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 2003 February show it to be still relatively bright, with m(F606W) = 21.4 and m(F814W) = 20.0 mag. Compared to 1994 December HST WFPC2 images, SN1986J shows a decline of only less than or similar to 1 mag in brightness over 8 years. Ground-based spectra taken in 1989, 1991, and 2007 show a 50% decline in H alpha emission between 1989 and 1991, and an order of magnitude drop between 1991 and 2007, along with the disappearance of He I line emissions during the period 1991-2007. The object's [O I] lambda lambda 6300, 6364, [O II] lambda lambda 7319, 7330 and [O III] lambda lambda 4959, 5007 emission lines show two prominent peaks near -1000 and -3500 km s(-1), 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 H alpha, He I, and [N II] lambda 5755 lines, and reverse-shock-heated O-rich ejecta on the facing expanding hemisphere, dominated by two large clumps generating two blueshifted emission peaks in the [O I], [O II], and [O III] lines.