Chandra Observations and Models of the Mixed-Morphology Supernova Remnant W44: Global Trends

We report on the Chandra observations of the archetypical mixed-morphology (or thermal composite) supernova remnant W44. As with other mixed-morphology remnants, W44's projected center is bright in thermal X-rays. It has an obvious radio shell but no discernible X-ray shell. In addition, X-ray-bright knots dot W44's image. The spectral analysis of the Chandra data shows that the remnant's hot bright projected center is metal-rich and that the bright knots are regions of comparatively elevated elemental abundances. Neon is among the affected elements, suggesting that ejecta contributes to the abundance trends. Furthermore, some of the emitting iron atoms appear to be underionized with respect to the other ions, providing the first potential 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 due to bulk mixing or thermal conduction, (2) evaporation of swept-up clouds, 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 tested against the Chandra spectrum for the remnant's projected center and found to be a good match. The evaporating-cloud model was constrained by the finding that the ionization parameters of the bright knots are similar to those of the surrounding regions. While both the entropy-mixed and the evaporating-cloud 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 (including that drawn from other studies) suggests that W44's remarkable morphology can be attributed to dust destruction and ejecta enrichment within an entropy-mixed, adiabatic-phase supernova remnant. The Chandra data prompt a new question—by what astrophysical mechanisms are the metals distributed so inhomogeneously in the supernova remnant.