Modelling the Winds from Carbon Rich AGB Stars : Implications for Stellar Evolution and the Chemical Evolution of Galaxies

Abstract: Stars on the AGB are known to lose mass at high rates (10-8 - 10-4 Msun/yr) through slow winds (20 km/s), enriching the inter stellar medium with newly synthesised elements. We explore how the mass loss for C-rich AGB stars depends on the basic stellar parameters by computing a large grid of wind models. Our first results confirm the existence of a critical wind regime and a threshold for mass loss due to dust-driven wind. The region in parameter space where dust-driven winds are possible is quite small, which will have a significant effect on stellar evolution - especially the evolution during the TP-AGB phase - as well as on nucleosynthesis. We have also considered the mass loss evolution during a He-shell flash in some detail. Starting from a stellar evolutionary track, we have computed the time evolution of the atmosphere and stellar wind during a flash event with detailed frequency-dependent radiation hydrodynamical models including dust formation and simulated the subsequent long-term dynamical evolution of the circumstellar envelope (CSE), including the formation of a so-called detached shell. Most important for the formation of a detached shell is the wind velocity evolution, i.e., whether di fferent wind phases are formed that may lead to wind-wind interactions. Our CSE model shows that a thin shell structure may be formed as a consequence of a rather short phase of intense mass loss in combination with a significant variation in the wind velocity. Finally, we have made a comparative study of stellar yields where we have considered a simplistic chemical evolution model: a so-called closed-box model for the evolution of metal-poor blue compact galaxies. For typical input parameters this model fails to reproduce the observed nitrogen abundance and He-fraction regardless of which yield sets we use. By increasing the number of high mass stars we can fit the data, but a wide spread in the predicted nitrogen abundances, still remains. This spread is mostly due to large variations in the nitrogen contribution from AGB stars. Our conclusion is therefore that the nitrogen-yield for AGB stars is the perhaps greatest uncertainty associated with the origin and evolution of nitrogen.