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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 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 intermediate-mass 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 may suggest that there are thick enough, clumpy Si-rich layers above the thick 56Ni-rich layers (>~ 1.2 Msun). The observed spectropolarimetric properties, combined with the photometric and spectroscopic properties, suggest that the progenitor of SN 2009dc has a super-Chandrasekhar mass, and that the explosion geometry is globally spherically symmetric, with clumpy distribution of intermediate-mass elements.