The trigger of the AGB superwind: the importance of carbon

TL;DR

This paper proposes a dual-trigger mechanism for the AGB superwind, driven either by carbon excess or luminosity, explaining observed differences in mass loss in stars of varying metallicities.

Contribution

It introduces a new dual-trigger model for the AGB superwind, supported by stellar evolution models and observational data.

Findings

Carbon-rich stars in low metallicity galaxies lose mass at lower luminosities.

Oxygen-rich stars have similar mass-loss rates at high luminosities across metallicities.

The dual-trigger model aligns with current observational constraints.

Abstract

The driving mechanism of the AGB superwind has become controversial in recent years. The efficacy of dust-driven mass loss has been queried. Spitzer observation of AGB stars in Local Group Galaxies show the surprising result that at low metallicity, AGB mass loss occurs at low luminosity, possibly lower than in the Galaxy, but only for carbon-rich stars. Oxygen-rich stars in the Galaxy and in lower metallicity galaxies have similar mass-loss rates only at high luminosities. To explain this dichotomy, we propose that the superwind has a dual trigger. The superwind starts either when sufficient excess carbon builds up for efficient formation of carbonaceous dust (which we propose occurs when $X_{\rm CO} = \rm (C - O)/ O_\odot = 0.1$), or when the luminosity reaches a value sufficient for a silicate-dust-driven wind (proposed at $L = 10^4 Z^{-4/3} \rm L_\odot)$. We show that this dual trigger fits the current observational constraints: the luminosity at which the superwind begins, and the predominance of carbon superwind star at low metallicity. We use stellar evolution models to check the consistency of our explanations and present detailed predictions of the luminosities at which the superwind is triggered for different metallicities and initial stellar masses.