A Sample of Type II-L Supernovae

TL;DR

This study characterizes Type II-L supernovae through analysis of light curves and spectra, proposing defining features and providing template light curves to distinguish them from Type II-P supernovae.

Contribution

It offers a detailed observational analysis of SNe II-L, clarifies their spectral and photometric characteristics, and provides standardized template light curves.

Findings

SNe II-L have hydrogen-deficient spectra with shallow Halpha absorption.

They exhibit faster line velocities and steeper decline rates than SNe II-P.

SNe II-L decline more than 0.5 mag in V band by day 50 post-explosion.

Abstract

What are Type II-Linear supernovae (SNe II-L)? This class, which has been ill defined for decades, now receives significant attention -- both theoretically, in order to understand what happens to stars in the ~15-25Mo range, and observationally, with two independent studies suggesting that they cannot be cleanly separated photometrically from the regular hydrogen-rich SNe II-P characterised by a marked plateau in their light curve. Here, we analyze the multi-band light curves and extensive spectroscopic coverage of a sample of 35 SNe II and find that 11 of them could be SNe II-L. The spectra of these SNe are hydrogen deficient, typically have shallow Halpha absorption, may show indirect signs of helium via strong OI 7774 absorption, and have faster line velocities consistent with a thin hydrogen shell. The light curves can be mostly differentiated from those of the regular, hydrogen-rich SNe II-P by their steeper decline rates and higher luminosity, and we propose as a defining photometric characteristic the decline in the V band: SNe II-L seem to decline by more than 0.5 mag from peak brightness by day 50 after explosion. Using our sample we provide template light curves for SNe II-L and II-P in 4 photometric bands.