Dynamic processes in Be star atmospheres. II. He I 2P-nD line formation in λ Eridani (outburst)

The He I λ6678 line of early Be stars generally shows violet (V) and red (R) emission whenever Hα emission is present, but its use as a diagnostic has been handicapped by a poor understanding of the processes that drive it into emission. In an attempt to address this problem we obtained three series of echelle spectra of the first two members of the singlet and triplet 2P-nD series of λ Eri (B2e) during 1992 November 3-5 at Kitt Peak. During these observations λ6678 showed substantial emission variability in both the wings and central profile, providing an opportunity to compare its behavior with that of the λ4922, λ5876, and λ4471 lines. We found that the responses of the lines were different in several respects. Whereas the emissions in the V wings of all four lines scaled together, the R wing of the λ4922 line invariably responded with increased absorption whenever the R wing of λ6678 line showed increased emission. These same trends occurred within the central photospheric profiles. The R-wing behavior shows that much, but not all, of the emission in λ6678 is caused by matter projected against the stellar disk. The excitation temperatures of the neighboring 2¹P transitions, λ6678 and λ4922, must be greater than and less than the photospheric continuum temperature, respectively. We have investigated departures from LTE for the He I spectrum in a variety of ad hoc, perturbed model atmospheres. We have found only one way to cause the source function of λ6678 to increase so strongly, namely, by increasing the atmospheric temperature in the line formation region to 30,000-40,000 K. When the kinetic temperature is this hot, the resonance λ584 transition becomes transparent, causing the 2¹P level to become underpopulated relative to other levels. Stimulated emission amplifies this condition, driving λ6678 selectively into emission. This effect was discovered by Auer and Mihalas for O3-O4 atmosphere models, but it has not been applied to active B stars. Our models suggest that λ6678 emission in Be stars can be used as a sensitive monitor of localized hot spots on these stars' surfaces. In particular they show that λ6678 emission originates in the plasma having a high (photosphere-like) density. Combining this finding with our observation that foreground material exhibits and predominant redshift implies at physical mechanism that incorporates both heat and violent downward motions penetrating well into the photosphere. The energies involved in heating the active portions of the atmosphere are too high to be produced by gravitational infall. This leaves magnetically induced flares among the few known processes on the surfaces of stars capable of sustaining this energy level.