Measuring Metallicities with HST/WFC3 photometry
  We quantified and calibrated the metallicity and temperature sensitivities of
colors derived from nine Wide Field Camera 3 (WFC3) filters aboard the Hubble
Space Telescope using Dartmouth isochrones and Kurucz atmospheres models. The
theoretical isochrone colors were tested and calibrated against observations of
five well studied galactic clusters: M92, NGC 6752, NGC 104, NGC 5927, and NGC
6791, all of which have spectroscopically determined metallicities spanning
-2.30 < [Fe/H] < +0.4. We found empirical corrections to the Dartmouth
isochrone grid for each of the following color magnitude diagrams (CMD)
(F555W--F814W, F814W), (F336W-F555W, F814W), (F390M-F555W, F814W) and
(F390W-F555W, F814W). Using the empirical corrections we tested the accuracy
and spread of the photometric metallicities assigned from CMDs and color-color
diagrams (which are necessary to break the age-metallicity degeneracy). Testing
three color-color diagrams [(F336W-F555W),(F390M-F555W),(F390W-F555W), vs
(F555W-F814W)], we found the colors (F390M-F555W) and (F390W-F555W), to be the
best suited to measure photometric metallicities. The color (F390W-F555W)
requires much less integration time, but generally produces wider metallicity
distributions, and, at very-low metallicity, the MDF from (F390W-F555W) is ~60%
wider than that from (F390M-F555W). Using the calibrated isochrones we
recovered the overall cluster metallicity to within ~0.1 dex in [Fe/H] when
using CMDs (i.e. when the distance, reddening and ages are approximately
known). The measured metallicity distribution function (MDF) from color-color
diagrams show this method measures metallicities of stellar clusters of unknown
age and metallicity with an accuracy of ~0.2 - 0.5 dex using F336W--F555W,
~0.15 - 0.25 dex using F390M-F555W, and ~0.2 - 0.4 dex with F390W-F555W, with
the larger uncertainty pertaining to the lowest metallicity range.
