During this phase the stellar atmosphere increases dramatically in size and the luminosity gets many orders of magnitude larger: the star becomes a red giant on the Asymptotic Giant Branch (AGB). This is also a phase of instability where variations of the luminosity and radius are commonly observed. In this phase, stars may lose a quite substantial amount of matter, up to 10-5 Mo per year, with velocities on the order of 5 to 20 km/s. This phase of mass loss may last for at most a million years. A circumstellar envelope develops: it may reach a size of one light-year and contain several solar masses, before getting diluted into the interstellar medium (ISM).
In the solar neighborhood, these stars contribute to 3/4 of the replenishment of the ISM, not accounting for the infall from the exterior of the Galaxy. The material flowing out of AGB stars is enriched in elements synthesized in their interiors. It is thus important to understand and describe more precisely the mecanisms of ejection from AGB stars. In their atmospheres dust grains form by condensation of the refractory elements contained in the gas. These grains absorb very efficiently the stellar photons and are pushed outwards by radiation pressure. They transfer the momentum to the gas which thereby is accelerated, producing the massive outflows.
T. Le Bertre (Observatory of Paris) and J. M. Winters (Technical University Berlin) have recently shown that it is possible to estimate the mass loss rates of these stars from color indices obtained in the near-infrared (this spectral range is explored systematically through the European project DENIS). Furthermore, with elaborate numerical simulations, accounting for the non-stationary hydrodynamics of the outflows, the circumstellar chemistry, and the formation of dust, they have been able to clarify the ejection mechanisms. A major result (Winters et al. 2000) is the discovery that AGB winds may split into 2 categories: in the first one (A in the accompanying figure), the outflows are driven by radiation pressure on dust. This comes out when enough dust is formed close to the star. In the second category (B), the mass loss rate is always smaller than 3 10-7 Mo per year, because the dust is not formed efficiently and the radiation pressure stays low. In these situations, the outflows are mainly due to the pulsations of the central stars and the velocities are small (less than 5 km/s).
These new results are important to describe the replenishment of the ISM and to estimate its enrichment in elements processed in the interiors of stars of mass comparable to the mass of the Sun.
Winters J.M., Le Bertre T., Jeong K.S., Helling C., Sedlmayr E.: 2000, "A systematic investigation of the mass loss mechanism in dust forming long-period variable stars", Astron. and Astrophys., accepted
Last update on 22 October 2013