Spectropolarimetric observations of supernovae

We briefly review the existing database of supernova spectropolarimetry, concentrating on recent data and on results from our group's research. Spectropolarimetry provides the only direct known probe of early-time supernova geometry. To obtain reliable conclusions, however, it is very important to correctly account for interstellar polarization. We find that Type IIn supernovae (SNe IIn) tend to be highly polarized, perhaps in part because of the interaction of the ejecta with an asymmetric circumstellar medium. In contrast, SNe II-P are not polarized much, at least shortly after the explosion. At later times, however, there is evidence for increasing polarization, as one views deeper into the expanding ejecta. Moreover, core-collapse SNe that have lost part (SN IIb) or all (SN Ib) of their hydrogen (or even helium; SN Ic) layers prior to the explosion tend to show substantial polarization; thus, the deeper we probe into core-collapse events, the greater the asphericity. There is now conclusive evidence that at least some SNe Ia are intrinsically polarized, although only by a small amount. Finally, SN spectropolarimetry provides the opportunity to study the fundamental properties of the interstellar dust in external galaxies. For example, we have found evidence for extremely high polarization efficiency for the dust along the line-of-sight to SN 1999gi in NGC 3184.