We have estimated in this paper the influence of gravitational and thermal settling, on the global parameters of low main-sequence stars. As we have introduced only microscopic flows in the computations except in convective zones, the stratification is probably overestimated.
We have shown that this physical process, if at work permanently, produces an effective temperature shift towards the red. For population II stars it exceeds generally 100 K.
Compared to the most reliable data available at present, the predicted shift is of the right order of magnitude, but probably slighly unsufficient to eliminate completely the discrepancy shown by Lebreton et al. (1997). However, as already mentioned, other effects could also be responsible for redshifts of that kind. So that more precise characteristic behaviors are needed to decipher among the different possibilities. Let us cite a few signatures of the microscopic diffusion.
Though very important improvement on the quality of the observational data on the nearest members of the old population have been obtained recently, they remain too coarse and too few to allow unambiguous tests. More objects with accurate fundamental parameters are needed to validate definitely this hypothesis.
A lot of progress is expected from the GAIA mission (Perryman et al. 1997), which will provide even more accurate distances and sometimes masses for a larger number of stars. But improvements in the determination of atmospheric parameters, as well as in methods of comparing observed and predicted quantities are also badly needed.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999