CLUMPS IN SUPERNOVA 1987A - THE H-ALPHA LINE

We investigate the fine structure of the Ha emission line from SN 1987A in the period days 115-673. We find at least five bumps in the radial velocity range - 2000 to + 1300 km s-1, with typical widths of 400-500 km s-1 and amplitudes 7-10 per cent above the local emission level. These bumps start showing up on day 115 +/- 5 and remain visible until day 523, at least one bump still being visible on day 673. Between days 115 and 523, the radial velocity of the bumps is approximately constant, with their amplitude gradually increasing. Between days 523 and 673, most bumps almost disappear, while one bump increases in relative strength. At that epoch, dust forms in the ejecta and changes the overall line profile considerably. Assuming that the bumps are due to local density enhancements (clumps) and that they suffer from dust extinction to a degree depending on their location in the ejecta, we show that the observed intensity contrast changes can be explained by selective dust extinction alone, if dust is distributed homogeneously in an inner shell of radius r(d) = 0.6 R(max) (where R(max) is the outer radius of the hydrogen shell) with dust optical depth tau = 3.5, and if the clumps are concentrated in the inner part of the hydrogen shell at average radius 0.5 R(max).