THE HE-I EMISSION-LINES OF SN 1987A

We model the evolution of the He I lines, especially He I 1.083 mu m and He I 2.058 mu m, from the envelope of SN 1987A during the first 3 yr after the explosion. The He I metastable 2(3)S and 2(1)S states are populated by fast electrons resulting from energy deposition by gamma rays and depopulated by electron collisions and two-photon decay. The 2.058 mu m line is excited mainly by nonthermal electrons at all times. The 1.083 mu m line is excited mainly by thermal electron impact for t less than or similar to 450 days and by recombination afterward. We can fit the observed evolution of He I 2.058 mu m with a model in which similar to 3 M(circle dot) of nearly pure helium and similar to 11 M(circle dot) of hydrogen mixed with primordial helium occupy the volume emitting most of the light (v less than or equal to 2500 km s(-1)). For t less than or similar to 450 days, trace carbon in the inner part of the helium suppresses the He I 2.058 mu m line by photoabsorbing He I lambda 584 and depopulating the 2(1)P(o) state. The observed 1.08 mu m line is a blend of He I 1.083 mu m and [S I] 1.082 mu m. We estimate that He I 1.083 mu m contributes similar to 40% of the observed flux for t less than or similar to 550 days and similar to 60% thereafter.