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

Abstract

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$0.2 keV) plasma with the average LMC abundance in a collisional ionization equilibrium. N70 encompasses the OB association LH114. Although N70 has a modest expansion velocity and essentially thermal radio emission, its diffuse X-ray luminosity ($\sim 6.1\times 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 ($\sim 1.9\times 10^{35}$ erg s${}^{-1}$) is also consistent with that of an evolved SNR. We therefore suggest that N185 is energized by a recent supernova.

arXiv comment11 pages, 9 figures, Published in ApJ

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