Spectropolarimetry of the Type Ic Supernova SN 2002ap in M74: More Evidence for Asymmetric Core Collapse

High-quality spectropolarimetric data of SN 2002ap were obtained with the ESOVery Large Telescope Melipal (+ FORS1) at 3 epochs that correspond to -6, -2,and +1 days for a V maximum of 9 Feb 2002. The polarization spectra show threedistinct broad features at 400, 550, and 750 nm that evolve in shape, amplitudeand orientation in the Q-U plane. The continuum polarization grows from nearlyzero to ~0.2 percent. The 750 nm feature is polarized at a level > 1 %. Weidentify the 550 and 750 nm features as Na I D and OI 777.4 nm moving at about20,000 km/s. The blue feature may be Fe II. We interpret the polarizationevolution in terms of the impact of a bipolar flow from the core that isstopped within the outer envelope of a carbon/oxygen core. Although thesymmetry axis remains fixed, as the photosphere retreats by different amountsin different directions due to the asymmetric velocity flow and densitydistribution, geometrical blocking effects in deeper, Ca-rich layers can leadto a different dominant axis in the Q-U plane. We conclude that the featuresthat characterize SN 2002ap, specifically its high velocity, can be accountedfor in an asymmetric model with a larger ejecta mass than SN 1994I such thatthe photosphere remains longer in higher velocity material. The characteristicsof "hypernovae" may be the result of orientation effects in a mildlyinhomogeneous set of progenitors, rather than requiring an excessive totalenergy or luminosity. In the analysis of asymmetric events with sphericallysymmetric models, it is probably advisable to refer to "isotropic equivalent"energy, luminosity, ejected mass, and nickel mass.Comment: AASTeX, 29 pages, 4 postscript figures, submitted to Apj, June 19, 200