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Astron. Astrophys. 348, 831-842 (1999)

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8. Conclusions

Central quasi-emission features seen in the absorption lines of some Be stars can be understood as being formed in the circumstellar disk. Purely photospheric models fail to reproduce the available observations. Both application of Hanuschik's (1995) parametrical model for shell line formation in a Keplerian disk and a mental principal components- and correlation-like analysis of the known properties of Be stars suggest that several circumstances strongly favour the exhibition of CQE's: The disk should be viewed edge-on, be optically relatively thin in the continuum, and have a small outer radius and little line broadening over and above the thermal width.

Under these conditions, CQE's are qualitatively explained as a line transfer effect in a rotating gaseous disk when also the finite size of the stellar disk is properly taken into account. CQE's only look like emission lines but in reality are due to the circumstance that in a rotating disk there is less line absorption and scattering at zero velocity than at slightly higher or lower velocities. Such a model and its application to CQE's has the potential for a quantitative tomography of the dynamics and structure of disks of Be stars. A promising target would be [FORMULA] Car, where in different shell lines CQE's were seen at different levels of prominence.

Another worthwhile extension of the present work would be towards stars with very narrow shell lines at very high spectral resolution in order to assess the degree of commonality of the CQE phenomenon to all Be shell stars. The narrowness of the lines certainly implies a difference in geometry or size of the disk itself or in its orientation with respect to the line of sight, or both. But there could also be dynamical differences. Since, in a Keplerian disk, very narrow shell lines require a large outer disk radius, Hanuschik's model does not generally predict CQE's for such stars, unless a line is formed only in the inner regions of the disk.

The apparent applicability of a Keplerian rotation-based model to CQE's also suggests two corollaries of more general relevance:

  1. Previous, often more indirect, inferences are re-inforced, that the disks of at least some Be stars are largely rotationally supported.

  2. Models for the formation of such disks, that do not include the necessary star-to-disk angular momentum transfer, are incomplete.

Furthermore, the large outward acceleration in wind-compressed disks is incompatible with the explanation presented above for CQE's and could pose an additional difficulty for the WCD model in its basic form.

Finally, the times when CQE's are present in a given star may be linked to phase transitions in the evolution of the circumstellar disk. CQE's seem to be more likely to occur during the early replenishment of the innermost regions of the disk.

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© European Southern Observatory (ESO) 1999

Online publication: August 13, 199