The Evolution of Late-time Optical Emission from SN 1986J
  We present late-time optical images and spectra of the Type IIn supernova SN 1986J. HST ACS/WFC images obtained in February 2003 show it to be still relatively bright with m(F606W) = 21.4 and m(F814W) = 20.0 mag. Compared against December 1994 HST WFPC2 images, SN 1986J shows a decline of only <1 mag in brightness over eight years. Ground-based spectra taken in 1989, 1991 and 2007 show a 50% decline in Halpha emission between 1989-1991 and an order of magnitude drop between 1991-2007, along with the disappearance of He I line emissions during the period 1991-2007. The object's [O I] 6300, 6364, [O II] 7319, 7330 and [O III] 4959, 5007 emission lines show two prominent peaks near -1000 km/s and -3500 km/s, with the more blueshifted component declining significantly in strength between 1991 and 2007. The observed spectral evolution suggests two different origins for SN 1986J's late-time optical emission: dense, shock-heated circumstellar material which gave rise to the initially bright Halpha, He I, and [N II] 5755 lines, and reverse-shock heated O-rich ejecta on the facing expanding hemisphere dominated by two large clumps generating two blueshifted emission peaks of [O I], [O II], and [O III] lines. 
D. Milisavljevic, R. Fesen, B. Leibundgut, R. Kirshner