A ∼ 40 YEAR VARIABILITY CYCLE IN THE LUMINOUS BLUE VARIABLE/WOLF-RAYET BINARY SYSTEM HD 5980?

Abstract

The massive Wolf-Rayet stellar system HD 5980 in the Small MagellanicCloud entered a sudden and brief ~ 1-3 mag eruptive state in themid-1990s. The cause of the instability is not yet understood, butmechanisms similar to those in luminous blue variables are suspected.Using a previously unreported set of spectroscopic data obtained in1955-1967 and recently acquired optical and HST/STIS spectra, we findthat (1) the brief eruptions of 1993 and 1994 occurred at the beginningof an extended (~ decades) high state of activity characterized by largeemission-line intensities; (2) the level of activity is currentlysubsiding; and (3) another strong emission-line episode appears to haveoccurred between 1960 and 1965, suggesting the possibility that thelong-term cyclical variability may be recurrent on a ~ 40 yeartimescale. These characteristics suggest the possible classification ofHD 5980 as an S Doradus-type variable. The effects due to binaryinteractions in the system are discussed, and we tentatively suggestthat the short duration and relatively hot spectral type (WN11/B1.5I)observed during maximum in the visual light curve may be attributed tothese interactions.