Bipolar Supernova Explosions

Abstract

We discuss the optical spectropolarimetry of several core-collapse supernovae, SN 1996cb (Type IIB), SN 1997X (Type Ic), and SN 1998S (Type IIn). The data show polarization evolution of several spectral features at levels from 0.5% to above 4%. The observed line polarization is intrinsic to the supernovae and not of interstellar origin. These data suggest that the distribution of ejected matter is highly aspherical. In the case of the Type IIn SN 1998S, the major-to-minor axis ratio must be larger than 2.5 if the polarization is 3% from an oblate spheroidal ejecta seen edge-on. A well-deÐned symmetry axis can be deduced from spectropolarimetry for SN 1998S, but the Type IIB events SN 1993J and SN 1996cb seem to possess much more complicated geometries with polarization position angles showing larger irregular variations across spectral features ; the latter may be associated with large-scale clumpiness of the ejecta. The observed degree of polarization of the Type Ic SN 1997X is above 4%. The data reveal a trend that the degree of polarization increases with decreasing envelope mass and with the depth within the ejecta. The high axial ratio of the ejecta is difficult to explain in terms of the conventional neutrino-driven corecollapse models for Type II explosions. Highly asymmetric explosion mechanisms such as the formation of bipolar jets during core collapse may be a necessary ingredient for models of all core-collapse supernovae.

Stellar Collapse and Gravitational Waves

“…even at the relatively low value z = 5 (corresponding to a luminosity distance ∼ 48 Gpc in the currently popular "concordance cosmology" [ Wang, Tegmark, & Zaldarriaga 2002]), the strain from bar modes peaks at frequencies less than 10 Hz, with a strain 8×10 −23 . This is well below the LIGO II threshold.…”

“…Progress and future understanding will only come as these models are further generalized, building in the rotation or core oscillations that seem needed (Fryer & Heger 2000) to explain asymmetries in supernovae (Wang et al 2001). Fully 3-D simulations, such as those from Fryer & Warren 2002, hold great promise for extending our knowledge of the collapse process.…”

Optical Observations of Core-Collapse Supernovae

“…We have studied SN IIn 1998S in some detail [69] (see also [70]); its optical spectrum is dominated by strong, multi-component emission lines, thought to be produced by an intense interaction between the supernova and its dense circumstellar environment. We combined our early-time (3 days after discovery) spectropolarimetric observation with total flux spectra spanning nearly 500 days.…”

“…The data are consistent with models in which the polarization is produced by an asymmetric He core configuration of material. It is interesting that the percent polarization may increase with decreasing envelope mass, along the sequence Type II, IIb, Ib, and Ic [72,70,73], suggesting that asymmetries in massive stars become more pronounced as one probes deeper into the core. SN 1999em, an extremely bright (m V ≈ 13.5) SN II-P, provided the rare opportunity to study the geometry of a "normal" core-collapse event at multiple epochs [74].…”

Spectropolarimetry of Type IIn SN2010jl: Peering Into the Heart of a Monster

“…The geometrical insight gained from spectropolarimetry may allow a critical approach to these open issues, especially for rare luminous type IIn's like SN2010jl, which could provide extreme tests of models. The only other type IIn SNe to be observed in polarized light were SN1998S (Leonard et al 2000;Wang et al 2001) and SN1997eg (Hoffman et al 2008) to provide points of comparison.…”