Publication Date:
2016-01-22
Description:
We present six epochs of spectropolarimetric observations and one epoch of spectroscopy of the Type Ib SN iPTF 13bvn. The epochs of these observations correspond to –10 to +61 d with respect to the r -band light-curve maximum. The continuum is intrinsically polarized to the 0.2–0.4 per cent level throughout the observations, implying asphericities of ~10 per cent in the shape of the photosphere. We observe significant line polarization associated with the spectral features of Ca ii ir3 , He i /Na i d , He i 6678, 7065, Fe ii 4924 and O i 7774. We propose that an absorption feature at ~6200 Å, usually identified as Si ii 6355, is most likely to be high-velocity H α at –16 400 km s –1 . Two distinctly polarized components, separated in velocity, are detected for both He i /Na i d and Ca ii ir3 , indicating the presence of two discrete line-forming regions in the ejecta in both radial velocity space and in the plane of the sky. We use the polarization of He i 5876 as a tracer of sources of non-thermal excitation in the ejecta; finding that the bulk of the radioactive nickel was constrained to lie interior to ~50–65 per cent of the ejecta radius. The observed polarization is also discussed in the context of the possible progenitor system of iPTF 13bvn, with our observations favouring the explosion of a star with an extended, distorted envelope rather than a compact Wolf–Rayet star.
Print ISSN:
0035-8711
Electronic ISSN:
1365-2966
Topics:
Physics
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