The Development and Behavior of an Active Region on/near the Photosphere of the B2[CLC]e[/CLC] Star μ Centauri

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Abstract
An unusual mass-loss event observed in the B2e star μ Centauri during the course of 5 days in 1994 April is described and discussed within the framework of contemporary ideas on the Be phenomenon. The onset of the activity occurred in less than 1 day and was characterized by variable emission in He I λ6678 that displayed a distinctive character. Unlike the transient microemission in He I that frequently occurs in μ Cen and other Be stars, the emission-line variations seen in this event took place more slowly in three discrete velocity intervals. On two occasions, violet (v)- and red (r)-shifted emission components declined on a timescale of 2 hr, while the emission at/near the line center increased. The short timescale and observed velocity behavior suggest that the site of the activity was near the photosphere. The possible importance of nonradial pulsations and magnetic fields in precipitating the event is discussed. A scenario is suggested to explain the observations in which material originating from an active site on the photosphere is injected into a slab. Layers in the active region become visible in He I λ6678 as the prevailing density builds to values favorable for the production of this emission line. It is estimated that the slab covered ~30% of the star. A 22% increase in the Hα emission strength by the final day of the observations indicates that the activity did indeed add material to the circumstellar disk.