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Session 119 - Active Stars.
Oral session, Thursday, January 16
Stars of a wide range of metallicity have been observed and analyzed in the spectral region of the BeII resonant doublet (\approx3130Åwith the aim of understanding the primary mechanism(s) of Be production during the galactic history and of investigating the structure of the outermost atmospheric layers in order to constrain which mixing and depletion mechanisms are more effective. We tested the different theoretical scenarios proposed so far to explain the formation of beryllium, namely if the classical cosmic-ray theory of spallation of interstellar CNO atoms by colliding protons and \alpha-particles, proposed 25 years ago by Reeves et al.(1970), is still a valid explanation. A reduced \chi^2 analysis has shown a correlation of [Be/H] with [Fe/H] that suggests a behaviour more akin to that of a primary element rather than a secondary one. This seems to favor the most recent view of creation of Be (and B) through the reversed mechanism, involving spallation between H and He interstellar atoms hit by accelerated CNO nuclei in the vicinity of supernovae (Duncan et al. 1992; Cassé et al. 1995). We find a real dispersion among different objects at almost the same metallicity, that might suggest the onset of some different mechanism involved in the galactic production of beryllium, additional destruction mechanisms, or, in the most recent scenario of Be formation, it might also reflect the presence of inhomogeneities in the interstellar medium surrounding active star forming regions. There is no plateau at the lowest metallicities observed in this sample which might indicate a primordial contribution to Be production. There is no suggestion of a break at the metallicity of [M/H]=-1.00 that usually marks the transition between halo and disk stars and no other change of slope seems necessary to fit satisfactorily our observed abundances, although a larger homogeneous sample of observations could test this better.
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Program listing for Thursday