DYNAMIC PROCESSES IN BE STAR ATMOSPHERES .4. COMMON ATTRIBUTES OF LINE-PROFILE DIMPLES

''Dimples'' are transient central absorption features flanked by weak emissions commonly seen in the He I lambda 6678 line profile of the mi ld B2e star lambda Eridani. Smith & Polidan have found that these feat ures can be reproduced with a model in which line photons are scattere d within an optically thick (in the line) slab elevated over the surfa ce of a rapidly rotating star. We have undertaken a series of simultan eous He I multiline observations of this star at the McMath, McDonald, Lick, David Dunlap, and Ritter Observatories to search for dimples in weak blue He I lines when they appear in lambda 6678. Four dimples we re found during 15 hr of multiobservatory monitoring. In three cases, a dimple was observed in a weak blue line of the same absorption serie s as lambda 6678. In the fourth instance, a dimple was observed only i n lambda 5876 and lambda 5015 lines that, like lambda 6678, are strong and have weak wings. A joint IUE/optical campaign demonstrated that t he He lambda 1640 line shows decreases in absorption and possible weak emissions just as new dimples appear in the lambda 6678 line. Our obs ervations confirm a previous report that dimples appear in the lambda 6678 line of four other Be stars. We also find that the resonance C iv double weakens when dimples appear, a result similar to that found fo r lambda Eri. Our data also disclosed that ''migrating subfeatures'' s imilar to those found in gamma Cas are present in the lambda 6678 line of the B5 star HR 1011. These features appear to be a more vigorous f orm of dimple activity than observed in a Eri and other mild Be stars. These findings lend support to the slab model as an explanation for t he dimple phenomenon. They also suggest that this activity is endemic to the class of mild Be stars. The appearance of dimples in the weak b lue He I lines suggests slab masses of at least 7 x 10(-13) M(.) for m ost dimples. The greatest enigma that characterizes classical Be stars is their highly variable and episodic mass-loss histories. Our estima tes of dimple-slab masses are high enough that this problem may be rem oved if the magnetic paradigm for Be activity is correct. In this pict ure, exospheric flares trigger explosive ablations of plasma from the upper photosphere. The evaporated mass is trapped by overlying closed magnetic field loops, where it cools, taking on characteristics of pro minence-like structures. If the loops were opened for any reason, this mass would be free to escape from the star at a rate consistent with mass-loss rates during active Be episodes. Then the essential differen ce between Be stars in active and inactive phases would be understood not as a difference in their mass release rates but rather in the prev ailing geometries of their surface fields.