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

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.

15 Pages, 16 Figures, 8 Tables, Accepted for publication in MNRAS, 2019 September 20

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