Tracing the Development of Dust around Evolved Stars: The Case of 47 Tuc

We observed mid-infrared (7.5-22 μm) spectra of asymptotic giant branch (AGB) stars in the globular cluster 47 Tuc with the Spitzer Space Telescope, and we find significant dust features of various types. A comparison of the characteristics of the dust spectra with the location of the stars in a (log P, K)-diagram shows that the dust mineralogy and the position on the AGB are related. A 13 μm feature is seen in spectra of low-luminosity AGB stars. More luminous AGB stars show a broad feature at 11.5 μm. The spectra of the most luminous stars are dominated by the amorphous silicate bending vibration centered at 9.7 μm. For 47 Tuc AGB stars, we conclude that early on the AGB dust (consisting primarily of Mg, Al, and Fe oxides) is formed. With further AGB evolution, amorphous silicates become the dominant species.