We present a model for the rotational evolution of a young , solar mass star interacting with an accretion disk .
The model incorporates a description of the angular momentum transfer between the star and disk due to a magnetic connection , and includes changes in the star ’ s mass and radius and a decreasing accretion rate .
The model also includes , for the first time in a spin evolution model , the opening of the stellar magnetic field lines , as expected to arise from twisting via star-disk differential rotation .
In order to isolate the effect that this has on the star-disk interaction torques , we neglect the influence of torques that may arise from open field regions connected to the star or disk .
For a range of magnetic field strengths , accretion rates , and initial spin rates , we compute the stellar spin rates of pre-main-sequence stars as they evolve on the Hayashi track to an age of 3 Myr .
How much the field opening affects the spin depends on the strength of the coupling of the magnetic field to the disk .
For the relatively strong coupling ( i.e. , high magnetic Reynolds number ) expected in real systems , all models predict spin periods of less than \sim 3 days , in the age range of 1–3 Myr .
Furthermore , these systems typically do not reach an equilibrium spin rate within 3 Myr , so that the spin at any given time depends upon the choice of initial spin rate .
This corroborates earlier suggestions that , in order to explain the full range of observed rotation periods of approximately 1 – 10 days , additional processes , such as the angular momentum loss from powerful stellar winds , are necessary .