The angular momentum evolution of low-mass stars
J. Bouvier 1, 2,
M. Forestini 1 and
S. Allain 1
Received 18 February 1997 / Accepted 23 April 1997
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 v sini for PMS, ZAMS and MS stars can be reasonably well reproduced by the model assuming solid-body rotation for stars in the mass-range from 0.5 to 1.1M . We deduce a median lifetime of 3 Myr for circumstellar accretion disks around pre-main sequence stars. By an age of 20 Myr, only 10% of the stars are still surrounded by disks. Stars with long disk lifetimes and low initial rotational velocities account for the large fraction of slow rotators (v sini 20 km s-1) observed in young clusters. On the opposite, stars with short-lived disk reach the ZAMS with velocities up to 200 km s-1 over the whole mass range investigated here.
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 moderate and fast rotators on the early main sequence requires physical processes that lead to the redistribution of angular momentum in stellar interiors on a timescale much shorter than evolutionary timescales, such as dynamical rotational instabilities, gravity waves or MHD torques. The evolution of slow rotators (v sini 10 km s-1), however, remains uncertain due to the currently uncomplete v sini distribution. Precise v sini measurements for slow rotators in young clusters, rather than upper limits, are required.
Key words: stars: rotation - stars: pre-main sequence
Send offprint requests to: J. Bouvier
© European Southern Observatory (ESO) 1997
Online publication: April 8, 1998