Intense Mass Loss from C-rich AGB Stars at low Metallicity?

TL;DR

This study shows that pulsation energy, rather than metallicity, primarily influences mass-loss rates in C-rich AGB stars, challenging previous assumptions about metallicity's role.

Contribution

The paper demonstrates that pulsation energy input governs mass-loss rates in AGB stars more than metallicity, emphasizing the importance of energy injection over chemical composition.

Findings

Mass-loss rate scales with pulsation kinetic energy input.

Lower metallicity does not necessarily mean lower mass loss if pulsation energy is similar.

Adjusting pulsation amplitude can compensate for metallicity effects.

Abstract

We argue that the energy injection of pulsations may be of greater importance to the mass-loss rate of AGB stars than metallicity, and that the mass-loss trend with metallicity is not as simple as sometimes assumed. Using our detailed radiation hydrodynamical models that include dust formation, we illustrate the effects of pulsation energy on wind properties. We find that the mass-loss rate scales with the kinetic energy input by pulsations as long as a dust-saturated wind does not occur, and all other stellar parameters are kept constant. This includes the absolute abundance of condensible carbon (not bound in CO), which is more relevant than keeping the C/O-ratio constant when comparing stars of different metallicity. The pressure and temperature gradients in the atmospheres of stars, become steeper and flatter, respectively, when the metallicity is reduced, while the radius where the atmosphere becomes opaque is typically associated with a higher gas pressure. This effect can be compensated for by adjusting the velocity amplitude of the variable inner boundary (piston), which is used to simulate the effects of pulsation, to obtain models with comparable kinetic-energy input. Hence, it is more relevant to compare models with similar energy-injections than of similar velocity amplitude. Since there is no evidence for weaker pulsations in low-metallicity AGB stars, we conclude that it is unlikely that low-metallicity C-stars have a lower mass-loss rate, than their more metal-rich counterparts with similar stellar parameters, as long as they have a comparable amount of condensible carbon.