The Expanding Photosphere Method Applied to SN 1992 AM At cz=14 600km/s

Abstract

We present photometry and spectroscopy of SN 1992am for five months following its discovery by the Calan/CTIO SN search. These data show SN 1992am to be type II-P, displaying hydrogen in its spectrum and the typical shoulder in its light curve. The photometric data and the distance from our own analysis are used to construct the supernova's bolometric light curve. Using the bolometric light curve, we estimate SN 1992am ejected approximately 0.30 M_sun_ of ^56^Ni, an amount four times larger than that of other well studied SNe II. SN 1992am's; host galaxy lies at a redshift of cz = 14600 km s ~, making it one of the most distant SNe II discovered, and an important application of the Expanding Photosphere Method. Since z = 0.05 is large enough for redshift-dependent effects to matter, we develop the technique to derive luminosity distances with the Expanding Photosphere Method at any redshift, and apply this method to SN 1992am, The derived distance, D = 180_-25_^+30^ Mpc, is independent of all other rungs in the extragalactic distance ladder. The redshift of SN 1992am's host galaxy is sufficiently large that uncertainties due to perturbations in the smooth Hubble flow should be smaller than 10%. The Hubble ratio derived from the distance and redshift of this single object is H_0_ = 81_-15_^+17^ km s^-1^ Mpc^-1^. In the future with more of these distant objects, we hope to establish an independent and statistically robust estimate of H_0_ based solely on type II supernovae.