ChandraObservations and Models of the Mixed‐Morphology Supernova Remnant W44: Global Trends

As with other mixed morphology remnants, W44's projected center is bright inthermal 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'shot, bright projected center is metal-rich and that the bright knots areregions of comparatively elevated elemental abundances. The neon abundance iselevated, suggesting that the center is rich in ejecta. Furthermore, some ofthe emitting iron atoms appear to be underionized with respect to the otherions, providing the first X-ray evidence for dust destruction in a supernovaremnant. We use the Chandra data to test the following explanations for W44'sX-ray bright center: 1.) entropy mixing from thermal conduction or bulk mixing,2.) cloud evaporation, and 3.) a metallicity gradient, possibly due to dustdestruction and ejecta enrichment. In these tests, we assume that the remnanthas evolved beyond the adiabatic evolutionary stage, which explains the X-raydimness of the shell. The entropy mixed model spectrum was found to be a goodmatch to the Chandra spectrum. The bright knots have similar levels ofionization as the surrounding regions, challenging the evaporating cloudsmodel. While both of these models are known to predict centrally bright X-raymorphologies, their predictions fall short of the observed brightness gradient.The resulting brightness gap can be largely filled in by emission from theextra metals in and near the remnant's projected center. The preponderance ofevidence suggests that W44's remarkable morphology can be attributed to dustdestruction and ejecta enrichment within an entropy mixed, adiabatic phasesupernova remnant.Comment: Accepted for publication in the August 20, 2004 issue of ApJ. 22 pages (in well spaced layout), references, captions, and figure