  As with other mixed morphology remnants, W44's projected center is bright in
thermal X-rays. It has an obvious radio shell, but no discernable X-ray shell.
X-ray bright knots dot W44's image. The Chandra data show that the remnant's
hot, bright projected center is metal-rich and that the bright knots are
regions of comparatively elevated elemental abundances. The neon abundance is
elevated, suggesting that the center is rich in ejecta. Furthermore, some of
the emitting iron atoms appear to be underionized with respect to the other
ions, providing the first X-ray evidence for dust destruction in a supernova
remnant. We use the Chandra data to test the following explanations for W44's
X-ray bright center: 1.) entropy mixing from thermal conduction or bulk mixing,
2.) cloud evaporation, and 3.) a metallicity gradient, possibly due to dust
destruction and ejecta enrichment. In these tests, we assume that the remnant
has evolved beyond the adiabatic evolutionary stage, which explains the X-ray
dimness of the shell. The entropy mixed model spectrum was found to be a good
match to the Chandra spectrum. The bright knots have similar levels of
ionization as the surrounding regions, challenging the evaporating clouds
model. While both of these models are known to predict centrally bright X-ray
morphologies, their predictions fall short of the observed brightness gradient.
The resulting brightness gap can be largely filled in by emission from the
extra metals in and near the remnant's projected center. The preponderance of
evidence suggests that W44's remarkable morphology can be attributed to dust
destruction and ejecta enrichment within an entropy mixed, adiabatic phase
supernova remnant.
