Enhanced abundances in three large-diameter mixed-morphology supernova remnants

We present an X-ray study of three mixed-morphology (MM) supernova remnants (SNRs) - HB 21, CTB 1, and HB 3 - using archival ASCA and ROSAT data. These data are complemented by archival Chandra X-Ray Observatory data for CTB 1 and XMM-Newton X-Ray Observatory data for HB 3. The spectra from HB 21 and HB 3 are well described with a single-temperature thermal plasma in ionization equilibrium, while a two-temperature thermal plasma is found in CTB 1. We found enhanced abundances in all three SNRs. The elemental abundance of Mg is clearly enhanced in CTB 1, while HB 21 has enhanced abundances of Si and S. The situation is not as clear in HB 3 - the plasma in this SNR either has significantly enhanced abundances of O, Ne, and Mg, or has marginally enhanced abundances of Mg and underabundant Fe. We discuss the plausibility of mixed-morphology SNR models for the three SNRs, and the presence of enhanced abundances. We revise a list of MM SNRs and their properties, compare the three SNRs studied here with other members of this class, and discuss the presence of enhanced elemental abundances in MM SNRs. We also report the ASCA detection of a compact source in the southern part of HB 3. The source spectrum is consistent with a power law with a photon index of ∼2.7, and an unabsorbed X-ray flux of ∼ 10^-12 ergs cm^-2 s^-1 in the 0.5-10.0 keV band. The column density toward this source differs from that toward the SNR, and it is therefore unlikely that they are related.