The Integrated Calcium II Triplet as a Metallicity Indicator:
  Comparisons with High Resolution [Fe/H] in M31 Globular Clusters
  Medium resolution (R=4,000 to 9,000) spectra of the near infrared Ca II lines
(at 8498, 8542, and 8662 A) in M31 globular cluster integrated light spectra
are presented. In individual stars the Ca II triplet (CaT) traces stellar
metallicity; this paper compares integrated CaT strengths to well determined,
high precision [Fe/H] values from high resolution integrated light spectra. The
target globular clusters cover a wide range in metallicity (from [Fe/H] = -2.1
to -0.2). While most are older than 10 Gyr, some may be of intermediate age
(2-6 Gyr). A handful (3-6) have detailed abundances (e.g. low [Ca/Fe]) that
indicate they may have been accreted from dwarf galaxies. Using various
measurements and definitions of CaT strength, it is confirmed that for GCs with
[Fe/H] < -0.4 and older than 2 Gyr the integrated CaT traces cluster [Fe/H] to
within about 0.2 dex, independent of age. CaT lines in metal rich GCs are very
sensitive to nearby atomic lines (and TiO molecular lines in the most metal
rich GCs), largely due to line blanketing in continuum regions. The [Ca/Fe]
ratio has a mild effect on the integrated CaT strength in metal poor GCs. The
integrated CaT can therefore be safely used to determine rough metallicities
for distant, unresolved clusters, provided that attention is paid to the limits
of the measurement techniques.
