CEPHEID CALIBRATION OF THE PEAK BRIGHTNESS OF SNE-IA V SN-1981B IN NGC-4536

Observations have been made with the Hubble Space Telescope between 19 94 June and August over a 67 day interval, comprising 17 epochs in the F555W band and 5 epochs in the F814W band. The target was a region of an outlying spiral arm of NGC 4536 [Sbc(s)I-II], a galaxy that was pa rent to the type Ia supernova SN 1981B. Seventy-three Cepheids were fo und with periods ranging between 5.4 and 65 days. The apparent distanc e modulus of NGC 4536 is (m - M)(AV) = 31.23 +/- 0.05 and (m - M)(AB) = 31.27 +/- 0.05. There is no statistically significant differential a bsorption between the Cepheids inside and outside the spiral arm, or b etween the Cepheids and the position of the supernova (SN). The result ing absolute magnitudes of SN 1981B are M(V)(max) = - 19.32 +/- 0.12 a nd M(B)(max) = -19.29 +/- 0.13 using the apparent modulus of NGC 4536 and the apparent magnitudes of SN 1981B as if there is no differential extinction between them. If we correct the SN for reddening, determin ed independently of the Cepheids, and then use the true modulus of NGC 4536 of (m - M)(o) = 31.10 +/- 0.13, the SN values are M(B)(max) = - 19.46 +/- 0.24 and M(V)(max) = - 19.44 +/- 0.21. Combining these calib rations with the three calibrations previously available for the SNe 1 895B, 1937C, and 1972E, determined in the first four papers of this se ries, gives the interim calibrations of [M(B)(max)] = -19.48 +/- 0.12 and [M(V)(max)] = -19.47 +/- 0.10. These require interim Hubble consta nts of H-0(B) = 55 +/- 3(internal) km s(-1) Mpc(-1), and H-0(V) = 58 /- 3(internal) km s(-1) Mpc(-1). Improvement is expected when the fift h and sixth calibrators, SN 1960F in NGC 4496A and SN 1990N in NGC 463 9, are added to the four available here. When these data are available , any putative decay rate-absolute magnitude correlation can also begi n to be studied directly from the Cepheid calibrations themselves.