The SN 1986G in Centaurus A.

Abstract

We present photometric and spectroscopic observations of SN 1986 G, spanning up to one year after the explosion. The overall photometric behavior of this SN Ia results rather normal. The maximum at B = 1245+/-0.05 was reached on May 11,1986, then the decline to the inflection point was relatively fast (β_B_ = 12.0 mag/100 d), but similar to other SNe. We show that the fast, early decline rate is not due, as suspected before, to the combined effect of the strong differential absorption and peculiar spectral evolution of this SN. Also, the late time decline rate resembles that of other SN Ia (γ_V_ = 1.58 mag/100 d). The particular position of the SN in the two-color diagram may be due to some undetected spectral peculiarity in the UV-visible gap of the available observations. Although the overall spectral evolution of SN 1986 G is typical of SN Ia, some peculiarities are pointed out. The expansion velocities of the envelope at different phases, deduced from the Si II 6355 A doublet, are among the slowest ever detected. The strong broad emission at about 4100 A, usually present in the SN Ia near-maximum spectra, is missing, affecting also the kinematic evolution of the envelope as derived from the Mg II 4481 A line. High S/N ratio late time spectra are presented, showing several features identified, by comparison with published synthetic spectra, as [Fe II], (Fe III] and [Co III] lines. The broad symmetric emission line at 6560 A, may be alternatively identified with Hα, indicating then the presence of an outer shell of ionized hydrogen. Different procedures have been used for determining the reddening suffered by the SN. All agree on a total color excess E(B - v) = 1.1 mag, somewhat higher than the previous determinations. Hence, the absorption- corrected absolute magnitude results are comparable to the average values for SN Ia.