Probing the Geometry and Circumstellar Environment of SN 1993 J in M 81 Hien

We have monitored the polarized radiation of the Type IIb SN 1993J in M81 over a period of 41 days, starting from 7 days after the explosion on 1993 March 27.5 (UT). Our data show clear evidence that the intrinsic continuum polarization of SN 1993J evolved from being essentially negligible on April 3–4, to a peak value of ∼ 1% in late April 1993, and started to decline by the middle of May. The polarized flux spectrum in late April strongly resembled spectra of Type Ib supernovae, with prominent He I lines but redshifted ∼ 3380 km s−1 relative to the total flux spectrum. These data are consistent with models of Höflich; they suggest that the polarization was most likely produced by either an asymmetric helium core configuration of material and/or flux, or scattering from an asymmetric circumstellar distribution of dusty material. A combination of electron and dust scattering, as well as a clumpy or stratified distribution of the emitting gas, are possible as the polarization mechanism of the continuum and emission lines. The latter interpretation is supported by the fact that 1–2 months after the explosion, the observed rotations of polarization position angle across prominent line features remain even after correction for effects of interstellar polarization. This indicates that emission lines of He I, Fe II, [O I], and H are all intrinsically polarized at position angles different from that of the continuum, with the non-Balmer lines generally being most highly polarized. If the supernova had an UCO/Lick Observatory, University of California, Santa Cruz, CA 95064; [email protected]. Current Address: Institute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, 7000 East Avenue, P.O. Box 808, L413, Livermore, CA 94550; [email protected]. Department of Astronomy, and Center for Particle Astrophysics, University of California, Berkeley, CA 947203411; [email protected]. Steward Observatory, University of Arizona, Tucson, AZ 85721; [email protected]. Space Astronomy Laboratory, University of Wisconsin, Madison, WI 53706. Current Address: Ritter Observatory, Dept. of Physics and Astronomy, University of Toledo, Toledo OH 43606; [email protected]. National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ 85726-6732; [email protected], [email protected].