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Astron. Astrophys. 334, 953-968 (1998)


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Full spectrum of turbulence convective mixing:
I. theoretical main sequences and turn-off for 0.6 [FORMULA] 15 [FORMULA]

Paolo Ventura 1, Anna Zeppieri 2, Italo Mazzitelli 1 and Francesca D'Antona 3

1 Istituto di Astrofisica Spaziale C.N.R., Via del Fosso del Cavaliere, I-00133 Roma, Italy
2 Collegio Leoniano, Anagni, Italy
3 Osservatorio Astronomico di Roma, I-00040 Monteporzio, Italy

Received 12 December 1997 / Accepted 6 March 1998

Abstract

We present the results of extensive evolutionary computations from the Zero Age Main Sequence to the Red Giants, for stars in the range [FORMULA] and with chemistry (Y, Z) = (0.274, 0.017). The novelty of these computations is in the general treatment of convection, namely:

1. convection as a whole is addressed in the Full Spectrum of Turbulence model (billions of eddy scales are considered) with the appropriate convective fluxes distribution, as opposed to the one-eddy Mixing Length Theory;

2. local chemical evolution also in the presence of convection is separately evaluated for each element, as a result of a process in which nuclear evolution and turbulent transport are fully coupled by means of a diffusive scheme (coupled-diffusion);

3. convective overshooting (when considered) is also addressed in the above coupled-diffusive scheme, assuming that the turbulent velocity exponentially vanishes outside the formally convective region according to an e-folding free parameter [FORMULA], tuned to fit observations.

After some tests on the small solar convective core in early main sequence, we discuss the effects of coupled-diffusion in the cores of larger mass stars, where the nuclear lifetimes of some (p-p and) CNO elements can be comparable to the mixing times. We also compute a full grid of tracks with a small amount of overshooting, finding that a unique free parameter can be suitable for the whole range of mass considered (solar and below solar included). Theoretical tracks are discussed, and isochrones are compared to the observational HR diagram for the Pleiades, finding an age [FORMULA] 120 Myr, consistent with that obtained from the candidate brown dwarf PPL 15. An age is also derived for the young cluster [FORMULA] Persei, for which a datation from the detection of lithium in brown dwarf candidates should be soon available. For completeness, and to facilitate comparisons with results by other authors, we also describe in details the ATON 2.0 code used for the present computations.

Key words: stars: evolution of – convection – numerical methods

Send offprint requests to: I. Mazzitelli

© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998

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