SPECTROPOLARIMETRY OF EXTREMELY LUMINOUS TYPE Ia SUPERNOVA 2009dc: NEARLY SPHERICAL EXPLOSION OF SUPER-CHANDRASEKHAR MASS WHITE DWARF

We present the first spectropolarimetric observations of a candidate of super-Chandrasekhar mass Type Ia supernova (SN): SN 2009dc. The observations were performed at 5.6 and 89.5 days after the B-band maximum. The data taken at the later epoch are used to determine the interstellar polarization. Continuum polarization is found to be small (< 0.3%), indicating that the explosion is nearly spherically symmetric. This fact suggests that a very aspherical explosion of the Chandrasekhar mass white dwarf is not a likely scenario for SN 2009dc. Polarization at the Si ii and Ca ii lines clearly shows a loop in the Q-U plane, indicating a non-axisymmetric, clumpy distribution of intermediatemass elements. The degree of line polarization at the Si and Ca line is moderate (0.5% ± 0.1% and 0.7%± 0.1%, respectively), but it is higher than expected from the trend of other Type Ia SNe. This suggests that there are thick enough, clumpy Si-rich layers above the thick Ni-rich layers ( ∼ > 1.2M⊙), and thus, the progenitor white dwarf mass exceeds the Chandrasekhar mass. Our results suggest that the progenitor of SN 2009dc has a super-Chandrasekhar mass, and that the explosion geometry is similar to that of normal Type Ia SNe, i.e., globally spherically symmetric, with clumpy distribution of intermediate-mass elements. Subject headings: polarization — supernovae: general — supernovae: individual (SN 2009dc) — supernovae: individual (SNe 2003fg, 2006gz, 1991T) — white dwarfs