Presentation 1 : None
Chemical Composition of Early Be Type Stars
Y. Frémat, R. Levenhagen, J. Zorec, N. Leister, A.-M. Hubert, M. Floquet, C. Neiner
Late O, B and early A type stars, dwarfs to giants which have shown at
least once emission in the hydrogen line series are
called Be stars. The common characteristics of these objects is the
high rotation which on average is at 80% of the critical one.
As these stars are near or likely in the main sequence, they can be
considered being still in a stable evolutionary stage and their
surface chemical abundance should, in principle, mirror the chemical
composition of the protostellar environment. Theoretical
models of the evolution of rotating stars predict, however, some
contamination of the atmosphere by yields owing to the CNO
cycle in the stellar core, favored by a turbulent diffusion of
elements. The abundance analysis of CNO elements done so far on
``normal" OB stars show some N enrichment and no change for C.
These stars are however low rotators, where the meridional
circulation is rather long ranging from 1e6 to 1e8 yr. This kind
of time scale is however smaller by a factor 1e2 in Be
stars, so that the expected mixing should, if exists, take place.
We present in this contribution the results obtained on the
abundance determination of a small number of Be stars with
Vsin(i) < 150 km s of CNO elements and He. We use for
that hemisphere-recomposed non-LTE models of stellar atmospheres
for highly rotating stars.
Presentation 2 : Poster
No-ideal effects on hydrogen populations
R. Rohrmann, J. Zorec, L. Cidale, N. Morell, Y. Frémat
Be stars, dwarfs to giant B type stars with emission lines, present frequently a second component of the Balmer discontinuity (BD) which can either be in emission or in absorption. The same Be star, depending on the epoch of observation, can show one or other aspect. This second BD is formed, like the emission lines, in a circumstellar envelope (CE). As the continuum radiation rises in regions of the CE which are rather close to the central star (R<3R*), a careful analysis of the characteristics of the second BD may bring useful information on the structure of the CE near the star and give new hints on the mechanisms leading to its formation. The characteristics of the second BD are probably dependent on the distribution properties of hydrogen atom populations. The so called no-ideal population phenomena may then be determinant. In the present contribution we study the aspect and the characteristics of the second BD in Be stars in terms of no-ideal population effects and discuss the physical properties of the regions where the corresponding radiation field is formed.