Possible Detection of Ovifrom the Large Magellanic Cloud Superbubble N70

Abstract

We present Far Ultraviolet Spectroscopic Explorer (FUSE ) observations toward four stars in the Large Magellanic Cloud (LMC) superbubble N70 and compare these spectra to those of four comparison targets located in nearby field and diffuse regions. The N70 sight lines show O VI ?1032 absorption that is consistently stronger than the comparison sight lines by ~60%. We attribute the excess column density (N = 1014.03 cm-2) to hot gas within N70, potentially the first detection of O VI associated with a superbubble. In a survey of 12 LMC sight lines, Howk and collaborators concluded that there was no correlation between ISM morphology and N. We present a reanalysis of their measurements combined with our own and find a clear difference between the superbubble and field samples. The five superbubbles probed to date with FUSE show a consistently higher mean N than the 12 nonsuperbubble sight lines, although both samples show equivalent scatter from halo variability. Possible ionization mechanisms for N70 are discussed, and we conclude that the observed O VI could be the product of thermal conduction at the interface between the hot, X-ray-emitting gas inside the superbubble and the cooler, photoionized material making up the shell seen prominently in H?. We calculate the total hydrogen density nH implied by our O VI measurements and find a value consistent with expectations. Finally, we discuss emission-line observations of O VI from N70.