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 characterist ics largely determine the nature of a star's variability in this cluster. N one of the stars with H alpha in absorption were found to be variables. On the other hand, all 16 stars identified as classical T Tauri stars (CTTSs) by their H alpha emission equivalent widths and the majority of the 49 weak T Tauri stars (WTTSs) in the part of the cluster we monitored showed evide nce 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.. Seve nteen of these are WTTSs and the other two are of unknown spectral class. T he 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 know n to define these features significantly by the IC 348 data alone. The thre e fastest rotators are also the three most massive stars in the periodic sa mple. 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. T his supports the canonical view that WTTS variability is primarily caused b y the rotation of a surface with large cool spots whose pattern is often st able for many rotation periods, while CTTS variability has an additional co mponent caused by accretion hot spots which typically come and go on shorte r timescales. Stars with significant infrared excess emission in this sampl e do tend to be CTTSs, while the WTTSs (including periodic ones, with one p ossible exception) show no infrared excess and, therefore, no evidence of d isks. Among the CTTSs, neither H alpha emission equivalent width nor infrar ed excess emission shows any correlation with degree of variability.