On the Correlation Between Metallicity and the X-Shaped Morphology of
  the Milky Way Bulge
  We demonstrate that failure to properly account for stellar evolution can
bias results in determinations of the spatial morphology of Galactic bulge
stars, focusing on the question of whether or not the X-shape is more
pronounced among the more metal-rich stars than among the metal-poor stars. We
argue that this trend, a result recently claimed by three separate groups, may
have been overestimated as it is relatively easier to detect a bimodality in
the distance distribution function at higher metallicities. This is due to
three factors. First, the intrinsic colour of red clump and red giant stars
vary with metallicity, at the level d(V-I)_{RC}/d\[M/H] ~ 0.25 mag dex^{-1},
and thus the ratio of red clump to red giant stars within a spectroscopic
sample will depend on the photometric selection of any investigation. Second,
the duration of ascent of the red giant branch goes down and the red clump
lifetime goes up as metallicity increases, which has the effect of increasing
the ratio of red clump to red giant stars by as much as ~33% over the range of
the bulge metallicity-distribution function. Finally, over the same metallicity
interval, the effective number of red giant branch bump stars is predicted to
increase by ~200%, and their presence becomes degenerate with the observational
parameters of the two red clumps, creating an illusory increase in
signal-to-noise for a second peak in the distance modulus distribution.
