Evidence for Companion-induced Secular Changes in the Turbulent Disk of a Be Star in the Large Magellanic Cloud MACHO Database

The light curve of a blue variable in the MACHO LMC database (FTS ID78.5979.72) appeared nearly unvarying for ~4 years (quasi-flat segment) butthen rapidly changed to become periodic with noisy minima for the remaining 4years (periodic segment); there are no antecedent indications of a gradualapproach to this change. Lomb Periodogram analyses indicate the presence of twodistinct periods of ~61 days and 8 days in both the quasi-flat and the periodicsegments. Minima of the periodic segment cover at least 50% of the orbitalperiod and contain spikes of light with the 8-day period; maxima do not showthis short period. The system typically shows maxima to be redder than minima.The most recent OGLE-III light curve shows only a 30-day periodicity. Thevariable's V and R magnitudes and color are those of a Be star, and recent setsof near infrared spectra four days apart, secured during the time of theOGLE-III data, show H-alpha emission near and at a maximum, confirming its Bestar characteristics. The model that best fits the photometric behaviorconsists of a thin ring-like circumstellar disk of low mass with four obscuringsectors orbiting the central B star in unison at the 61-day period. The centralstar peers through the three equi- spaced separations between the four sectorsproducing the 8-day period. The remainder of the disk contains hydrogen inemission making maxima appear redder. A companion star of lower mass in aninclined and highly eccentric orbit produces an impulsive perturbation near itsperiastron to change the disk's orientation, changing eclipses from partial tocomplete within ~ 10 days.Comment: 42 pages, 14 figures, and 2 tables Submitted to AJ v3: Title changed, figures added, model modifie