Physical Nature of the [S II]-bright Shell Nebulae N70 and N185

Abstract

N70 and N185 are two large (>=100 pc in diameter) shell nebulae in the Large Magellanic Cloud (LMC). Their high [S II]/Hα 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 (~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 (~6.1 × 10³⁵ 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 (~1.9 × 10³⁵ erg s^-1) is also consistent with that of an evolved SNR. We therefore suggest that N185 is energized by a recent supernova.