Chandra Observations and Models of the Mixed-Morphology Supernova   Remnant W44: Global Trends
  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. 
R. L. Shelton, K. D. Kuntz, and R. Petre