Optical and Ultraviolet Observations of a Low-velocity Type II Plateau Supernova 2013am in M65

Optical and ultraviolet observations for the nearby type II plateau supernova (SN IIP) 2013am in the nearby spiral galaxy M65 are presented in this paper. The early spectra are characterized by relatively narrow P-Cygni features, with ejecta velocities much lower than observed in normal SNe IIP (i.e., ~2000 km s^-1 versus ~5000 km ^-1 in the middle of the plateau phase). Moreover, prominent Ca II absorptions are also detected in SN 2013am at relatively early phases. These spectral features are reminiscent of those seen in the low-velocity and low-luminosity SN IIP 2005cs. However, SN 2013am exhibits different photometric properties, having shorter plateau phases and brighter light curve tails if compared to SN 2005cs. Adopting R_V = 3.1 and a mean value of total reddening derived from the photometric and spectroscopic methods (i.e., E(B - V) = 0.55 ± 0.19 mag), we find that SN 2013am may have reached an absolute V-band peak magnitude of -15.83 ± 0.71 mag and produced an ⁵⁶Ni mass of 0.016^+0.010_-0.006 M _⊙ in the explosion. These parameters are close to those derived for SN 2008in and SN 2009N, which have been regarded as "gap-filler" objects linking the faint SNe IIP to the normal ones. This indicates that some low-velocity SNe IIP may not necessarily result from the low-energetic explosions. The low expansion velocities could be due to a lower metallicity of the progenitor stars, a larger envelope mass ejected in the explosion, or the effect of viewing angle where these SNe were observed at an angle away from the polar direction.