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Detection of Broad H alpha Emission Lines in the Late-Time Spectra of a Hydrogen-Poor Superluminous Supernova

Yan, L and Quimby, R and Ofek, E and Gal-Yam, A and Mazzali, PA and Perley, DA and Vreeswijk, PM and Leloudas, G and de Cia, A and Masci, F and Cenko, SB and Cao, Y and Kulkarni, SR and Nugent, PE and Rebbapragada, UD and Wozniak, PR and Yaron, O (2015) Detection of Broad H alpha Emission Lines in the Late-Time Spectra of a Hydrogen-Poor Superluminous Supernova. The Astrophysical Journal, 814 (2). ISSN 1538-4357

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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 (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. 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.

Item Type: Article
Uncontrolled Keywords: 0201 Astronomical And Space Sciences, 0305 Organic Chemistry, 0306 Physical Chemistry (Incl. Structural)
Subjects: Q Science > QB Astronomy
Q Science > QC Physics
Q Science > QD Chemistry
Divisions: Astrophysics Research Institute
Publisher: American Astronomical Society
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Date Deposited: 21 Feb 2017 09:29
Last Modified: 06 Sep 2017 16:30
DOI or Identification number: 10.1088/0004-637X/814/2/108
URI: http://researchonline.ljmu.ac.uk/id/eprint/5614

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