Refined Classifications of Two Stripped-Envelope SNe

ATel #7821; Isaac Shivvers, Alexei V. Filippenko
on 21 Jul 2015; 20:24 UT
Credential Certification: Isaac Shivvers (ishivvers@berkeley.edu)

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We report refined classifications of two stripped-envelope supernovae (SNe) based upon spectra (330-1000 nm) obtained on UT July 20 with the 3-m Shane reflector (+ Kast) at Lick Observatory. Classifications were performed though cross-correlation with a library of supernova spectra using the "SuperNova IDentification" code (SNID; Blondin & Tonry 2007, Ap.J. 666, 1024) including the updated template spectra of Silverman et al. (2012, MNRAS, 425, 1789) and Liu & Modjaz (2014, arXiv:1405.1437). Host-galaxy information was retrieved through the NASA/IPAC Extragalactic Database (NED).

 
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Name                 | Host        | z      | Type  | Phase | Discovery | Notes 
ASASSN-15mj          | Anon        | 0.0344 | Ib    | -8.0  | ATel #7790| (1) 
PSNJ22460504-1059484 | NGC 7371    | 0.0089 | Ib    | -6.0  | K.Itagaki | (2) 
 

Notes:
(1) Classified as a possible Type II SN at an early phase by ATel #7796 from a spectrum taken UT July 13.9 showing a blue continuum with a weak H-alpha feature. Our spectrum shows weak hydrogen absorption along with helium absorption features, similar to that of the Type Ib SN 2004gq about 1 week before maximum brightness.
(2) Classified as a possible Type IIb SN by ATel #7787 from a spectrum taken UT July 11.07 showing relatively strong H-alpha absorption along with weak He absorption features. Our spectrum shows a much-diminished H-alpha feature with pronounced He absorption lines, similar to that of the Type Ib SN 2005bf about 1 week before maximum brightness or the Type Ib SN 1999ex near maximum. This object was also discovered by PanSTARRS as PS15bgt.

For both of these SNe, our spectra obtained on UT July 20 clearly indicate a Type Ib (rather than IIb) SN classification. The time-dependent nature of these classifications highlights the difficulty of distinguishing between Type IIb and Ib SNe without a full spectral sequence and emphasizes that the boundary between these stripped-envelope subclasses is not well defined --- some Type Ib SNe show weak hydrogen at early times.