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Session 78 - Stars - Young and Old, Large and Small.
Display session, Wednesday, January 15
Metropolitan Ballroom,
Time-serial observations of HeI lines in \lambda Eri exhibit brief emissions even when this B2e star is relatively quiescent. At these times the triplet/singlet \lambda5876/\lambda6678 emission ratio is close to one. The rapidity and low velocity of these emissions argues that they arise against the background of the stellar disk. Yet triplet and singlet emission in equal proportions runs contrary to expectation from either contemporary non-LTE atmosphere or nebular recombination theory.
To address the cause of the equal triplet/singlet emissions, we have simulated HeI emission from various kinds of model slabs well above the star's surface. We find that illumination by a source of EUV/X-ray flux depletes HeI column density such that very little HeI is formed in the slab. A more interesting outcome results for slabs with high densities (\sim10^12 cm^-3) and moderately high optical thickness. Under these assumptions modest amounts of emission can be reproduced in roughly equal amounts in both singlet and triplet lines. The key to producing this emission is for slab atoms to feel the slab's own Lyman continuum radiation. This condition causes \lambda584 and other resonance lines to overpopulate the first few excited levels, a condition augmented by recombination preferentially to the upper states. This combination serves as a ``Lyman-pumped recombination" and may well explain the emissions in many Be stars during a variety of phases. Our results are consistent with HeI studies from planetary nebulae, symbiotic variables, and AGN, and may have a bearing on other "detached atmosphere" problems as well.
