A critical examination of hypernova remnant candidates in M101. I. MF 83

The supernova remnant (SNR) candidate MF 83 in M101 is coincident with a ve ry luminous X-ray source. Based on the high X-ray luminosity, it has been s uggested that MF 83 is a "hypernova remnant" requiring an explosion energy about 2 orders of magnitude higher than normal supernovae. We have analyzed high-quality ground-based and Hubble Space Telescope observations of MF 83 , and find that MF 83 is a star formation region, consisting of a large ion ized gas shell and four H II regions along its periphery. Continuum images show OB associations in these H II regions and within the large shell. The shell has an expansion velocity of similar to 50 km s(-1) and a diameter of similar to 270 pc. The optical properties of this shell in MF 83 are simil ar to those of X-ray-bright superbubbles in the Large Magellanic Cloud. If the X-ray emission is indeed diffuse, the implied thermal energy in MF 83 i s high, a few times 10(52) ergs. This amount of thermal energy requires a l arge number of concentrated supernova explosions or one powerful explosion. Future X-ray observations with a high angular resolution are needed to res olve the diffuse emission and point sources in MF 83, in order to determine more accurately the thermal energy in the shell interior and its required explosion energy.