EXPANDING PHOTOSPHERES OF TYPE-II SUPERNOVAE AND THE EXTRAGALACTIC DISTANCE SCALE

We use the Expanding Photosphere Method to determine distances to 10 Type II supernovae. The effects of asymmetries, extinction, and flux dilution are explored. Using empirical evidence and time-dependent, spherical models which treat H and He in non-LTE, we show that blackbody corrections caused by flux dilution are small for Type II supernovae in the infrared, and in the optical when their color temperatures are less than 6000 K. The extinction to a Type II-P supernova can be estimated from its light curve: the uncertainty introduced into a distance measurement due to extinction is usually less than 10%. Correcting for extinction and flux dilution, we derive distances to 10 supernovae: SN 1968L, SN 1969L, SN 1970G, SN 1973R, SN 1979C, SN 1980K, SN 1987A, SN 1988A, SN 1990E, and SN 1990ae. The distance measurements span a wide range, 50 kpc to 120 Mpc, which is unique among the methods for establishing the extragalactic distance scale. The distances measured to SN 1970G in M101 and SN 1987A in the LMC are in good agreement with distances determined from Cepheid variable stars. Our distance to the Virgo Cluster, 22 +/- 3 Mpc, is larger than recent distance estimates made using surface brightness fluctuations, planetary nebula luminosity functions, and the Tully-Fisher method. Using the distances determined from these Type II supernovae, we derive a value of H-0 = 60 +/- 10 km s-1 Mpc-1. This value is subject to errors caused by local deviations in the Hubble flow but will soon be improved by applying the Expanding Photosphere Method to several distant Type II supernovae.