Astron. Astrophys. 363, 605-616 (2000)

Lithium and mass loss in massive AGB stars in the Large Magellanic Cloud

P. Ventura ¹, F. D'Antona ¹ and I. Mazzitelli <sup>2

1</sup> Osservatorio Astronomico di Roma, 00040 Monte Porzio Catone (Rome), Italy (paolo@coma.mporzio.astro.it)
² Istituto di Astrofisica Spaziale C.N.R., Via Fosso del Cavaliere, 00133 Rome, Italy (aton@hyperion.ias.rm.cnr.it)

Received 21 April 2000 / Accepted 5 September 2000

Abstract

The aim of this work is to use full evolutionary models to derive observational constraints on the mass loss rate of the upper Asymptotic Giant Branch (AGB) stars. The observations used to constrain the models are: i) the relative number of luminous Lithium rich AGBs in the Magellanic Clouds, with respect to the total number of AGBs populating the luminosity range ; ii) the s-process enhancement of the same sample. The calibration of the mass loss rate we obtain gives feedbacks on the interpretation of observational data of obscured AGBs, and allows us to provide consistent lithium yields for these stars, to be used to constrain the galactic chemical evolution.

We find that: a) we can put lower and upper limits to the mass loss rate during the AGB phase; b) after a "visible" phase, the models evolve into a phase of strong mass loss, which can be identified with the obscured OH/IR stars accessible only in the infrared; the models well reproduce the Period- loci of the obscured AGBs (Wood et al. 1992); c) the most massive AGBs (mass of progenitors, hereinafter , ) are extremely luminous ( to -7.5); d) The lithium yield increases with the mass loss rate and with the total stellar mass, being maximum for AGB stars close to the lower limit for carbon semi-degenerate ignition. However, the mass loss calibration obtained in this work implies that massive AGBs do not contribute significantly to the lithium enrichment of the interstellar medium.

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

Send offprint requests to: P. Ventura

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Contents

• 1. Introduction
• 2. Macro physics input
  • 2.1. Diffusive scheme and overshooting
  • 2.2. Symmetric overshooting
  • 2.3. Convection
  • 2.4. Pulsation periods
  • 2.5. Mass loss
  • 2.6. Model observability
• 3. Model results
  • 3.1. Mass and luminosity at which HBB is achieved
  • 3.2. Lithium yields
  • 3.3. Mass loss and the duration of the optically bright phase
  • 3.4. Mass loss and Lithium evolution
• 4. Numeric simulations of the optically bright AGB phase
• 5. The obscured phase
  • 5.1. Pulsation period evolution
  • 5.2. Other mass loss formulations
  • 5.3. Comparison with other mass loss informations
• 6. Overshooting
  • 6.1. Symmetric overshooting
  • 6.2. The influence of
• 7. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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