DYNAMIC PROCESSES IN BE STAR ATMOSPHERES .1. DIMPLE FORMATION IN THE HE-I LAMBDA-6678 LINE OF LAMBDA-ERIDANI

'' Dimples '' are ephemeral absorption features flanked by incipient b lue/red emission in the He I lambda6678 line profiles of lambda Eri an d several similar mild classical Be stars. They develop rapidly (appro ximately 15 minutes), last 2-4 hr, and are observed in this line on th e average at least once per night, making them the most ubiquitous of the ''spectral transients '' appearing in the line profiles of at leas t some mild Be stars. Their formation may well be related to the unkno wn instability-causing mass-loss episodes in Be stars. In this paper w e discuss simultaneous IUE and optical (lambda6678) time-serial observ ations of lambda Eri, together with near-simultaneous UV continuum obs ervations made by Voyager 1, in 1990 October. We find several examples of weakenings of the C IV, N V resonance lines that coincide with the appearance of lambda6678 dimples. The absence of variations in other UV lines and in the UV continuum at the same time (IUE) or nearly the same time (Voyager) argues against dimples being caused by thermal var iations from the underlying star. We suggest instead that the resonanc e line weakenings are caused by non-LTE effects associated with the co ndensation of high density structures at some elevation over the star. We present a simple model of an opaque (in the line only), essentiall y stationary slab which backscatters lambda6678 line radiation into a surrounding ''penumbral'' region. Lambda6678 photons are scattered a s econd time in this region back into the observer's line of sight and i n the process acquire the local projected doppler shift from rotation. We examine several attributes of this mechanism and find that slabs t ypically have a projected radius of about 1 R., an elevation of approx imately 0.10R, and a density of approximately 2 x 10(11) cm-3. Our mo dels can also match dimples resembling '' P Cygni '' features by permi tting our model slabs to rise or fall and at the same time to move ret rograde or prograde with respect to the stellar surface. Slabs would p robably produce too little emission to be easily detected in the Halph a profile. Their detection in strong He I lines seems the best stategy among early Be stars. Slabs contain enough mass (> 10(-14) M.) and de velop over such a large area and so rapidly as to greatly limit the nu mber of mechanisms that could form them. Their rapid development spati ally over the star precludes a purely thermal origin, e.g., from the p ropagation of a shock at the speed of sound. Yet, in terms of bulk mot ion dimples represent an exclusively low vertical-velocity phenomenon. Arguments can be advanced that dimples are not the results of ballist ic tossings of blobs or of orbiting circumstellar ejecta. One viable m echanism for their formation requires that dimple-slabs condense from ambient rarefied material via a supersonically propagating instability . If this velocity is understood in terms of an Alfven wave, a case ca n be made that magnetic fields of greater-than-or-equal-to 100 G can e xist in small regions above lambda Eri and many other mild classical B e stars.