## The angular momentum evolution of low-mass stars
^{1} Laboratoire d'Astrophysique, Observatoire de Grenoble,
Université Joseph Fourier, B.P. 53, F-38041 Grenoble Cedex 9,
France (jbouvier@laog.obs.ujf-grenoble.fr)^{2} Canada-France-Hawaii Telescope Corporation, P.O. Box
1597, Kamuela, HI 96743, USA
We present a model for the evolution of surface rotation of stars in the mass range from 0.5 to 1.1M , from their first appearance in the HR diagram as T Tauri stars up to the age of the Sun. The model is based on 3 assumptions: i) nearly solid-body rotation, ii) pre-main sequence disk locking, iii) wind braking. The initial conditions and the calibration of the braking law are completely determined from observations. The model includes only 2 adjustable parameters: the distribution of disk lifetimes in the pre-main sequence and the velocity at which saturation of the angular momentum losses due to the stellar wind occurs. We review all the observational results, including the most recent
ones, that can be used to constrain the models, as well as theoretical
work that puts limits onto the parameter space. We show that the
currently available distributions of In agreement with other models, we find that a mass-dependent saturation velocity for the angular momentum losses is required to account for the longer spin down timescale of lower mass stars on the zero-age main sequence. We argue that this assumption provides an alternative to the hypothesis of radiative core-convective envelope decoupling, which has been used in other models. Both the rapid spin down of fast rotators on the ZAMS and its mass-dependency are accounted for in the present solid-body rotation models. In particular, we show that the model predicts a distribution of rotational periods at the age of the Hyades for 0.5-1.1M stars that is in close agreement with the observations. We conclude that the observed evolution of
## Contents- 1. Introduction
- 2. Model assumptions
- 3. Model parameters
- 3.1. Physics of the stellar evolution models
- 3.2. Initial conditions
- 3.3. Angular momentum loss
- 3.4. Disk lifetimes
- 4. Effect of the parameters
- 5. Observational constraints
- 6. Model vs observations
- 6.1. 1 M model
- 6.2. 0.8 M model
- 6.3. 0.5 M model
- 6.4. Rotational period distribution at the age of the Hyades
- 6.5. Summary
- 7. Discussion
- 8. Conclusion
- Appendix
- References
© European Southern Observatory (ESO) 1997 Online publication: April 8, 1998 |