DISCOVERY OF CEPHEIDS IN NGC-5253 - ABSOLUTE PEAK BRIGHTNESS OF SN-IA-1895B AND SN-IA-1972E AND THE VALUE OF H(0)

Observations with the Hubble Space Telescope between 1993 May 31 and 1 993 July 19 in 20 epochs in the F555W passband and five epochs in the F785LP passband have led to the discovery of 14 Cepheids in the Amorph ous galaxy NGC 5253. The apparent V distance modulus is (m - M)AV = 28 .08 +/- 0.10 determined from the 12 Cepheids with normal amplitudes. T he distance modulus using the F785LP data is consistent with the V val ue to within the errors. Five methods used to determine the internal r eddening are consistent with zero differential reddening, accurate to a level of E(B - V) < 0.05 mag, over the region occupied by Cepheids a nd the two supernovae (SNe) produced by NGC 5253. The apparent magnitu des at maximum for the two SNe in NGC 5253 are adopted as B(max) = 8.3 3 +/- 0.2 mag for SN 1895B, and B(max) = 8.56 +/- 0.1 and V(max) = 8.6 0 +/- 0.1 for SN 1972E which is a prototype SN of Type Ia. The apparen t magnitude system used by Walker (1923) for SN 1895B has been correct ed to the modern B scale and zero point to determine its adopted B(max ) value. The absolute magnitudes for the two SNe determined from these data are M(B)(max) = - 19.80 +/- 0.28 for SN 1895B and M(B)(max) = - 19.55 +/- 0.23, and M(v)(max) = - 19.50 +/- 0.21 for SN 1972E. Combini ng these data with M(B)(max) = - 19.65 +/- 0.18 and M(v)(max) = - 19.6 4 +/- 0.13 determined earlier for SN 1937C in IC 4182 (Sandage et al. 1992; Saha et al. 1994; Schaefer 1994) gives the current mean calibrat ion of SNe Ia by this Cepheid method as [M(B)(max)] = - 19.65 +/- 0.13 , based on three determinations, and [M(v)(max)] = - 19.60 +/- 0.11, b ased on two determinations. The resulting Hubble constants determined using these absolute magnitudes and a recent version of the Hubble dia grams in B and in V for SNe Ia are H0(B) = 50 +/- 8 km s-1 Mpc-1, and H0(V) = 54 +/- 8 km s-1 Mpc-1, where the quoted errors are external va lues based on the assumption that the uncertainty of the adopted mean absolute magnitudes of SNe Ia is as large as 0.3 mag.