Relative Ages of Nine Inner Milky Way Globular Clusters from Proper
  Motion Cleaned Color-Magnitude Diagrams
  Our picture of the age-metallicity relation for Milky Way globular clusters
(MWGCs) is still highly incomplete, and the majority of MWGCs lack
self-consistent age measurements. Here, we exploit deep, homogenous multi-epoch
Hubble Space Telescope (HST) imaging of nine MWGCs located towards the inner
Milky Way to measure their relative ages, in most cases for the first time. Our
relative age measurements are designed to be directly comparable to the large
set of MWGC ages presented by VandenBerg et al. (2013, V13), using identical
filters, evolutionary models, and bolometric corrections, extended to the
higher extinction values relevant to our target clusters. Adopting the V13 MWGC
age scale, our relative age measurements imply that our target clusters are
consistently very old, with a mean age of 12.9$\pm$0.4 Gyr, with the exception
of the young metal-rich MWGC NGC 6342. We perform two tests to validate the
precision of our methodology, and discuss the implications of our target
cluster loci in the MWGC age-metallicity plane. In addition, we use our fully
self-consistent bolometric corrections to assess the systematic impact of
variations in the total-to-selective extinction ratio $R_{V}$ on relative age
measurements.
