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P. A. Mazzali (MPA Garching, INAF-Trieste)
The recent great advances in observational facilities, and the increased effort dedicated to Supernova observations have, among other exciting discoveries, revealed that the outflow of material in many, possibly even in all Supernovae, is not spherical. Here we review the evidence in the Type Ic SNe linked to GRBs and in Type Ia SNe.
The connection between Ray Bursts and highly energetic (30-50 foe isotropic equivalent) Type Ic SNe ("Hypernovae") is well established, after the cases of GRB980425/SN1998bw, GRB030329/SN2003dh, and GRB031203/SN2003lw. Evidence for asymmetry in the SN ejecta comes both from polarimetry and from the line profiles, and it fits well with the highly beamed nature of the GRB phenomenon. In particular, the nebular Fe lines in GRB/SNe are broader than the O line, a situation which requires an aspherical explosion. This is confirmed by the recent observation of double peaks in the O line of a bright SN Ic, which can be interpreted as an aspherical explosion viewed off-axis. Less powerful SNe Ic with the spectroscopic signatures of Hypernovae also exist (e.g. SN2002ap). These may also be asymmetric, as indeed may be normal SNeIc, although probably to a lesser degree.
Unlike core-collapse SNe, in SNe Ia we probably are not confronted with global asphericities, but rather with a non homogeneous distribution of properties. Spectroscopically, evidence for asphericity is best found at the earliest times, suggesting that it affects mostly the outer part of the ejecta, unlike SNe Ib/c. Evidence that fingers or more likely blobs of burned material are ejected at high velocities is seen in the form of high-velocity absorption in the strongest spectral lines (Ca II IR, Si II), and is confirmed by polarization measurements. Although our understanding is still at a very preliminary stage, this evidence may be key to determine the detailed properties of SN Ia explosions.
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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.