AGB stars
(Cioni, Gledhill)
Variability and chemistry of AGB stars
The luminosity of AGB stars varies periodically with long periods and large amplitudes. Long term microlensing programs (i.e. EROS, MACHO and OGLE) have provided light-curves for many AGB stars in the Magellanic Clouds and in the Bulge. At least four period-magnitude relations were discovered (i.e. Wood et al. 1999, Cioni et al. 2001, 2003). All but one relation is associated to a different mode of radial pulsation which changes from overtone to fundamental mode for ageing stars. Mira variables, important distance indicators, are luminous fundamental mode pulsators. This result has been confirmed by comparing interferometric angular diameters with models of large amplitude pulsating atmospheres. The yet unexplained relation is perhaps occupied by binary stars with an AGB companion. Wood and Cioni are investigating this possibility; data from a spectroscopic monitoring campaign with FLAMES at the VLT are being analysed. The goal is to combine photometric and radial velocity variations in order to constrain the nature of those stars with an unexplained type of large amplitude variability. They represent about 25% variable AGB stars in the Large Magellanic Cloud.
AGB stars produce a considerable fraction of the heavy elements present in the interstellar medium which set the basis for the formation of a new generation of stars. High-resolution spectrographs operating on large telescope allow us to directly measure the abundance of these elements. Although the spectra of AGB stars are dominated by molecular bands, often preventing the analysis of individual atomic lines, the constant development of model atmospheres and spectral synthesis for these complex and dynamical objects shows a very active field of research likely to bring exiting results in the near future.
Imaging polarimetry of AGB stars
AGB stars lose mass at a rather high rate. Thus, they are among the major contributors of heavy elements to the interstellar medium for the formation of new stars and planets. They are important sources of elements produced via the s-process and of dust which forms in the outer part of their atmosphere. AGB stars are also the precursors of beautiful, mostly a-spherical PNe, but the transition is poorly understood: is the shape the result of binary interaction or is it due to intrinsic properties of the AGB progenitor? Recent results indicate that circumstellar envelopes already become highly a-spherical during the AGB phase, possibly under the influence of a strong magnetic field. Polarimetry of a large sample of stars can provide important clues about when the AGB star envelopes become spherical, as polarisation is a measure of asymmetry of the scattering particles around the star.
Imaging polarimetry of the Magellanic Cloud, to which distances are sufficiently accurate to measure the degree of linear polarisation, have been obtained from La Silla using the EFOSC2 instrument (optical) while a collaboration has been established to use the SIRPOL instrument (near-infrared) to significantly contribute to the investigation of a large sample of stars. This sample will allow us to study the relation between the degree of polarisation and several key stellar parameters providing crucial constraints for the models that describe the formation of a-spherical PN. Since the new opportunity offered by wide-field imaging polarimetry includes stars of different types (not only evolved giants) other aspects of stellar structure and evolution, such as the formation of massive stars, will be constrained.