THE LUMINOUS ECLIPSING SMC-OB-5980 BEFORE AND DURING THE RECENT LBV-LIKE OUTBURST - AN EXTREME CASE OF COLLIDING WINDS(WN BINARY HD)

The 1994 LBV-(luminous blue variable)like outburst of one of the lumin ous, hot components of the binary HD 5980 made it the brightest star i n the Small Magellanic Cloud for an interval of 5 months. The most int riguing question to arise from this event is the following: Why did th e HD 5980 spectrum change from an H-poor WN3 with veiled OB absorption lines about 20 yr ago to an H-rich WN11 without central absorption li nes during the outburst? In an attempt to answer this apparent enigma, we present and analyze new phase-dependent spectroscopic, polarimetri c, and light-curve observations. Together with other published data, t hese new observations allow us to improve the orbital parameters consi derably, except for the radial velocity amplitudes and hence the masse s, which are only roughly constrained. Especially important in HD 5980 is the strong collision of the two nearly equal pre-outburst winds. T he emission-line spectrum generated by the collision tends to mask the underlying line spectra of both components when the system is relativ ely quiescent. We argue that the pre-erupting system consists of a ver y luminous but moderately massive H-rich O type supergiant, possibly w ith emission lines, and a low-mass, H-poor, relatively faint WN compan ion, whose lines are mostly drowned out by wind collision emission, th e spectrum of which largely imitates that of a WNE star. It was the O supergiant that erupted in a normal way as an H-rich, visually bright WN11 star. In this way, the need for peculiar evolutionary scenarios ( e.g., rapid evolution from a faint, low-mass, H-poor WNE star to a lum inous, H-rich WNL star) is avoided.