  The Advanced Camera for Surveys on board the Hubble Space Telescope has been
used to obtain deep, high-resolution images of the intermediate-age star
cluster NGC 1846 in the Large Magellanic Cloud. We present new color-magnitude
diagrams (CMDs) based on F435W, F555W, and F814W imaging. We test the
previously observed broad main sequence turnoff region for "contamination" by
field stars and (evolved) binary star systems. We find that while these impact
the number of objects in this region, none can fully account for the large
color spread. Our results therefore solidify the recent finding that stars in
the main sequence turnoff region of this cluster have a large spread in color
which is unrelated to measurement errors or contamination by field stars, and
likely due to a ~300 Myr range in the ages of cluster stars. An unbiased
estimate of the stellar density distribution across the main sequence turnoff
region shows that the spread is fairly continuous rather than strongly bimodal
as suggested previously. We fit the CMDs with several different sets of
theoretical isochrones, and determine systematic uncertainties for population
parameters when derived using any one set of isochrones. We note a degeneracy
between age and [alpha/Fe], which can be lifted by matching the shape
(curvature) of the full red giant branch in the CMD. We find that stars in the
upper part of the main sequence turnoff region are more centrally concentrated
than those in any other region of the CMD, including more massive red giant
branch and asymptotic giant branch stars. We consider several possible
formation scenarios which account for the unusual features observed in the CMD
of NGC 1846.
