Watching the Stars go `Round and `Round

With the PMS nature of these X--ray sources well characterized, I examine the periodic nature of only the stars which were associated with X--ray sources. I use simulated data to demonstrate that for stars with perfectly sinusoidal behavior, accurate periods can be found in a cases of signal to noise greater than 2. that the

For more realistic stars, which do not vary in brightness (along our line--of--sight) exactly as a sine wave, the signal to noise must be higher by a factor of two or more. I also discuss the expected color changes due to spot modulation and find that the allowed variations cover a very limited range in phase space. I find rotation periods for five stars at confidence of greater than 99.9/ greater than 99/ at much lower confidence.

Finally, the relation between the data presented here and other data is discussed. The sampling rate of the data presented here allows for detection of shorter periods than previously reported for T Tauri stars. I note the bimodal distribution of rotation periods which other authors have reported. I further note that the ratio of slow rotators to fast rotators seems to change as a function of the age of the star forming region. Rotation period and various observables are compared. I find weak correlations between slow rotation and high variability and between slow rotation and IR color excess. Both these correlations support the hypothesis that stars with disks are slow rotators. However, there is also a large fraction of slow rotators which do not show evidence for disks.