Spitzer spectra of evolved stars in omega Centauri and their low-metallicity dust production

TL;DR

This study investigates dust production in 14 metal-poor giant stars in omega Centauri using Spitzer spectra, revealing that dust formation is limited in the most metal-poor stars and dominated by metallic iron grains in certain cases.

Contribution

First spectral study of dust production in a metal-poor ([Fe/H] < -1) globular cluster population, highlighting the role of metallic iron dust in low-metallicity stars.

Findings

Only the relatively metal-rich stars show silicate emission.

Most metal-poor stars do not produce detectable dust.

Metallic iron dominates dust in metal-poor, oxygen-rich stars.

Abstract

Dust production is explored around 14 metal-poor ([Fe/H] = -1.91 to -0.98) giant stars in the Galactic globular cluster omega Centauri using new Spitzer IRS spectra. This sample includes the cluster's post-AGB and carbon stars and is thus the first representative spectral study of dust production in a metal-poor ([Fe/H] < -1) population. Only the more metal rich stars V6 and V17 ([Fe/H] = -1.08, -1.06) exhibit silicate emission, while the five other stars with mid-infrared excess show only a featureless continuum which we argue is caused by metallic iron dust grains. We examine the metallicity of V42, and find it is likely part of the metal-rich population ([Fe/H] ~ -0.8). Aside from the post-AGB star V1, we find no star from the cluster's bulk, metal-poor ([Fe/H] < -1.5) population - including the carbon stars - to be producing detectable amounts of dust. We compare the dust production to the stars' H-alpha line profiles obtained at the Magellan/Clay telescope at Las Campanas Observatory, finding pulsation shocking in the strongest pulsators (V6, V17 and V42), but evidence of outflow in all other stars. We conclude that the onset of dust production does not signify a fundamental change in the material leaving the star. Our data add to a growing body of evidence that metallic iron dominates dust production in metal-poor, oxygen-rich stars, but that dust is probably not the primary accelerant of winds in this mass-metallicity regime.