Dust production and mass loss in the Galactic globular cluster 47 Tucanae

TL;DR

This study investigates dust production and mass loss in 47 Tucanae, identifying metallic iron grains as key opacity sources and suggesting pulsations may drive stellar winds independently of radiation pressure.

Contribution

It provides new insights into dust composition and mass-loss mechanisms in globular cluster stars, highlighting the role of iron grains and pulsations.

Findings

Metallic iron grains likely dominate opacity in stellar winds.

Mass-loss rates may be overestimated due to grain properties.

Dusty mass loss onset is linked to stellar variability.

Abstract

Dust production among post-main-sequence stars is investigated in the Galactic globular cluster 47 Tucanae (NGC 104) based on infrared photometry and spectroscopy. We identify metallic iron grains as the probable dominant opacity source in these winds. Typical evolutionary timescales of AGB stars suggest the mass-loss rates we report are too high. We suggest that this is because the iron grains are small or elongated and/or that iron condenses more efficiently than at solar metallicity. Comparison to other works suggests metallic iron is observed to be more prevalent towards lower metallicities. The reasons for this are explored, but remain unclear. Meanwhile, the luminosity at which dusty mass loss begins is largely invariant with metallicity, but its presence correlates strongly with long-period variability. This suggests that the winds of low-mass stars have a significant driver that is not radiation pressure, but may be acoustic driving by pulsations.