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Session 61 - New Views of the Magellanic Clouds.
Display session, Wednesday, June 12
Bright diffuse X-ray emission has been detected in large H II complexes, such as the Carina Nebula in the Galaxy and the 30 Doradus nebula in the LMC. This indicates the existence of hot (10^6 K) gas; however, it is unclear whether stellar winds or supernova remnants dominate the heating. To resolve this issue, we have selected N66, the largest H II region in the Small Magellanic Cloud (SMC), for a detailed case study. N66 contains an OB association with unevolved O3 members. This strongly implies that supernovae have not yet occurred and that the X-ray-emitting gas is predominantly heated by stellar winds. The young evolutionary stage of N66 is further supported by the small velocity widths of the H\alpha line profile observed in the H II region. Therefore, the X-ray emission of N66 can unambiguously provide insight into the stellar wind heating of hot gas.
We have analyzed the soft X-ray emission of N66, using both a proprietary ROSAT HRI observation (50 ks) and an archival ROSAT PSPC observation (26.6 ks). We find that the X-ray emission from the direction of N66 is dominated by a known foreground supernova remnant(SNR), 0057-7226, and that no obvious X-ray emission is detected outside the projected SNR boundary. The PSPC count rate of the SNR is (4.1\pm 0.1)\times 10^-3 counts s^-1 arcmin^-2, while the 3-\sigma upper limit for the H II region is 1.0\times 10^-4 counts s^-1 arcmin^-2. This upper limit is similiar to those of the X-ray-dim superbubbles in the LMC. However, it is lower than the observed X-ray count rate \sim 8\times 10^-3 counts s^-1 arcmin^-2 of Carina Nebula and (4-8)\times 10^-3 counts s^-1 arcmin^-2 of 30 Doradus by 1\sim2 orders of magnitude.
The aforementioned disparity in X-ray surface brightness is intrinsic, since N66 has the smallest extinction among the three. This suggests that the X-ray-bright H II complexes cannot be powered solely by stellar winds. SNR shocks must have provided additional heating.
Program listing for Wednesday