X-ray activity and rotation of the young stars in IC 348
  The physical origin of the strong magnetic activity in T Tauri stars and its relation to stellar rotation is not yet well understood. We investigate the relation between the X-ray activity, rotation, and Rossby number for a sample of 82 young stars in the ~3 Myr old cluster IC 348. We use the data of four Chandra observations of IC 348 to derive the X-ray luminosities of the young stars. The young stars in IC 348 show no correlation between X-ray activity and rotation period. Considering the Rossby numbers, nearly all IC 348 stars are in the saturated regime of the activity-rotation relation defined by main-sequence stars. Searching for possible super-saturation effects, we find a marginal (but statistically in-significant) trend that the stars with the smallest Rossby numbers show slightly lower X-ray activity levels. We compare the dispersion of fractional X-ray luminosities of the stars in the saturated rotation regime in IC 348 to that seen in younger and older stellar populations. The scatter seen in the ~3 Myr old IC 348 is considerably smaller than for the ~1 Myr old ONC, but, at the same time, considerably larger than the dispersion seen in the ~30 Myr old cluster NGC 2547 and in main-sequence stars. The results of our X-ray analysis of IC 348 show that neither the rotation rates nor the presence/absence of circumstellar disks are of fundamental importance for determining the level of X-ray activity in TTS. Our results suggest that the scatter of X-ray activity levels shown by the fast-rotating members of young clusters decreases with the age of the stellar population. We interpret this as a signature of the changing interior structure of PMS stars and the consequent changes in the dynamo mechanisms that are responsible for the magnetic field generation. 
Frauke Alexander and Thomas Preibisch
astro-ph.SR