BARIUM AND OTHER S-PROCESS ELEMENTS IN THE EARLY-TIME SPECTRUM OF SN-1987A

Modelling the optical spectrum of SN 1987A as it appeared 2-3 weeks after outburst allows a determination of the abundance and distribution of s-process elements (Ba, Sr, Sc) in the ejecta. A Monte Carlo spectrum synthesis code is employed for this purpose. In particular, the problem of a possible overabundance of barium is studied by means of an NLTE code which calculates the occupation numbers for the Ba II levels and the relative abundances of Ba II and Ba III in the supernova envelope using rates derived explicitly from the Monte Carlo calculations, and uses them to obtain the optical depth of the Ba II lines used in the spectrum synthesis. New photoionisation cross-sections have also been computed for the Ba II levels and used in the NLTE code. This procedure, which effectively includes the influence of line-blocking on the excitation and ionisation balance of barium, leads to a large increase in the predicted Ba II/Ba III ratio, which is a consequence of the blocking of far-UV photons by lines of Ca II, Si II, Fe I and Fe II. This leads to an increase of the Ba II line strengths. Although an overabundance of barium is still necessary to fit the observed lines, the overabundance factor is now smaller than the values obtained in several previous investigations, and is in agreement with evolutionary calculations. The best fit of the observed s-process elements' lines is obtained for Ba and Sr abundances increased by factors of 5 and 2, respectively, over the adopted (1/2.75 solar) LMC metal abundance, while no Sc overabundance is needed. These overabundance factors give the best fits to both the March 15, 1987, spectrum, where the Ba II 6141 angstrom line had reached maximum intensity, and the March 8, 1987, spectrum, where this, and other s-process elements' lines were still rather weak. Additionally, it is found, from observational and computational evidence, that a confinement of the s-process elements is also required in order to match the observed wavelength of these lines, which are consistently less blueshifted than the others (e.g. H-alpha). For both considered dates, the s-process elements are found to be confined to velocities corresponding to an enclosed mass of 89% of the progenitor's mass, consistent with nucleosynthesis calculations.