Ionization freeze-out and barium problem in supernova 1987A
  We have shown that in the atmosphere of SN 1987A level populations of hydrogen are mostly controlled by an ionization freeze-out and ion-molecular processes up to 40 days. The ionization freeze-out effects are important for normal SNe II-P as well. The time dependent effects and the mutual neutralization between H^- and H^+ may result in a non-monotonic radial dependence of the Sobolev optical depth for H-alpha and then in a blue emission satellite of H-alpha observed in SN 1987A spectra at Bochum event phase. The relative abundance of molecular hydrogen about 0.0001-0.001 is high enough for the photospheric phase of SN 1987A. We emphasize that the poor knowledge of the far UV radiation field in the envelope of SN 1987A still forbids a truly reliable estimate of the Ba abundance but well-constructed and tested models may improve this situation. 
V. P. Utrobin and N. N. Chugai