Optical and Ultraviolet Observations of A Low-Velocity Type II-Plateau Supernova 2013am in M65

TL;DR

This paper presents optical and ultraviolet observations of the low-velocity Type II-Plateau supernova 2013am, revealing spectral features similar to low-luminosity SNe IIP but with distinct photometric properties, suggesting diverse explosion mechanisms.

Contribution

It provides detailed observational analysis of SN 2013am, highlighting its unique spectral and photometric characteristics and implications for supernova explosion models.

Findings

SN 2013am has lower ejecta velocities (~2000 km/s) compared to typical SNe IIP.

SN 2013am exhibits shorter plateau phases and brighter light-curve tails than similar low-velocity SNe.

Estimated $^{56}$Ni mass is about 0.016 solar masses, linking it to the gap-filler supernovae.

Abstract

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., $\sim$2000 km s$^{-1}$ vs. $\sim$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\pm$0.19 mag), we find that SN 2013am may have reached an absolute $V$-band peak magnitude of $-15.83\pm0.71$ mag, and produced a $^{56}$Ni mass of $0.016^{+0.010}_{-0.006} solar masses 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, and the low expansion velocities could be due to a lower metallicity of the progenitor stars, a larger envelope mass ejected in the explosion, or that is was observed at an angle that is away from the polar direction.