Physical Nature of the [S II]-Bright Shell Nebulae N70 and N185
  N70 and N185 are two large ($\ge$100 pc in diameter) shell nebulae in the
Large Magellanic Cloud (LMC). Their high [\ion{S}{2}]/H$\alpha$ ratios rival
those of supernova remnants (SNRs), but they are not confirmed as SNRs. To
study their physical nature, we have obtained \emph{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 ($\sim$0.2 keV) plasma with the average LMC
abundance in a collisional ionization equilibrium. N70 encompasses the OB
association LH114. Although N70 has a modest expansion velocity and essentially
thermal radio emission, its diffuse X-ray luminosity ($\sim6.1\times10^{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 ($\sim1.9\times10^{35}$ erg
s$^{-1}$) is also consistent with that of an evolved SNR. We therefore suggest
that N185 is energized by a recent supernova.
