Dust is forming along the red giant branch of 47 TUC

TL;DR

This study provides strong evidence that dust forms along the red giant branch of 47 Tuc, using infrared excess measurements and high-resolution imaging to improve detection sensitivity and refute previous claims of artifacts.

Contribution

It introduces a more sensitive infrared diagnostic (K-8) for detecting circumstellar dust and confirms dust formation along the RGB with high-resolution imaging, challenging prior artifact-based interpretations.

Findings

Infrared excess (K-8) effectively detects circumstellar dust.

Dust forms along the entire RGB of 47 Tuc.

High-resolution images confirm genuine dust signatures.

Abstract

We present additional evidence that dust is really forming along the red giant branch (RGB) of 47 Tuc at luminosities ranging from above the horizontal branch to the RGB-tip (Origlia et al. 2007). The presence of dust had been inferred from an infrared excess in the (K-8) color, with K measured from high spatial resolution ground based near-IR photometry and "8" referring to Spitzer-IRAC 8 micron photometry. We show how (K-8) is a far more sensitive diagnostic for detecting tiny circumstellar envelopes around warm giants than colors using only the Spitzer-IRAC bands, for example the (3.6-8) color used by Boyer et al. (2010). In addition, we also show high resolution HST-ACS I band images of the giant stars which have (K-8) color excess. These images clearly demonstrate that Boyer et al (2010) statement that our detections of color excess associated with stars below the RGB-tip arise from blends and artefacts is simply not valid.