Dust driven mass loss from carbon stars as a function of stellar parameters - I. A grid of Solar-metallicity wind models

TL;DR

This study presents a comprehensive grid of dynamic models for dust-driven winds in carbon stars, highlighting the critical role of stellar parameters and carbon abundance in mass loss, and providing a practical mass-loss routine for stellar evolution models.

Contribution

It introduces a detailed grid of 900 wind models covering key stellar parameters, emphasizing the importance of carbon abundance and grain size effects, and offers a new mass-loss prescription for stellar evolution.

Findings

Mass-loss rates depend strongly on stellar temperature, mass, luminosity, and carbon abundance.

A mass-loss threshold exists below which dust-driven winds cannot form.

Dust grain sizes may exceed the small particle approximation, affecting wind properties.

Abstract

[Abridged] We have computed a grid of 900 numeric dynamic model atmospheres (DMAs) using a well-tested computer code. This grid of models covers most of the expected combinations of stellar parameters, which are made up of the stellar temperature, the stellar luminosity, the stellar mass, the abundance of condensible carbon, and the velocity amplitude of the pulsation. The resultant mass-loss rates and wind speeds are clearly affected by the choice of stellar temperature, mass, luminosity and the abundance of available carbon. In certain parts of the parameter space there is also an inevitable mass-loss threshold, below which a dust-driven wind is not possible. Contrary to some previous studies, we find a strong dependence on the abundance of free carbon, which turns out to be a critical parameter. Furthermore, we have found that the dust grains that form in the atmosphere may grow too large for the commonly used small particle approximation of the dust opacity to be strictly valid. This may have some bearing on the wind properties, although further study of this problem is needed before quantitative conclusions can be drawn. The wind properties show relatively simple dependences on stellar parameters above the mass-loss threshold, while the threshold itself is of a more complicated nature. Hence, we chose not to derive any simplistic mass-loss formula, but rather provide a mass-loss prescription in the form of an easy-to-use FORTRAN routine. Since this mass-loss routine is based on data coming from an essentially self-consistent model of mass loss, it may therefore serve as a better mass-loss prescription for stellar evolution calculations than empirical formulae. Furthermore, we conclude that there are still some issues that need to be investigated, such as the role of grain-sizes.