Globular Cluster Calibration of the Peak Brightness of the Type Ia Supernova 1992A and the Value of Ho

Abstract

We have determined the absolute magnitude at maximum light of SN 1992A by using the turn-over magnitude of the Globular Cluster Luminosity Function of its parent galaxy, NGC 1380. A recalibration of the peak of the turn-over magnitude of the Milky Way clusters using the latest HIPPARCOS results has been made with an assessment of the complete random and systematic error budget. The following results emerge: a distance to NGC 1380 of 18.6 +/- 1.4 Mpc, corresponding to (m-M)=31.35 +/- 0.16, and an absolute magnitude of SN 1992A at maximum of M_B(max)= -18.79 +/- 0.16. Taken at face value, SN 1992A seems to be more than half a magnitude fainter than the other SNeI-a for which accurate distances exist. We discuss the implications of this result and present possible explanations. We also discuss the Phillips'(1993) relationship between rate of decline and the absolute magnitude at maximum, on the basis of 9 SNeI-a, whose individual distances have been obtained with Cepheids and the Globular Cluster Luminosity Function. The new calibration of this relationship, applied to the most distant SNe of the Calan-Tololo survey, yields Ho=62 +/- 6 km/s/Mpc.