DETECTION OF BROAD Hα EMISSION LINES IN THE LATE-TIME SPECTRA OF A HYDROGEN-POOR SUPERLUMINOUS SUPERNOVA

• Published in: The Astrophysical journal Vol. 814; no. 2; p. 108
• Main Authors: Yan, Lin, Quimby, R., Ofek, E., Gal-Yam, A., Mazzali, P., Perley, D., Vreeswijk, P. M., Leloudas, G., Cia, A. de, Masci, F., Cenko, S. B., Cao, Y., Kulkarni, S. R., Nugent, P. E., Rebbapragada, Umaa D., Woźniak, P. R., Yaron, O.
• Format: Journal Article
• Language: English
• Published: United States Institute of Physics (IOP) 01.12.2015
• Subjects: ASTRONOMY AND ASTROPHYSICS; iPTF13ehe; PTF12dam; SN2007bi; supernovae
• ISSN: 1538-4357; 1538-4357
• DOI: 10.1088/0004-637X/814/2/108

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 (Erad ~ 1051 erg, Ekin ~ 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. In this paper, 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. Finally, we estimate that at least ~15% of all SLSNe-I may have late-time Balmer emission lines.