Luminous Type II supernovae for their low expansion velocities

TL;DR

This paper studies a subset of luminous Type II supernovae with low expansion velocities, revealing that ejecta-CSM interaction significantly influences their spectral and photometric properties, and estimating their rarity among all SNe II.

Contribution

It identifies and characterizes a new class of luminous SNe II with low expansion velocities driven by ejecta-CSM interaction, providing insights into their properties and frequency.

Findings

LLEV SNe II are 2-4% of all normal SNe II.

Ejecta-CSM interaction lasts 4-11 weeks post explosion.

Interaction dominates spectral features over progenitor metallicity.

Abstract

We present optical and near-IR data of three Type II supernovae (SNe II), SN 2008bm, SN 2009aj, and SN 2009au. These SNe display the following common characteristics: signs of early interaction of the ejecta with circumstellar material (CSM), blue $B\!-\!V$ colours, weakness of metal lines, low expansion velocities, and $V$-band absolute magnitudes 2-3 mag brighter than those expected for normal SNe II based on their expansion velocities. Two more SNe reported in the literature (SN 1983K and LSQ13fn) share properties similar to our sample. Analysing this set of five SNe II, which are luminous for their low expansion velocities (LLEV), we find that their properties can be reproduced assuming ejecta-CSM interaction that lasts between 4-11 weeks post explosion. The contribution of this interaction to the radiation field seems to be the dominant component determining the observed weakness of metal lines in the spectra rather than the progenitor metallicity. Based on hydrodynamic simulations, we find that the interaction of the ejecta with a CSM of ~3.6 M$_\odot$ can reproduce the light curves and expansion velocities of SN 2009aj. Using data collected by the Chilean Automatic Supernova Search, we estimate an upper limit for the LLEV SNe II fraction to be 2-4 per cent of all normal SNe II. With the current data-set, it is not clear whether the LLEV events are a separated class of SNe II with a different progenitor system, or whether they are the extreme of a continuum mediated by CSM interaction with the rest of the normal SN II population.