Temperature evolution of the envelope of SN 1987A

We present a theoretical derivation of the H _α line luminosity of the expanding envelope of SN 1987A from the theory of hydrogen recombination lines. A remarkable deviation of our calculated H _α light curve from the observed light curve was found when a constant temperature was assumed. From the deviation we easily derive the temperature evolution. The temperature actually rises after day 500 and this may be explained as follows: as the shell expands, the electron and ion densities rapidly fall, greatly reducing the recombination cooling rate, while heating continues.