A Variability Study of Pre-Main-Sequence Stars in the Extremely Young Cluster IC 348

The extremely young cluster IC 348 has been monitored in the Cousins I band with a 0.6 m telescope at Wesleyan's Van Vleck Observatory. Photometry of 150 stars was obtained on 76 images taken on 27 separate nights during the period 1998 December through 1999 March. As expected, spectral characteristics largely determine the nature of a star's variability in this cluster. None of the stars with Hα in absorption were found to be variables. On the other hand, all 16 stars identified as classical T Tauri stars (CTTSs) by their Hα emission equivalent widths and the majority of the 49 weak T Tauri stars (WTTSs) in the part of the cluster we monitored showed evidence of variability. Nineteen stars were found to be periodic, with periods ranging from 2.24 to 16.2 days and masses ranging from 0.35 to 1.1 M⊙. Seventeen of these are WTTSs and the other two are of unknown spectral class. The period distribution is remarkably similar to what is found in the Orion Nebula cluster for stars in the same mass range. Namely, it is bimodal with peaks at 2–3 days and 7–8 days, although there are not enough periods known to define these features significantly by the IC 348 data alone. The three fastest rotators are also the three most massive stars in the periodic sample. It is striking that none of the known CTTSs were found to be periodic even though they are more highly variable than the WTTSs in the cluster. This supports the canonical view that WTTS variability is primarily caused by the rotation of a surface with large cool spots whose pattern is often stable for many rotation periods, while CTTS variability has an additional component caused by accretion hot spots which typically come and go on shorter timescales. Stars with significant infrared excess emission in this sample do tend to be CTTSs, while the WTTSs (including periodic ones, with one possible exception) show no infrared excess and, therefore, no evidence of disks. Among the CTTSs, neither Hα emission equivalent width nor infrared excess emission shows any correlation with degree of variability.