Dust is Forming Along the Red Giant Branch of 47 Tuc

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. 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-Infrared Array Camera (IRAC) 8 μm 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. In addition, we also show high-resolution Hubble Space Telescope Advanced Camera for Surveys I-band images of the giant stars that have (K - 8) color excess. These images clearly demonstrate that the Boyer et al. statement that our detections of color excess associated with stars below the RGB tip arise from blends and artifacts is simply not valid.

This work is based on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA through an award issued by JPL/Caltech.