SN2012ab: A Peculiar Type IIn Supernova with Aspherical Circumstellar Material

Abstract

We present photometry, spectra, and spectropolarimetry of supernova (SN) 2012ab, mostly obtained over the course of $\sim 300$ days after discovery. SN 2012ab was a Type IIn (SN IIn) event discovered near the nucleus of spiral galaxy 2MASXJ12224762+0536247. While its light curve resembles that of SN 1998S, its spectral evolution does not. We see indications of CSM interaction in the strong intermediate-width emission features, the high luminosity (peak at absolute magnitude $M=-19.5$), and the lack of broad absorption features in the spectrum. The H$\alpha$ emission undergoes a peculiar transition. At early times it shows a broad blue emission wing out to $-14,000$ km ${\mathrm{s}}^{-1}$ and a truncated red wing. Then at late times ($>$ 100days) it shows a truncated blue wing and a very broad red emission wing out to roughly $+20,000$ km ${\mathrm{s}}^{-1}$. This late-time broad red wing probably arises in the reverse shock. Spectra also show an asymmetric intermediate-width H$\alpha$ component with stronger emission on the red side at late times. The evolution of the asymmetric profiles requires a density structure in the distant CSM that is highly aspherical. Our spectropolarimetric data also suggest asphericity with a strong continuum polarization of $\sim 1-3$% and depolarization in the H$\alpha$ line, indicating asphericity in the CSM at a level comparable to that in other SNe IIn. We estimate a mass-loss rate of $\dot{M}=0.050{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ for $v_{\mathrm{pre}}=100$km${\mathrm{s}}^{-1}$ extending back at least 75yr prior to the SN. The strong departure from axisymmetry in the CSM of SN 2012ab may suggest that the progenitor was an eccentric binary system undergoing eruptive mass loss.

18 pages, 12 figures

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