Optical Spectroscopy of Supernova Remnants in M81 and M82
  We present spectroscopy of 28 SNR candidates as well as one H II region in
M81, and two SNR candidates in M82. Twenty six out of the M81 candidates turn
out to be genuine SNRs, and two in M82 may be shocked condensations in the
galactic outflow or SNRs. The distribution of [N II]/H{\alpha} ratios of M81
SNRs is bimodal. M81 SNRs are divided into two groups in the spectral line
ratio diagrams: an [O III]-strong group and an [O III]-weak group. The latter
have larger sizes, and may have faster shock velocity. [N II]/H{\alpha} ratios
of the SNRs show a strong correlation with [S II]/H{\alpha} ratios. They show a
clear radial gradient in [N II]/H{\alpha} and [S II]/H{\alpha} ratios: dLog ([N
II]/H{\alpha})/dLog R = -0.018 {\pm} 0.008 dex/kpc and dLog ([S
II]/H{\alpha})/dLog R = -0.016 {\pm} 0.008 dex/kpc where R is a deprojected
galactocentric distance. We estimate the nitrogen and oxygen abundance of the
SNRs from the comparison with shock-ionization models. We obtain a value for
the nitrogen radial gradient, dLog(N/H)/dLogR = -0.023 {\pm} 0.009 dex/kpc, and
little evidence for the gradient in oxygen. This nitrogen abundance shows a few
times flatter gradient than those of the planetary nebulae and H II regions. We
find that five SNRs are matched with X-ray sources. Their X-ray hardness colors
are consistent with thermal SNRs.
