Rosa Williams

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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 con- firmed 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.

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