Astron. Astrophys. 333, 629-643 (1998)

Modelling the angular momentum evolution of low-mass stars with core-envelope decoupling

S. Allain

Laboratoire d'Astrophysique, Observatoire de Grenoble, URA CNRS 708, B.P. 53, F-38041 Grenoble Cedex 9, France (Stephanie.Allain@obs.ujf-grenoble.fr)

Received 5 June 1997 / Accepted 22 January 1998

Abstract

We present a model of angular momentum evolution for stars in the mass range 0.5 - 1.1 , during their early stages of evolution. The model is based upon the following hypothesis: a constant surface rotational period during star-disk interaction, angular momentum loss through magnetic wind, and differential rotation parameterized with a constant coupling time. We investigate the effect of the different parameters, the initial velocity at the T Tauri age, the disk lifetime, the magnetic braking law, and we discuss the effect of introducing a core-envelope decoupling. The angular momentum transfer is parameterized by the use of a coupling time scale , which controls the exchanges of angular momentum between the - fast-rotating - radiative core and the convective envelope, both supposed to rotate as solid bodies. We present evolutionary tracks of a single star through the pre-main sequence and the main-sequence, for different masses and different coupling time-scales. We conclude that rapid rotators require solid-body rotation, and ZAMS slow rotator require a strong differential decoupling with a characteristic coupling time about 100 Myr.

Key words: stars: rotation stars: late-type stars: pre-main sequence stars: interiors

Send offprint requests to: S. Allain

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Contents

• 1. Introduction
• 2. Observational constraints
• 3. Description of the model
  • 3.1. Evolutionary models
  • 3.2. Hypotheses
  • 3.3. Equations of evolution
• 4. Effects of varying the parameters
  • 4.1. Disk lifetime
  • 4.2. Initial velocity
  • 4.3. The braking law
• 5. Core-envelope decoupling
  • 5.1. Choice of the coupling time
  • 5.2. Short coupling time
  • 5.3. Intermediate
  • 5.4. Long
• 6. Discussion and conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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