2019ApJ...871..102N

We study the spectroscopic evolution of superluminous supernovae (SLSNe) later than 100 days after maximum light. We present new data for Gaia16apd and SN 2017egm and analyze these with a larger sample comprising 41 spectra of 12 events. The spectra become nebular within 2-4 e-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 SNe Ic by a prominent O I λ7774 line and higher ionization 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 five "eigenspectra" capture >=70% of the variance, while a clustering analysis shows no clear evidence for multiple SLSN subclasses. Line velocities are 5000-8000 km s^–1 and show stratification of the ejecta. O I λ7774 likely arises in a dense inner region that also produces calcium emission, while [O I] λ6300 comes from farther out until 300-400 days. The luminosities of O I λ7774 and Ca II suggest significant clumping, in agreement with previous studies. Ratios of [Ca II] λ7300/[O I] λ6300 favor progenitors with relatively massive helium cores, likely >=6 M_☉, though more modeling is required here. SLSNe with broad light curves show the strongest [O I] λ6300, suggesting larger ejecta masses. We show how the inferred velocity, density, and ionization structure point to a central power source.