Spitzer spectroscopy of mass-loss and dust production by evolved stars in globular clusters

Abstract

We have observed a sample of 35 long-period variables (LPVs) and four Cepheid variables in the vicinity of 23 Galactic globular clusters using the Infrared Spectrograph on the Spitzer Space Telescope. The LPVs in the sample cover a range of metallicities from near solar to about 1/40th solar. The dust mass-loss rate (MLR) from the stars increases with pulsation period and bolometric luminosity. Higher MLRs are associated with greater contributions from silicate grains. The dust MLR also depends on metallicity. The dependence is most clear when segregating the sample by dust composition, less clear when segregating by bolometric magnitude, and absent when segregating by period. The spectra are rich in solid-state and molecular features. Emission from alumina dust is apparent across the range of metallicities. Spectra with a 13 μm dust emission feature, as well as an associated feature at 20 μm, also appear at most metallicities. Molecular features in the spectra include H2O bands at 6.4-6.8 μm, seen in both emission and absorption, SO2 absorption at 7.3-7.5 μm, and narrow emission bands from CO2 from 13.5 to 16.8 μm. The star Lyngå 7 V1 has an infrared spectrum revealing it to be a carbon star, adding to the small number of carbon stars associated with Galactic globular clusters. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.