Spitzer Spectroscopy of Mass Loss and Dust Production by Evolved Stars
  in Globular Clusters
  We have observed a sample of 35 long-period variables and four Cepheid
variables in the vicinity of 23 Galactic globular clusters using the Infrared
Spectrograph on the Spitzer Space Telescope. The long-period variables in the
sample cover a range of metallicities from near solar to about 1/40th solar.
The dust mass-loss rate from the stars increases with pulsation period and
bolometric luminosity. Higher mass-loss rates are associated with greater
contributions from silicate grains. The dust mass-loss rate 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-um dust emission feature, as well as an
associated feature at 20 um, also appear at most metallicities. Molecular
features in the spectra include H_2O bands at 6.4-6.8 um, seen in both emission
and absorption, SO_2 absorption at 7.3-7.5 um, and narrow emission bands from
CO_2 from 13.5 to 16.8 um. The star Lynga 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.
