1 Introduction

The circumstellar envelopes (CSEs) of asymptotic giant branch (AGB) stars are the results of extensive stellar mass loss. This is one of the most important processes affecting the final evolution of low- and intermediate-mass stars. The CSEs may also be one of the principal seed beds for the formation of (more or less) complex molecules and dust grains, and they are among the main contributors to the (nuclear-processed) gas mass return in the Galaxy. Hence, an understanding of the circumstellar environment is astrophysically very important (Olofsson 1999). Even though the general phenomenon of mass loss on the AGB is well established there remains a number of unsolved problems. One of them is the temporal variations of the mass loss on different time scales, and the mechanisms responsible for this. In this paper we are concerned with variations in individual stars on time scales of $\lesssim$10³ to about 10⁴ years.

In a few cases, the circumstellar material has been observed in the form of completely detached gas/dust shells of large size, $\gtrsim$10 $\hbox{$^{\prime\prime}$ }$. In particular, CO radio line observations reveal this phenomenon for a number of carbon stars: R Scl, U Ant, S Sct (Olofsson et al. 1996); TT Cyg (Olofsson et al. 1996, 2000); U Cam (Lindqvist et al. 1996, 1999). Also, detached dust shells have been detected around a handful of AGB and post-AGB stars (Waters et al. 1994; Izumiura et al. 1996, 1997; Hashimoto et al. 1998; Speck et al. 2000). In addition, multiple-shell structures have been seen in scattered light towards the high mass loss rate carbon star IRC+10216 and some post-AGB objects (Harpaz et al. 1997; Kwok et al. 1998; Sahai et al. 1998; Mauron & Huggins 1999, 2000). These data suggest more or less episodic mass loss on a number of different time scales (Olofsson et al. 1990; Schröder et al. 1999), and effects of interacting winds and interactions between gas and dust in winds may also be present (Olofsson et al. 2000; Steffen & Schönberner 2000; Simis et al. 2001).

In González Delgado et al. (2001, hereafter Paper I) we succeeded in imaging two of these detached shells in scattered stellar light, R Scl and U Ant; a novel way of obtaining independent data at an angular resolution much higher than that usually obtainable at radio wavelengths. The images exhibit brightness distributions in the form of relatively uniform-intensity disks, centred on the stars, with sharp outer radii at 21 $\hbox{$^{\prime\prime}$ }$ and 43 $\hbox{$^{\prime\prime}$ }$ in the case of R Scl and U Ant, respectively. For U Ant, the CO radio line data and the optical images are in good agreement in terms of the size of the detached shell, but the latter reveal new structures not present in the former. For R Scl the light appears to be scattered in a shell, which is significantly larger than the one detected in CO.

An interesting way of continuing this study is to obtain also polarimetric information on the scattered light. The advent of modern imaging techniques has opened up the possibility to obtain images in polarised light. Such data have been used to investigate the morphology of the CSE around IRC+10216 at infrared wavelengths. They show an axi-symmetric reflection nebula with the central star acting as the source of illumination (Tamura et al. 1988; Kastner & Weintraub 1994). Also, the light scattered in the extended dust envelopes around two supergiants, $\alpha$ Ori and $\mu$ Cep, taken in UBV filters, shows indications of polarisation (Le Borgne et al. 1986; Mauron & Le Borgne 1986; Le Borgne & Mauron 1989).

Imaging polarimetry has proven to be a very efficient observational mode for resolving the dust around bright central stars on small angular scales, regions usually unreachable by conventional imaging. Gledhill et al. (2001) used this technique in a study of a sample of proto-planetary nebulae (PPNe). It allowed them to classify the envelope morphologies on the basis of the polarisation characteristics. It is also a good tool for determining the position of the illuminating stars in PPNe. Thus, the presence of a self-luminous source in the centre of the Egg Nebula, as derived from direct and polarimetric images obtained with HST/NICMOS, led to the identification of a binary companion to the central star, which irradiates the nebula (Sahai et al. 1998; Weintraub et al. 2000).

In this paper we study the polarisation of optical light scattered in the circumstellar environments around the two carbon stars presented in direct images in Paper I, R Scl and U Ant. For these objects the imaging polarimetry allows a further study of the nature of the scattered light and the structure of the shells.

 

 

Table 1: Observations performed with EFOSC2 on the ESO 3.6 m telescope.

Star	Epoch	Pixel size	Filters	Coronograph	Template star
R Scl	Nov. 00	0 $\hbox{$.\!\!^{\prime\prime}$ }$32	F59, F77	Needle mask	HR 4565
U Ant	March 01	0 $\hbox{$.\!\!^{\prime\prime}$ }$32	F59, F77	Spot on kevlar spiders	HR 1877