1995A&A...303..118M

The problem of the dramatic difference in the intensity of the Ba II lines between SN 1987A (type II-pec) and normal SNe II-P in the photospheric epoch is studied by means of synthetic spectra computations. The evolution of the Ba II 6141A line in SN 1987A, including both the period between days 20 and 80, when the line was very strong, and epochs before and after that, when the line was weaker, is well reproduced for the Ba overabundance factor f=X/X_i=5 (metal abundance X_i=(1/2.75)xsolar for the LMC), in agreement with previous calculations for the early phase. It is found that the Ba overabundance is necessary for all layers in the ejecta with velocity between 1500 and 9000km/s, which include almost all the ejected mass. On the other hand, the Ba II lines in normal SNe II-P at the same epochs are well reproduced assuming a Ba overabundance factor 1<f<2 (for a solar metal abundance), which is consistent with nucleosynthesis predictions for the production of s-process elements. Since this implies that the absolute Ba abundance (Ba/H) is very similar in SN 1987A and in SNe II-P, then it must be concluded that the much greater strength of the Ba II 6141A line in SN 1987A, in particular between days 20 and 80, is to be attributed to the different atmospheric parameters in the two types of SNe,which favour the formation of strong Ba II lines in SN 1987A and not in SNe II-P. It is shown that the primary cause for the strength of the Ba II lines in SN 1987A is the low luminosity of this SN, i.e. the compactness of the pre-SN star, which reduces the ionization degree of Barium. However, the problem remains why Ba is overabundant (f≃5) in SN 1987A but not in SNe II-P.