SN 2009kn - the twin of the Type IIn supernova 1994W

TL;DR

SN 2009kn exhibits spectral features of a Type IIn supernova with photometric evolution similar to Type IIP, providing insights into its explosion mechanism, circumstellar interaction, and dust formation, and comparing it to the similar SN 1994W.

Contribution

This study provides detailed photometric and spectroscopic data of SN 2009kn, testing models of Type IIn supernovae and comparing its properties with SN 1994W to understand their nature.

Findings

SN 2009kn shows narrow Balmer lines with P Cygni profiles.

The light curve suggests radioactive decay of 56Co as energy source.

Higher 56Ni mass estimate than SN 1994W, with dust formation observed.

Abstract

We present an optical and near-infrared photometric and spectroscopic study of supernova (SN) 2009kn spanning ~1.5 yr from the discovery. The optical spectra are dominated by the narrow (full width at half-maximum ~1000 km s^-1) Balmer lines distinctive of a Type IIn SN with P Cygni profiles. Contrarily, the photometric evolution resembles more that of a Type IIP SN with a large drop in luminosity at the end of the plateau phase. These characteristics are similar to those of SN 1994W, whose nature has been explained with two different models with different approaches. The well-sampled data set on SN 2009kn offers the possibility to test these models, in the case of both SN 2009kn and SN 1994W. We associate the narrow P Cygni lines with a swept-up shell composed of circumstellar matter and SN ejecta. The broad emission line wings, seen during the plateau phase, arise from internal electron scattering in this shell. The slope of the light curve after the post-plateau drop is fairly consistent with that expected from the radioactive decay of 56Co, suggesting an SN origin for SN 2009kn. Assuming radioactivity to be the main source powering the light curve of SN 2009kn in the tail phase, we infer an upper limit for 56Ni mass of 0.023 M_sun. This is significantly higher than that estimated for SN 1994W, which also showed a much steeper decline of the light curve after the post-plateau drop. We also observe late-time near-infrared emission which most likely arises from newly formed dust produced by SN 2009kn. As with SN 1994W, no broad lines are observed in the spectra of SN 2009kn, not even in the late-time tail phase.