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

SN 2002es is a peculiar subluminous Type Ia supernova (SN Ia) with a combination of observed characteristics never before seen in a 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 (~6000 km/s) more typical of the 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 (Dm15(B)=1.28 +/- 0.04 mag), making it a significant outlier in the light-curve width vs. luminosity relationship. We estimate a 56Ni mass of 0.17 +/- 0.05 M_sun synthesized in the explosion, relatively low for a 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 56Ni mass, indicating that either the light curve was not completely powered by 56Ni 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 the progenitor of SN 2002es is likely from an old stellar population. We also present a less extensive dataset for SN 1999bh, an object which shares similar observed 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. We find that current theoretical models are unable to explain the observed of characteristics of SN 2002es.