The Type IIn Supernova 2002kg: The Outburst of a Luminous Blue Variable Star in NGC 2403

TL;DR

SN 2002kg in NGC 2403 is identified as a luminous blue variable star undergoing an outburst, resembling LBV S Doradus, and exemplifies the class of SN impostors with diverse energetic outbursts in massive-star evolution.

Contribution

This paper demonstrates that SN 2002kg is a luminous blue variable star in outburst, providing insights into pre-supernova massive star behavior and expanding understanding of SN impostors.

Findings

SN 2002kg's properties differ from normal supernovae.

V37's initial mass is estimated to be >~ 40 M_sun.

The outburst is less energetic than eta Carinae-like eruptions.

Abstract

We show that Supernova (SN) 2002kg in NGC 2403, initially classified as Type II-narrow (IIn), has photometric and spectroscopic properties unlike those of normal SNe. Its behavior, instead, is more typical of highly massive stars which experience the short-lived luminous blue variable (LBV) phase toward the end of their lives. The star, in fact, most resembles the LBV S Doradus in outburst. The precursor of SN 2002kg is the irregular, bright blue variable star 37 (V37), catalogued by Tammann & Sandage in 1968. Using high-quality ground-based, multi-band images we can constrain the initial mass of V37 to be M_ini >~ 40 M_sun. We find that, although the spectra indicate a nitrogen enhancement, possibly revealing the products of CNO processing by V37 in the ejecta, the star lacks a substantial LBV nebula. The outburst from SN 2002kg/V37 is not nearly as energetic as the giant eruptions of the eta Carinae-like variables, such as SN 1954J/V12, also in NGC 2403. SN 2002kg/V37, however, is among a growing number of ``SN impostors'' exhibiting a broad range of outburst energetics during a pre-SN phase of massive-star evolution.