SN 1987A: After the Peak

Now that the light curve of SN 1987A has passed its peak and begun to decline, more accurate analysis of the explosion can be carried out and better predictions made of the future evolution. Observations continue to be consistent with the explosion of a star that, on the main sequence, had a mass of 19 +/- 3 M_sun_ and, at the time of explosion, a helium core mass of 6 +/- 1 M_sun_. The iron core mass was 1.45 +/- 0.15 M_sun_, and a neutron star of 1.40 +/- 0.15 M_sun_ (gravitational mass) has been formed releasing 2-3 x 10^53^ ergs of neutrinos (all flavors). The kinetic energy of an explosion required to reproduce the light curve and other key data was E_exp_ ~ 8 x 10^50^ (M_env_/5 M_sun_) ergs, with M_env_ the hydrogen envelope mass at the time of the explosion. Envelope masses less than 3 M_sun_ and greater than 14 M_sun_ are not allowed, and favored values are 5-10 M_sun_. The nitrogen-rich shell discovered around SN 1987A by the IUE suggests that significant mass loss has occurred, but the extent of the required mass loss depends upon how much the observed N/C ratio exceeds 10 times solar. Since it is doubtful at present that a star which still retains >~ 3 M_sun_ of envelope will move to the blue because of mass loss alone, other models employing low metallicity are explored. Low-metallicity (Z_sun_/4) models (with no mass loss) of 15 and 20 M_sun_ stars are presented, both of which burn helium as red supergiants and move back to the blue shortly before exploding. A similarly evolved 25 M_sun_ star does not move back to the blue. The light curve of the supernova is now powered by the decay of 0.07 M_sun_ of radioactive ^56^Co as has been the case since late March when the hydrogen envelope was traversed. By September the characteristic decay rate, 0.01 mag per day (bolometric), has shown conclusively the radioactive nature of the light curve. The heavy elements ejected, 1.5 +/- 0.5 M_sun_, are all moving slowly, <~ 1500 km s^-1^, as a consequence of the deceleration of the core explosion by the hydrogen envelope. Thus the energy input by ^56^Ni and ^56^Co decay has dynamical consequences with important implications for Rayleigh-Taylor instability. This mixing plus that which occurs as the reverse shock propagates into the exploding debris aids in producing a good fit to the light curve and may be partly responsible for the earlier than expected appearance of X-rays. The velocity of the slowest moving hydrogen, <~2100 km s^-1^ by observations, restricts the column depth of the helium core to ~5 x 10^4^ g cm^-2^ at age 10^6^ s. This constrains the expected flux of γ-lines which should peak during the first half of 1988 with a typical flux of 5 (+/- a factor of 2) x 10^-4^ photons cm^-2^ s^-1^ (847 keV). Other details of velocity, composition, and light curve evolution are given.