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 ( > 100 days ) 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 { M } _ { \odot } \mathrm { yr ^ { -1 } } for v _ { pre } = 100 km \mathrm { s ^ { -1 } } extending back at least 75 yr 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 .