Detection of Broad Hα Emission Lines in the Late-time Spectra of a Hydrogen-poor Superluminous Supernova

December 2015 • 2015ApJ...814..108Y

Authors • Yan, Lin • Quimby, R. • Ofek, E. • Gal-Yam, A. • Mazzali, P. • Perley, D. • Vreeswijk, P. M. • Leloudas, G. • De Cia, A. • Masci, F. • Cenko, S. B. • Cao, Y. • Kulkarni, S. R. • Nugent, P. E. • Rebbapragada, Umaa D. • Woźniak, P. R. • Yaron, O.

Abstract • iPTF13ehe is a hydrogen-poor superluminous supernova (SLSN) at z = 0.3434, with a slow-evolving light curve and spectral features similar to SN2007bi. It rises in 83-148 days to reach a peak bolometric luminosity of ∼1.3 × 10⁴⁴ erg s^-1, then decays slowly at 0.015 mag day^-1. The measured ejecta velocity is ∼ 13,000 km s^-1. The inferred explosion characteristics, such as the ejecta mass (70-220 M_⊙), and the total radiative and kinetic energy (E_rad ∼ 10⁵¹ erg, E_kin ∼ 2 × 10⁵³ erg), are typical of slow-evolving H-poor SLSN events. However, the late-time spectrum taken at +251 days (rest, post-peak) reveals a Balmer Hα emission feature with broad and narrow components, which has never been detected before among other H-poor SLSNe. The broad component has a velocity width of ∼4500 km s^-1 and a ∼300 km s^-1 blueward shift relative to the narrow component. We interpret this broad Hα emission with a luminosity of ∼2 × 10⁴¹ erg s^-1 as resulting from the interaction between the supernova ejecta and a discrete H-rich shell, located at a distance of ∼4 × 10¹⁶ cm from the explosion site. This interaction causes the rest-frame r-band LC to brighten at late times. The fact that the late-time spectra are not completely absorbed by the shock-ionized H-shell implies that its Thomson scattering optical depth is likely ≤1, thus setting upper limits on the shell mass ≤30 M_⊙. Of the existing models, a Pulsational Pair Instability supernova model can naturally explain the observed 30 M_⊙ H-shell, ejected from a progenitor star with an initial mass of (95-150) M_⊙ about 40 years ago. We estimate that at least ∼15% of all SLSNe-I may have late-time Balmer emission lines.

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Frank Masci

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