Ejecta in Evolved SNRs

Evidence of enhanced metal abundances is becoming commonplace in evolved remnants. ASCA observations revealed overabundances of Si, S, and Fe near the center of the Cygnus Loop [105]. The observed abundance ratios were shown to be consistent with a Type II supernova from a progenitor with mass of about 25 [email protected]

At least four evolved, shell-like SNRs in the LMC show evidence for ejecta near their centers. DEM L71, thought to be 5 000-years-old, shows Fe-rich ejecta in its interior, presumably heated by the reverse shock [56]. The Fe:O ratio is larger than 5:1. Dynamical arguments suggest that the total ejecta mass is ~1.5Mq; the Fe mass based on the spectral fits is —0.8 [email protected] Thus the X-ray measurements strongly support a Type Ia origin for this remnant. N49B (0525-66.0), with an estimated age of 104 yrs, shows enhanced Mg and Si in the center, but no evidence for either enhanced O or Fe [110]. Two other shell-like 104-year-old SNRs, 0548-70.4 and 0534-69.9, were found to have central enhancements of O, Mg, Si, S, and Fe [52]. The O:Fe ratios support a Type Ia origin for both remnants.

Some, but not all, MM remnants show evidence for residual ejecta. Spatially resolved spectroscopy along the long axis of the MM remnant W44 using Chandra reveals that the remnant's hot bright projected center is rich in Mg, Si, and Ne, and that the bright knots are regions of comparatively elevated elemental abundances [147]. The fact that Ne is among the elements with enhanced abundance indicates that ejecta contribute to the abundance trends. An underabundance of Fe from those same regions provides the first potential X-ray evidence for dust destruction in a supernova remnant. Additionally, Yokogawa et al. found evidence for abundance enhancements in three evolved (older than 104 yrs) SMC SNRs, all of which appear to be MM remnants [181]. W49B shows enhanced metal abundances throughout [68], but its evolutionary state and its membership in the MM class are a matter of debate. Rho and Petre suggest it is not, largely because of the presence of ejecta [133]; Kawasaki et al. argue it is because of plasma properties [78]. The evolved, small-diameter remnant G347.7+0.2, which has all the attributes of MM remnants except a centrally filled X-ray morphology, shows slightly enhanced Si throughout, and a S enhancement on its western side [91].

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