IS SN 1987A ENTERING A NEW PHASE OF EVOLUTION

Optical spectra of the ejecta of SN 1987A taken at the AAT now cover s even years of evolution. In recent years, SN 1987A has been in a phase known as freeze-out. The timescales for recombination have exceeded t hose of energy deposition, and the ionisation structure has become fix ed. During this phase, cooling is slow and the optical spectrum has be en extremely stable. Our latest spectrum, however, shows significant c hange. [FeI] and [FeII] emission from iron-rich clumps has dominated t he optical emission from the supernova over the last four years. All t he [FeII] features have disappeared in our latest spectrum from Decemb er 1993 and model fits of [FeI] features indicate that these clumps ha ve cooled to the critical temperature of 1000 K. They may be entering a phase of rapid cooling known as the infrared catastrophe. In additio n, emission at high velocities has strengthened, in line with the pred ictions of freeze-out. SN 1987A may be entering a new, and previously unobserved, phase in supernova evolution.