DYNAMIC PROCESSES IN BE-STAR ATMOSPHERES .5. HELIUM LINE EMISSIONS FROM THE OUTER ATMOSPHERE OF LAMBDA-ERIDANI

The He I lines of the mild B2e star lambda Eri often exhibit rapid, sm all-amplitude emissions that can occur at random places in their photo spheric lines, even when the star is in a ''nonemission state.'' New s imultaneous observations of the triplet lambda 5876 and singlet lambda 6678 lines show that the emission ratio for these lines is near unity , contrary to the predictions of either non-LTE model atmospheres or n ebular recombination theory. Several He I emission events point to the formation of short-lived structures near the star's surface. On 1995 September 12 the line lambda 6678 exhibited a strong (0.13I(cont)) emi ssion lasting some 20 minutes. The rapid decay of this feature implies a density of greater than or equal to 10(11.5) cm(-3) for an emitting plasma structure near the cm star. This value is consistent with dens ity estimates for slabs which may be responsible for ephemeral ''dimpl es'' in this star's He I lines on other occasions. We argue that photo spheric helium emissions during Ha-quiescent phases are caused by fore ground material and ask what mechanism might produce these features ag ainst the stellar background. To answer this question we have simulate d He I line emission from model slabs having various properties and su spended over the star. We find that illumination by a source of extrem e-ultraviolet (EUV) or X-ray flux depletes the He I column density so that it is difficult to form observable He I lines. A more interesting set of conditions occur for slabs with high densities (similar to 10( 12) cm(-3)) and moderately large optical thicknesses in optical He I l ines. Under these modified assumptions modest amounts of emission can be reproduced in singlet and triplet lines, and in the observed ratio. The key to producing this emission is for the slab to feel its own Ly man continuum radiation. This condition causes lambda 584 and other re sonance lines to partially depopulate the ground state and to overpopu late the first few excited levels, ensuring that the departure coeffic ients of relevant atomic levels approach common values. The second nec essary ingredient is a high density, which tends to equalize the depar ture coefficients of excited levels through recombinations and through redistribution of electrons among the l-sublevels. The combination is a kind of ''Lyman-pumped recombination'' because it relies on the Lym an continuum being marginally optically thick. Our results are consist ent with studies of He I emission from planetary nebulae, symbiotic va riables, and active galactic nuclei (AGNs), and may have a bearing on other ''detached atmospheres'' problems as well. This study appears to be the first application of such a recombination mechanism to a quasi -photospheric setting.