X-ray activity and rotation of the young stars in IC 348

Context. The physical origin of the strong magnetic activity in T Tauri stars and its relation to stellar rotation is not yet well-understood.
Aims: We investigate the relation between the X-ray activity, rotation, and Rossby number for a sample of young stars in the ≈3 Myr old cluster IC 348.
Methods: We use the data of four Chandra observations of IC 348 to derive the X-ray luminosities of the young stars. Basic stellar parameters and rotation rates are collected from the literature. This results in a sample of 82 X-ray detected stars with known rotation periods. We determine the Rossby numbers (i.e. the ratio of rotation period to convective turnover time) of 76 of these stars from stellar structure- and evolution-models for pre-main sequence stars.
Results: The young stars in IC 348 show no correlation between X-ray activity and rotation period. For 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 have slightly lower X-ray activity levels. There are no significant differences in the X-ray activity level for stars of different spectral types and no relation between spectral type and Rossby number is seen. In addition, for stars belonging to different IR-classes, no significant differences are present for the X-ray activity level as well as for their Rossby numbers. We compare the dispersion in the 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 [σ(log (L_X/L_bol)) = 0.43] is considerably smaller than for the ≈ 1 Myr old Orion nebula cluster [σ(log (L_X/L_bol)) = 0.63], but, at the same time, considerably larger than the dispersion seen in the ≈ 30 Myr old cluster NGC 2547 [σ(log (L_X/L_bol)) = 0.24] and in main-sequence stars.
Conclusions: 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 in the X-ray activity levels for the rapidly 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 pre-main sequence stars and the consequent changes in the dynamo mechanisms that are responsible for the magnetic field generation.