Supernova 1987A - Six weeks later

One month after its explosion on February 23. 316 UT, SN 1987A continues to increase in luminosity after remaining on a slowly rising plateau of approximate constant brightness for 4 weeks. This is consistent with the behavior we predicted in Paper I. Although there are some (controversial) indications that Sk -69ᵈeg^ 202 did not explode, a number of properties of the supernova thus far strongly constrain the progenitor to be a blue supergiant in the mass range 12-~30 Mₛun_, with better agreement for stars in the 15-20 Mₛun_ range, i. e. , a star having properties much like those we derived for Sk -69ᵈeg^ 202 in Paper I. Two possible reasons for a blue presupernova star are discussed in greater detail: mass loss and low metallicity. Optical, hard X-ray and γ-ray, and γ-line light curves are given for several models consistent with observations to date. The best fit to the optical data is obtained using a 15-20 Mₛun_ model that lost about half of its mass prior to exploding. The radius of the star was 2-5 x 10¹2^ cm, and the explosion kinetic energy was 1-3 x 10⁵1^ ergs. The recent brightening indicates that the supernova has entered a stage where radioactivity is the dominant source of power for the optical display. This may bode well for γ-line astronomy.