PHYSICAL NATURE OF THE [S II]-BRIGHT SHELL NEBULAE N70 AND N185

N70 and N185 are two large (>= 100 pc in diameter) shell nebulae in the Large Magellanic Cloud (LMC). Their high [S II]/H alpha ratios rival those of supernova remnants (SNRs), but they are not confirmed as SNRs. To study their physical nature, we have obtained XMM-Newton X-ray observations and high-dispersion long-slit echelle spectroscopic observations of these two nebulae. The X-ray spectra of both nebulae can be well interpreted with an optically thin thermal (similar to 0.2 keV) plasma with the average LMC abundance in a collisional ionization equilibrium. N70 encompasses the OB association LH114. Although N70 has a modest expansion velocity and essentially thermal radio emission, its diffuse X-ray luminosity (similar to 6.1 x 10(35) erg s(-1)) is higher than that from a quiescent superbubble with N70's density, size, and expansion velocity; thus, N70 is most likely a superbubble that is recently energized by an interior SNR. N185 does not contain any known OB association, and its X-ray luminosity is an order of magnitude lower than expected if it is a quiescent superbubble. N185 has nonthermal radio emission and has high-velocity material expanding at nearly 200 km s(-1), similar to many known SNRs in the LMC. Its X-ray luminosity (similar to 1.9 x 10(35) erg s(-1)) is also consistent with that of an evolved SNR. We therefore suggest that N185 is energized by a recent supernova.