The Low-velocity, Rapidly Fading Type Ia Supernova 2002es

Abstract

SN 2002es is a peculiar subluminous Type Ia supernova (SN Ia) with a combination of observed characteristics never before seen in an SN Ia. At maximum light, SN 2002es shares spectroscopic properties with the underluminous SN 1991bg subclass of SNe Ia, but with substantially lower expansion velocities(similar to 6000 km s(-1)) more typical of the peculiar SN 2002cx subclass. Photometrically, SN 2002es differs from both SN 1991bg-like and SN 2002cx-like supernovae. Although at maximum light it is subluminous (M-B = -17.78 mag), SN 2002es has a relatively broad light curve (Delta m(15)(B) = 1.28 +/- 0.04 mag), making it a significant outlier in the light-curve width versus luminosity relationship. We estimate a Ni-56 mass of 0.17 +/- 0.05 M-circle dot synthesized in the explosion, relatively low for an SN Ia. One month after maximum light, we find an unexpected plummet in the bolometric luminosity. The late-time decay of the light curves is inconsistent with our estimated Ni-56 mass, indicating that either the light curve was not completely powered by Ni-56 decay or the ejecta became optically thin to gamma-rays within a month after maximum light. The host galaxy is classified as an S0 galaxy with little to no star formation, indicating that the progenitor of SN 2002es is likely from an old stellar population. We also present a less extensive data set for SN 1999bh, an object which shares similar photometric and spectroscopic properties. Both objects were found as part of the Lick Observatory Supernova Search, allowing us to estimate that these objects should account for 2.5% of SNe Ia within a fixed volume. Current theoretical models are unable to explain the observed characteristics of SN 2002es.