HYDROGEN MOLECULES IN SN 1987A

The observations of CO and SiO in the infrared spectrum of SN 1987A clearly indicate that molecules can form in the debris of a supernova explosion. Since H-2 is not easily observable we compute its abundance theoretically. For conditions typical of the inner (v<2500 km s(-1)) envelope of SN 1987A, the fraction of H that is in molecular form rises to similar to 1% by t similar to 800 days. For t<500 days the formation is dominated by the gas-phase reactions H + H+ --> H-2(+) + hv; H-2(+) + H --> H-2 + H+. Thereafter, the formation is dominated by the reactions H + e --> H- + hv; H- + H --> H-2 + e. At early times the H- may absorb similar to 10%-30% of visible photons, contributing to the apparent paucity of H alpha emission. For t>1000 days the abundance of H-2 ''freezes out'' due to the slowing of all reactions. The opacity of the supernova envelope in the range 912 < lambda < 1400 Angstrom (the upper limit depending on temperature) is dominated by resonance scattering in the Lyman and Werner bands of H-2. The resulting fluorescence emission bands of H-2 in the range 1150 < A < 1650 Angstrom may be observable in the UV spectra of supernovae at late times.