The multi-faceted Type II-L supernova 2014G from pre-maximum to nebular phase

TL;DR

This study provides detailed multi-wavelength observations of supernova 2014G, revealing complex spectral evolution, evidence of pre-explosion mass loss, and insights into the progenitor's mass and explosion characteristics.

Contribution

It offers a comprehensive analysis of SN 2014G from early to nebular phases, including spectral features, light curve behavior, and progenitor modeling, highlighting interactions with asymmetric circumstellar material.

Findings

Early high ionisation emission lines indicate pre-explosion mass loss.

Steeper radioactive tail decline suggests gamma-ray escape.

Spectral features imply asymmetric, possibly bipolar CSM interaction.

Abstract

We present multi-band ultraviolet, optical, and near-infrared photometry, along with visual-wavelength spectroscopy, of supernova (SN) 2014G in the nearby galaxy NGC 3448 (25 Mpc). The early-phase spectra show strong emission lines of the high ionisation species He II/N IV/C IV during the first 2-3 d after explosion, traces of a metal-rich CSM probably due to pre-explosion mass loss events. These disappear by day 9 and the spectral evolution then continues matching that of normal Type II SNe. The post-maximum light curve declines at a rate typical of Type II-L class. The extensive photometric coverage tracks the drop from the photospheric stage and constrains the radioactive tail, with a steeper decline rate than that expected from the $^{56}$Co decay if $\gamma$-rays are fully trapped by the ejecta. We report the appearance of an unusual feature on the blue-side of H$\alpha$ after 100 d, which evolves to appear as a flat spectral feature linking H$\alpha$ and the O I doublet. This may be due to interaction of the ejecta with a strongly asymmetric, and possibly bipolar CSM. Finally, we report two deep spectra at ~190 and 340 d after explosion, the latter being arguably one of the latest spectra for a Type II-L SN. By modelling the spectral region around the Ca II, we find a supersolar Ni/Fe production. The strength of the O I $\lambda\lambda$6300,6363 doublet, compared with synthetic nebular spectra, suggests a progenitor with a zero-age main-sequence mass between 15 and 19 M$_\odot$.