paper

Nebular-phase spectra of superluminous supernovae: physical insights from observational and statistical properties

arXiv:1808.00510 · doi:10.3847/1538-4357/aaf470

Abstract

We study the spectroscopic evolution of superluminous supernovae (SLSNe) later than 100 days after maximum light. We present new data for Gaia16apd and SN2017egm, and analyse these with a larger sample comprising 41 spectra of 12 events. The spectra become nebular within 2-4 -folding times after light curve peak, with the rate of spectroscopic evolution correlated to the light curve timescale. Emission lines are identified with well-known transitions of oxygen, calcium, magnesium, sodium and iron. SLSNe are differentiated from other Type Ic SNe by a prominent O I 7774 line and higher-ionisation states of oxygen. The iron-dominated region around 5000 Å is more similar to broad-lined SNe Ic than to normal SNe Ic. Principal Component Analysis shows that 5 `eigenspectra' capture 75% of the variance, while a clustering analysis shows no clear evidence for multiple SLSN sub-classes. Line velocities are 5000--8000 km/s, and show stratification of the ejecta. O I 7774 likely arises in a dense inner region that also produces calcium emission, while [O I] 6300 comes from further out until 300--400 days. The luminosities of O I 7774 and Ca II suggest significant clumping, in agreement with previous studies. Ratios of [Ca II]7300/[O I]6300 favour progenitors with relatively massive helium cores, likely M, though more modelling is required here. SLSNe with broad light curves show the strongest [O I] 6300, suggesting larger ejecta masses. We show how the inferred velocity, density and ionisation structure point to a central power source.

arXiv commentAccepted in ApJ, updated to match accepted version

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