Dense, Fe‐rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?

We present observations of two LMC supernova remnants (SNRs), DEM L238 andDEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright centralemission, surrounded by a faint shell, is present in both remnants. The centralemission has an entirely thermal spectrum dominated by strong Fe L-shell lines,with the deduced Fe abundance in excess of solar and not consistent with theLMC abundance. This Fe overabundance leads to the conclusion that DEM L238 andDEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emissionoriginates in gas swept up and heated by the blast wave. A standard Sedovanalysis implies about 50 solar masses in both swept-up shells, SNR agesbetween 10,000 and 15,000 yr, low (< 0.05 cm^-3) preshock densities, andsubluminous explosions with energies of 3x10^50 ergs. The central Fe-richsupernova ejecta are close to collisional ionization equilibrium. Theirpresence is unexpected, because standard Type Ia SNR models predict faintejecta emission with short ionization ages. Both SNRs belong to a previouslyunrecognized class of Type Ia SNRs characterized by bright interior emission.Denser than expected ejecta and/or a dense circumstellar medium around theprogenitors are required to explain the presence of Fe-rich ejecta in theseSNRs. Substantial amounts of circumstellar gas are more likely to be present inexplosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similarSNRs could be remnants of ``prompt'' Type Ia explosions with young (~100 Myrold) progenitors.Comment: 24 pages, 8 figures, ApJ, in pres