SIMBAD references

Measurements of rotational periods of pre-main-sequence stars in several young (∼1 Myr) open clusters reveal a uniform trend. Stars with masses below 0.25 M_☉ show a bimodal period distribution with fast and slow rotators clustered around 2 and 8 day periods, respectively, while the period distribution of low-mass stars lacks the slow-rotating component. In one popular interpretation of this observational result, the slow rotators are identified as ``disk-locked'' stars whose periods are fixed to the orbital periods at the inner edge of the accretion disk; the fast rotators are assumed to be stars that have lost their connection to the disk. We argue that this scenario can account for observations if the mass accretion rate in the disk declines with time. We construct a simple model for the period evolution in T Tauri stars (TTSs) that includes realistic prescriptions for the mass accretion rate, radius evolution, and transition between strong and weak accretion disk coupling. Using this model to simulate the cluster period distribution, we can qualitatively reproduce the observed results, but only if the accretion is allowed to continue after the disk and star are no longer locked. When the contribution from accretion is ignored, the unlocked stars tend to rotate slower than the locked population. Our model predicts a very strong mass segregation between the two peaks in the bimodal distribution, with lower mass stars and higher mass stars being the fast and slow rotators, respectively. This effect is not readily apparent in the data, although some of the discrepancy might be due to the uncertainties associated with measurements of stellar masses. Improved observational constraints on TTS masses could help support or rule out our prediction that the rotational periods of TTSs with masses above 0.25 M_☉ are dependent on stellar mass.