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Astron. Astrophys. 340, 463-475 (1998)

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Envelope burning over-luminosity: a challenge to synthetic TP-AGB models

P. Marigo 1, 2

1 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, D-87540 Garching bei München, Germany
2 Department of Astronomy, University of Padova, Vicolo dell'Osservatorio 5, I-35122 Padova, Italy

Received 7 May 1998 / Accepted 10 September 1998


Until recently synthetic AGB models had not taken into account the break-down of the core mass-luminosity ([FORMULA]) relation due to the occurrence of envelope burning in the most massive ([FORMULA] for Pop. II and [FORMULA] for Pop. I) and luminous ([FORMULA]) stars.

Marigo et al. (1998) made the first attempt to consistently include the related over-luminosity effect (i.e. above the [FORMULA] relation) in synthetic TP-AGB calculations. The method couples complete envelope integrations with analytical prescriptions, these latter being presently updated with the highly detailed relations by Wagenhuber & Groenewegen (1998).

In this paper the reliability of the solution scheme is tested by comparison with the results of complete evolutionary calculations for a [FORMULA] AGB star undergoing envelope burning (Blöcker & Schönberner 1991; Blöcker 1995).

Indeed, the method proves to be valid as it is able to reproduce with remarkable accuracy several evolutionary features of the [FORMULA] star (e.g. rate of brightening, luminosity evolution as a function of the core mass and envelope mass for different mass-loss prescriptions) as predicted by full AGB models.

Basing on the new solution method, we present extensive synthetic TP-AGB calculations for stars with initial masses of [FORMULA] and [FORMULA], and three choices of the initial metallicity, i.e. [FORMULA], [FORMULA], and [FORMULA]. Three values of the mixing-length parameter are used, i.e. [FORMULA]. We investigate the dependence of envelope burning on such stellar parameters (M, Z, and [FORMULA]). The comparison between different cases gives hints on the interplay between envelope burning over-luminosity and mass loss, and related effects on TP-AGB lifetimes.

Key words: stars: mass-loss – stars: AGB and post-AGB – stars: evolution

Send offprint requests to: Paola Marigo (paola@mpa-garching.mpg.de)

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998