The extent of mixing in stellar interiors - Evolutionary models and tests based on the HR diagrams of 34 open clusters

Details The extent of mixing in stellar interiors - Evolutionary models and tests based on the HR diagrams of 34 open clusters The extent of mixing in stellar interiors - Evolutionary models and tests based on the HR diagrams of 34 open clusters

Jan 1981

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1981a%26a....93..136m&link_type=abstract

Astronomy and Astrophysics, vol. 93, no. 1-2, Jan. 1981, p. 136-149.

Astronomy and Astrophysics

Astrophysics

222

Hertzsprung-Russell Diagram, Open Clusters, Stellar Evolution, Stellar Interiors, Stellar Models, Stellar Structure, Chemical Composition, Convective Flow, Turbulent Diffusion, Turbulent Mixing

Scientific paper

New evolutionary models are calculated completing the grids of models with non-local treatment of convection in stellar interiors in the range of 1-9 Msun; 3 sets of models with different extension of the internal mixing are considered. The corresponding isochrones are established with the appropriate bolometric corrections and Teff scales for the Mυ vs. (B-V) and Mυ vs. (U-B) diagrams. Attention is paid to the expected number of stars on the upper part of the sequences and to the spurious effects of unsolved binaries and rapidly rotating stars.
Photoelectric data for 34 clusters in the age range of Pleiades to Hyades, with well reduced measurements and analysed membership, colour excess, and distance moduli, are used for comparison with the theoretical time-lines. The data grouped in composite sequences of clusters of very similar ages allow to avoid the usual difficulties due to the sparseness of the upper sequences in the relatively young clusters.
The detailed comparisons of sequences and time-lines clearly demonstrate that, as a general rule, the core-hydrogen burning phase extends much farther (0. 0.8 mag) from the reference sequence than is indicated by standard evolutionary models. No change of initial helium or metal contents Y and Z is able to bring agreement. These results suggest that the zone of mixing in stellar interiors is more extended than the one usually considered: expressed in mass fraction of the classical core defined by Schwarzschild's criterion, the additional zone of extended mixing may reach about 20-40%. Very tentatively we suggest that this fraction seems to be increasing with mass in the studied range of 2 Msun. In addition to the possibility of convective overshoot, various other physical mechanisms such as the slow mixing resulting from turbulent diffusion (cf. Schatzman et al., 1980) and the circulation due to meridional currents could contribute to the observed differences.

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