Is Dust Forming on the Red Giant Branch in 47 Tuc?

TL;DR

This study re-evaluates dust formation in 47 Tuc using Spitzer data, finding that previous detections were affected by artifacts and that dust is mainly present in the coolest, most luminous variable stars near the RGB tip.

Contribution

It clarifies the true dust-producing stars in 47 Tuc, correcting earlier claims by addressing observational artifacts and stellar blending issues.

Findings

Dust is likely only produced by the coolest, most luminous variable stars.

Stars below about 1 magnitude from the RGB tip do not produce detectable dust.

Previous infrared excess detections were caused by observational artifacts.

Abstract

Using Spitzer IRAC observations from the SAGE-SMC Legacy program and archived Spitzer IRAC data, we investigate dust production in 47 Tuc, a nearby massive Galactic globular cluster. A previous study detected infrared excess, indicative of circumstellar dust, in a large population of stars in 47 Tuc, spanning the entire Red Giant Branch (RGB). We show that those results suffered from effects caused by stellar blending and imaging artifacts and that it is likely that no stars below about 1 mag from the tip of the RGB are producing dust. The only stars that appear to harbor dust are variable stars, which are also the coolest and most luminous stars in the cluster.