  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 relative to the total flux spectrum. These data are consistent with models
of Hoflich; 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 oblate geometry,
our data are consistent with a small viewing angle (i.e., more or less
equator-on), although the degree of asphericity that gave rise to the
polarization at early times is probably smaller (minor to major axis ratio >
0.7) than has been previously suggested.

Astrophysics