SN 2015U: A Rapidly Evolving and Luminous Type Ibn Supernova

TL;DR

SN 2015U is a luminous, rapidly evolving Type Ibn supernova with spectra dominated by narrow helium lines, best explained by shock breakout from dense circumstellar material caused by recent mass loss from its progenitor.

Contribution

This study provides detailed photometric, spectroscopic, and spectropolarimetric observations of SN 2015U, offering new insights into its shock breakout mechanism and progenitor mass loss history.

Findings

SN 2015U's luminosity was powered by shock interaction, not radioactive decay.

Spectral analysis revealed helium-rich, hydrogen-free features.

The supernova's properties suggest a recent, extreme mass-loss event from the progenitor.

Abstract

Supernova (SN) 2015U (also known as PSN J07285387+3349106) was discovered in NGC 2388 on 2015 Feb. 11. A rapidly evolving and luminous event, it showed effectively hydrogen-free spectra dominated by relatively narrow helium P-Cygni spectral features and it was classified as a SN Ibn. In this paper we present photometric, spectroscopic, and spectropolarimetric observations of SN 2015U, including a Keck/DEIMOS spectrum (resolution $\approx$ 5000) which fully resolves the optical emission and absorption features. We find that SN 2015U is best understood via models of shock breakout from extended and dense circumstellar material (CSM), likely created by a history of mass loss from the progenitor with an extreme outburst within $\sim$1-2 yr of core collapse (but we do not detect any outburst in our archival imaging of NGC 2388). We argue that the high luminosity of SN 2015U was powered not through $^{56}$Ni decay but via the deposition of kinetic energy into the ejecta/CSM shock interface. Though our analysis is hampered by strong host-galaxy dust obscuration (which likely exhibits multiple components), our dataset makes SN 2015U one of the best-studied Type Ibn supernovae and provides a bridge of understanding to other rapidly fading transients, both luminous and relatively faint.