2007MNRAS.381..201M

We present spectropolarimetric observations of the peculiar Type Ib/c SN 2005bf, in MCG+00-27-005, from 3600-8550 Å. The SN was observed on 2005 April 30.9, 18 d after the first B-band light-curve maximum and 6 d before the second B-band light-curve maximum. The degree of the interstellar polarization (ISP), determined from depolarized emission lines in the spectrum, is found to be large with p_max(ISP) = 1.6 per cent and, 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 per cent. Polarizations of 3.5 and 4 per cent are observed for absorption components of Ca II H& K and infrared triplet, and 1.3 per cent for He i 5876 Å and Fe II. On the Q-U plane clear velocity-dependent loop structure is observed for the He i 5876 Å 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 -15 000 km/s. The low degree of polarization observed at Hβ suggests that the polarization observed for the other Balmer lines (∼0.4 per cent 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, 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 7774 Å, the delayed excitation and, hence, observability of He i and, potentially, the varied geometries of He and Ca.