Spectropolarimetry of the Type Ib/c SN 2005bf⋆

We present spectropolarimetric observations of the peculiar Type Ib/c SN 2005bf, in MCG+00-27-005, from 3600-8550Å. The SN was observed on 2005 April 30.9, 18 days after the first B-band light-curve maximum and 6 days before the second B-band light-curve maximum. The degree of the Interstellar Polarization, determined from depolarized emission lines in the spectrum, is found to be large with pmax(ISP ) = 1.6% and θ(ISP ) = 149. ◦7 ± 4.0, but this may be an upper limit on the real value of the ISP. After ISP subtraction, significant polarization is observed over large wavelength regions, indicating a significant degree of global asymmetry, ∼> 10%. Polarizations of 3.5% and 4% are observed for absorption components of Ca II H&K and IR triplet, and 1.3% for He I 5876Å and Fe II. On the Q−U plane clear velocity-dependent loop structure is observed for the He I 5876Å line, suggestive of departures from an axial symmetry and possible clumping of the SN ejecta. Weak High Velocity components of Hα, Hβ and Hγ are observed, with velocities of −15 000km s . The low degree of polarization observed at Hβ suggests that the polarization observed for the other Balmer lines (∼ 0.4% above the background polarization) may rather be due to blending of Hα and Hγ with polarized Si II and Fe II lines, respectively. We suggest a model in which a jet of material, that is rich in Ni, has penetrated the C-O core, but not the He mantle. The jet axis is tilted with respect to the axis of the photosphere. This accounts for the lack of significant polarization of O I 7774Å, the delayed excitation and, hence, observability of He I and, potentially, the varied geometries of He and Ca.