ULTRAVIOLET OBSERVATIONS OF THE ERUPTION IN THE SMALL MAGELLANIC CLOUD WOLF-RAYET SYSTEM HD 5980

摘要

We present results of IUE observations (AA1200-3200) of the erupting system HD 5980 obtained in 1994 November and December. The 1994 spectrum presents emission lines from ions such as N Ⅲ, Si Ⅱ-Ⅲ and Al Ⅲ which were absent in 1991. There is a systematic tendency towards higher degrees of ionization over the 44 days covered by the present observations. At the start of the observations on HJD 2449674 the UV continuum flux at 1850 A is a factor of ～2 larger than in 1991, declining by 30% in 18 days, followed by a tendency to once again increase. FES magnitudes indicate a rapid decline in visual brightness after maximum in the eruption. Wind speeds as high as ～1700 km s~(-1) are measured although the P Cygni absorption components are highly peculiar, containing several emission-like "bumps," the most prominent of which lies at 920 km s~(-1). We conclude that the eruption occurred in the primary star of the 19.3 day orbital pair, the one classified as WN4 by Breysacher et al. and which now is displaying an LBV-type event. The phenomena in HD 5980 provide evidence supporting evolutionary scenarios in which pulsationally unstable, H-rich WN stars precede the LBV phase.