PROBING THE GEOMETRY AND CIRCUMSTELLAR ENVIRONMENT OF SN 1993J IN M81

We have monitored the polarized radiation of the Type lIb 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 intrin sic continuum polarization of SN 1993J evolved from being essentially negligible on April 3-4, to a peak value of similar to 1% in late Apri l 1993, and started to decline by the middle of May. The polarized flu x spectrum in late April strongly resembled spectra of Type Ib superno vae, with prominent He I lines but redshifted similar to 3380 km s(-1) relative to the total flux spectrum. These data are consistent with m odels of Hoflich; they suggest that the polarization was most likely p roduced by either an asymmetric helium core configuration of material and/or flux, or scattering from an asymmetric circumstellar distributi on of dusty material. A combination of electron and dust scattering, a s 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 mon ths after the explosion, the observed rotations of polarization positi on angle across prominent line features remain even after correction f or 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-Bal mer lines generally being most highly polarized. If the supernova had an oblate geometry, our data are consistent with a small viewing angle (i.e., more or less equator-on), although the degree of asphericity t hat gave rise to the polarization at early times is probably smaller ( minor to major axis ratio greater than or similar to 0.7) than has bee n previously suggested.