Ejecta and the Late Stages of Stellar Evolution

Abstract

The late stages of stellar evolution are characterised by prodigious mass-loss, particularly on the Asymptotic Giant Branch (AGB), resulting in the build up of a circumstellar envelope of gas and dust. In this way, the majority of stars (» 0:8 M¯ < M < » 8 M¯) will have shed their excess mass by the end of the AGB, allowing them to evolve to hotter temperatures to become the central stars of planetary nebulae (PN). With the expulsion of so much dust into the circumstellar environment, polarization (and polarimetry) becomes a key techmique in the study of these objects. At wavelengths shortwards of 5 ¹m, AGB and post-AGB objects (those in transition to the PN phase are often termed proto-PN) are seen predominantly by dust-scattered light, and are therefore polarized. Imaging polarimetry may be used to investigate the geometry of the dust shells, the distribution of dust therein (and hence the massloss rate evolution on the AGB) and also the nature of the scattering particles themselves (the dust grains). A particularly enduring puzzle concerns the origin of the complex structures seen in PN, which originate from what are presumed to be spherically symmetric outflows on the AGB. Polarimetry is the natural tool with which to search for and investigate the origin of asymmetric structure, such as bipolar outflows, which reveal themselves in the form of significant net linear polarization. At wavelengths longer than 5 ¹m, thermal emission from warm (150 K) dust prevails. This too may be polarized, if the grains are non-spherical and aligned, for example by a magnetic field. The possible role of magnetic fields in driving and collimating asymmetric outflows from evolved stars, in the form of magnetised winds, has received recent attention, and a number of theoretical models have been proposed. The ability of polarimetry to detect these fields and to determine their structure will offer vital observational tests for the models. An alternative approach to determining field configurations and strengths is the use of polarimetric imaging of maser emission at radio wavelengths. The use of polarimetry in studies of evolved stars and their ejecta, over a wide range of wavelengths and spatial resolutions will be reviewed.