A Critical Examination of Hypernova Remnant Candidates in M 101 . I . MF 83

The SNR candidate MF83 in M101 is coincident with a very luminous Xray source. Based on the high X-ray luminosity, it has been suggested that MF83 is a “hypernova remnant” requiring an explosion energy about two orders of magnitude higher than normal supernovae. We have analyzed high-quality ground-based and HST observations of MF83, and find that MF83 is a star formation region, consisting of a large ionized 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 ∼50 km s and a diameter of ∼270 pc. The optical properties of this shell in MF83 are Visiting astronomer, Kitt Peak National Observatory, National Optical Astronomy Observatories, operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the National Science Foundation. Visiting astronomer, Michigan-Dartmouth-MIT Observatory. Hubble Fellow.