On the Connection between Mass Loss and Evolution of C-rich AGB stars

TL;DR

This paper investigates the mass loss processes of carbon-rich AGB stars, aiming to improve stellar evolution models by detailed simulations of mass loss during He-shell flashes and the circumstellar envelope evolution.

Contribution

It presents detailed radiation-hydrodynamical models of mass loss during He-shell flashes and explores the limitations of simple mass loss prescriptions in stellar evolution.

Findings

Existing prescriptions differ significantly from detailed models.

Mass loss evolution during He-shell flashes is complex and non-linear.

Simple parameterisations are inadequate in certain stellar parameter regions.

Abstract

The mass loss properties of carbon AGB stars are not very well constrained at present. A variety of empirical or theoretical formulae with different parameterisations are available in the literature and the agreement between them is anything but good. These simple mass loss prescriptions are nontheless used in many models of stellar evolution without much consideration of their applicability in various cases. We present here an on-going project aiming at a better description of the mass loss, that could be used to improve stellar evolution models -- especially the evolution during the TP-AGB phase. As a first step, we have considered the mass loss evolution during a He-shell flash. Using stellar parameters sampled from a stellar evolutionary track, we have computed the time evolution of the atmospheric layers and wind acceleration region during a flash event with detailed frequency-dependent radiation-hydrodynamical models including dust formation. Based on these results, we have also simulated the subsequent long-term dynamical evolution of the circumstellar envelope (CSE), including the formation of a detached shell. We find that existing simple mass loss prescriptions imply different mass loss evolutions compared with our model. The second step of the project deals with the dependence of mass loss on the basic stellar parameters. At the moment we are computing a large grid of wind models for C-rich AGB stars. Preliminary results show that simple parameterisations are difficult to obtain in certain regions of the parameter space considered, due to strong non-linearities in the wind mechanism.