Spectroscopic fine-structure in Supernova 1987A.

We investigate the evolution of strong spectral lines in SN 1987A from fully resolved, high-S/N, flux calibrated spectra obtained at the 0.61 m Bochum telescope. The Balmer lines Hα-Hδ exhibit fine- structure in their absorption troughs, appearing around day 20 and fading after day ~ 30. This fine-structure appears as a well-developed peak in Hα (the Bochum event), as a rather broad flux excess in Hβ and as weak flux gradient changes in Hγ and Hδ. The fine-structure is responsible for the transient filling-up, i.e. disappearance of Hβ and Hγ between days 30 and 70. Related fine-structure is also observed in Na I-D, with a redside bump similar to the equivalent Hα feature. Flux gradient changes are observed also in Ca II λ 8600. As a consequence, the Bochum event is not an isolated Hα phenomenon, but rather a general spectroscopic fine-structure event in SN 1987A. Typical radial velocities of the fine-structure are slightly larger than the photospheric velocity. As the sudden appearance of fine-structure is accompanied by a sudden reincrease of the net integral flux of Hα, being indicative for additional heating of the ejecta by radioactive decay of mixed-up ^56^Co in the layers penetrating the photosphere, we conclude that the fine-structure event is due to a local change in the excitation and ionization conditions, probably caused by ^56^Co-rich layers just above the photosphere. Similar observations in SNe 1985L and 1985P imply that mixing-up is of general importance in type-II supernovae.