A sample of Type II-L supernovae

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 ~15-25 M range, and observationally, with two independent studies suggesting that they cannot be cleanly separated photometrically from the regular hydrogenrich SNe II-P characterized by a marked plateau in their light curve. Here, we analyse the multiband 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 Hα absorption, may show indirect signs of helium via strong O I λ7774 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 to define them based on their decline in the V band: SNe II-L 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 four photometric bands.