SN 2015an: a normal luminosity type II supernova with low expansion velocity at early phases

TL;DR

SN 2015an is a typical luminosity Type II supernova with unusual spectral features, low expansion velocity, and signs of ejecta-circumstellar interaction, providing insights into progenitor properties and explosion dynamics.

Contribution

This study provides detailed photometric and spectroscopic analysis of SN 2015an, highlighting its atypical spectral features and low expansion velocity, and models its explosion parameters.

Findings

Recombination phase lasts up to ~120 days with a higher decline rate.

Spectral analysis shows low Hα expansion velocity and early high velocity component.

Ejecta-circumstellar interaction indicated by high velocity features.

Abstract

We present the photometry and spectroscopy of SN 2015an, a Type II Supernova (SN) in IC 2367. The recombination phase of the SN lasts up to $\sim$120 d, with a decline rate of 1.24 mag/100d, higher than the typical SNe IIP. The SN exhibits bluer colours than most SNe II, indicating higher ejecta temperatures. The absolute $V$-band magnitude of SN 2015an at 50 d is $-$16.83$\pm$0.04 mag, pretty typical for SNe II. However, the $^{56}$Ni mass yield, estimated from the tail $V$-band light curve to be 0.021$\pm$0.010 M$_\odot$, is comparatively low. The spectral properties of SN 2015an are atypical, with low H$\alpha$ expansion velocity and presence of high velocity component of H$\alpha$ at early phases. Moreover, the continuum exhibits excess blue flux up to $\sim$50 d, which is interpreted as a progenitor metallicity effect. The high velocity feature indicates ejecta-circumstellar material interaction at early phases. The semi-analytical modelling of the bolometric light curve yields a total ejected mass of $\sim$12 M$_\odot$, a pre-supernova radius of $\sim$388~R$_\odot$ and explosion energy of $\sim$1.8 foe.