On the diversity of Super-luminous Supernovae: Ejected mass as the dominant factor

M. Nicholl, S. J. Smartt, A. Jerkstrand, C. Inserra, S. A. Sim, T. -W. Chen, S. Benetti, M. Fraser, A. Gal-Yam, E. Kankare, K. Maguire, K. Smith, M. Sullivan, S. Valenti, D. R. Young, C. Baltay, F. E. Bauer, S. Baumont, D. Bersier, M. -T. Botticella, M. Childress, M. Dennefeld, M. Della Valle, N. Elias-Rosa, U. Feindt, L. Galbany, R. Kotak, L. Le Guillou, G. Leloudas, P. Mazzali, R. McKinnon, J. Polshaw, D. Rabinowitz, S. Rostami, R. Scalzo, E. Schmelzer, B. P. Schmidt, S. Schulze, J. Sollerman, F. Taddia, F. Yuan

Published 2015-03-11Version 1

H-poor super-luminous supernovae (SLSNe) are a rare and poorly understood class of explosion. We assemble the largest sample (24) of such objects to date, with griz light curves and optical spectra. We parameterize the light curve through rise and decline timescales, finding that these are highly correlated. Magnetar-powered models reproduce the correlation, with the diversity in rise and decline driven by the diffusion timescale. Circumstellar interaction models can exhibit a similar rise-decline relation, but for only a narrow density range, which may be problematic for these models. We see a similar correlation in normal SNe Ibc (powered by 56Ni), though SLSNe rise and decline more slowly, and their peak luminosity requires an additional energy source. We find that SLSN light curves are approximately 3.5 mag brighter and 3 times broader than SNe Ibc, but that the intrinsic shapes are similar. Some SLSNe (2007bi-like) have very broad light curves, possibly indicating two progenitor channels, but statistical tests do not distinguish separate populations in our sample. The spectral evolution is also presented. Velocities measured from the Fe II 5169 line are similar for SLSNe and SNe Ic, suggesting that the difference in diffusion time is dominated by the ejected mass. If the opacities in SLSNe are similar to other SNe Ibc, then the average ejected mass in SLSNe is higher by more than a factor of two. Assuming kappa = 0.1 cm2/g, we estimate a mean (median) SLSN ejecta mass of ~10 Msun (6 Msun), with a range of ~3-30 Msun, though doubling the opacity would bring the mass estimates in line with other SNe Ibc. The velocities of many SLSNe are constant, indicating a dense shell of ejecta. We conclude that the most probable mechanism for generating SLSNe is the explosion of a star similar to, but more massive than, a typical SN Ic progenitor, powered by an engine such as a magnetar.