The trigger of the asymptotic giant branch superwind: the importance of carbon

The driving mechanism of the asymptotic giant branch (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 shows 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-CO = (C-O)/O-circle dot = 0.1], or when the luminosity reaches a value sufficient for a silicate dust-driven wind (proposed at L = 10(4)Z(-4/3) L-circle dot). 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.