Guillermo Garcia-Segura

Ultraviolet interstellar absorption lines in the LMC: Searching for hidden SNRs

Strong x-ray emission detected in Large Magellanic Cloud (LMC) superbubbles has been explained as the result of interior supernova remnants (SNRs) hitting the dense superbubble shell. Such SNRs cannot be found using conventional criteria. We thus investigate the possibility of using the interstellar absorption properties in the ultraviolet (UV) as a diagnostic of hidden SNR shocks. The International Ultraviolet Explorer (IUE) archives provide the database for this pilot study. They contain high-dispersion spectra of several stars in x-ray bright superbubbles. To distinguish the effects of SNR shocks from those of local stellar winds and a global hot halo around the LMC, we included control objects in different environments. We find that almost all interstellar absorption properties can be explained by the interstellar environment associated with the objects. Summarizing the two most important results of this study: (1) a large velocity shift between the high-ionization species (C IV and Si IV) and the low-ionization species (S II, Si II, and C II*) is a diagnostic of hidden SNR shocks; however, the absence of a velocity shift does not preclude the existence of SNR shocks; (2) there is no evidence that the LMC is uniformly surrounded by hot gas; hot gas is preferentially found associated with large interstellar structures like superbubbles and supergiant shells, which may extend to large distances from the plane.

Hidden supernova remnants in the large Magellanic cloud H II complex N44

We have obtained ROSAT PSPC observations of N44, one of the largest H II complexes in the Large Magellanic Cloud. The X-ray emission mostly fails within the ionized shell structures in N44. We find that one faint shell is a classical supernova remnant overlooked by previous surveys. If we model the two largest shells as pressure-driven superbubbles, the predicted X-ray luminosity falls far below the observed value. Instead, we show that off-center supernova remnants hitting superbubble shells can explain the excess X-ray emission.

High Resolution IRAS Images of Large Shells Around Wolf-Rayet Stars

High-resolution IRAS images, processed by HIRES, are ideal for the search of large, cold, fossil shell structures around Wolf-Rayet stars. In our preliminary effort, we have confirmed the large, l.°5 – 2°, ring structures around NGC 6888 and RCW 58, which are clearly shown in the HIRES processed images. We interpret them as fossil bubbles blown by the main-sequence progenitors of the WR stars, although these large shells were previously suggested to be evolved supernova remnants.