimated for the detached shells and found to be consistent with the thermal-pulse-formation scenario.