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S. E. Robinson (RIT), K. J. Mighell (NOAO/KPNO)
We present our analysis of archival Hubble Space Telescope WFPC2 observations in the F555W (V) and F814W (I) filters of the intermediate-age metal-rich populous star cluster Terzan 7. We compare our VI color-magnitude diagram with the theoretical Y2 isochrones of Yi et al. and Padua 2000 isochrones of Girardi et al. We find that the Y2 theoretical isochrones with the Lejeune color transformations are able to replicate the morphological features present in our HST photometry from the main-sequence turnoff to just below the horizontal branch, which is the brightest limit of our data. The Y2 theoretical isochrones with the Green color transformations are able to reproduce the morphology of the lower main-sequence of Terzan 7, but only after being shifted by a small blueward offset of 0.02 mag. Comparing the Y2 isochrones with the Padua 2000 models at a metallicity of z=0.004 and an age of 10 Gyr, we find that the Padua 2000 models systematically predict hotter effective temperatures for main-sequence turnoff stars and cooler effective temperatures for stars at the lower end of the red giant branch. We determine that Terzan 7 is 8 +/- 0.5 Gyr old with a metallicity of Z=0.007 ([Fe/H]=-0.43 +/- 0.12 dex), which is in agreement with spectroscopic estimates based on the calcium II triplet lines, but is significantly more metal-rich than previously determined from photometric methods.
Sarah Robinson's research was supported by the NOAO/KPNO REU Program, funded by the National Science Foundation. Mighell was supported by a grant from the National Aeronautics and Space Administration (NASA), S-67046-F, awarded by the Long-Term Space Astrophysics program of NASA's Office of Space Science.
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The author(s) of this abstract have provided an email address for comments about the abstract: mighell@noao.edu