Dynamic Processes in Be Star Atmospheres. IV. Common Attributes of Line Profile ``Dimples''
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Abstract
"Dimples" are transient central absorption features flanked by weak emissions commonly seen in the He I λ6678 line profile of the mild B2e star λ Eridani. Smith & Polidan have found that these features can be reproduced with a model in which line photons are scattered within an optically thick (in the line) slab elevated over the surface of a rapidly rotating star. We have undertaken a series of simultaneous 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 ilines when they appear in λ6678. Four dimples were found during 15 hr of multiobservatory monitoring. In three cases, a dimple was observed in a weak blue line of the same absorption series as λ6678. In the fourth instance, a dimple was observed only in λ5876 and λ5015 lines that, like λ6678, are strong and have weak wings. A joint lUE/optical campaign demonstrated that the He II λ1640 line shows decreases in absorption and possible weak emissions just as new dimples appear in the λ6678 line. Our observations confirm a previous report that dimples appear in the λ6678 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 for λ Eri. Our data also disclosed that "migrating subfeatures" similar to those found in γ Cas are present in the λ6678 line of the B5 star HR 1011. These features appear to be a more vigorous form of dimple activity than observed in λ Eri and other mild Be stars. These findings lend support to the slab model as an explanation for the dimple phenomenon. They also suggest that this activity is endemic to the class of mild Be stars. The appearance of dimples in the weak blue He I lines suggests slab masses of at least 7 x 10-13 Msun for most dimples. The greatest enigma that characterizes classical Be stars is their highly variable and episodic mass-loss histories. Our estimates of dimple-slab masses are high enough that this problem may be removed if the magnetic paradigm for Be activity is correct. In this picture, 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 prominence-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 massloss rates during active Be episodes. Then the essential difference between Be stars in active and inactive phases would be understood not as a difference in their mass release rates but rather in the prevailing geometries of their surface fields.
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