Mapping the Interstellar Reddening and Extinction towards Baade's Window
  Using Minimum Light Colors of ab-type RR Lyrae Stars. Revelations from the
  De-reddened Color-Magnitude Diagrams
  We have obtained repeated images of 6 fields towards the Galactic bulge in 5
passbands (u, g, r, i, z) with the DECam imager on the Blanco 4m telescope at
CTIO. From over 1.6 billion individual photometric measurements in the field
centered on Baade's window, we have detected 4877 putative variable stars. 474
of these have been confirmed as fundamental mode RR Lyrae stars, whose colors
at minimum light yield line-of-sight reddening determinations as well as a
reddening law towards the Galactic Bulge which differs significantly from the
standard R_V = 3.1 formulation. Assuming that the stellar mix is invariant over
the 3 square-degree field, we are able to derive a line-of-sight reddening map
with sub-arcminute resolution, enabling us to obtain de-reddened and extinction
corrected color-magnitude diagrams (CMD's) of this bulge field using up to 2.5
million well-measured stars. The corrected CMD's show unprecedented detail and
expose sparsely populated sequences: e.g., delineation of the very wide red
giant branch, structure within the red giant clump, the full extent of the
horizontal branch, and a surprising bright feature which is likely due to stars
with ages younger than 1 Gyr. We use the RR Lyrae stars to trace the spatial
structure of the ancient stars, and find an exponential decline in density with
Galactocentric distance. We discuss ways in which our data products can be used
to explore the age and metallicity properties of the bulge, and how our larger
list of all variables is useful for learning to interpret future LSST alerts.
