2004AJ....127.1602C

The extremely young cluster IC 348 has been monitored photometrically over five observing seasons from 1998 December to 2003 March in Cousins I with a 0.6 m telescope at Van Vleck Observatory. Twenty-eight periodic variables and 16 irregular variables have been identified. The variability study is most sensitive for stars with I<14.3 mag; at that brightness level, we find that 24 of the 27 known pre-main-sequence (PMS) cluster members in the monitored field are variables, illustrating the value of photometric monitoring for identifying PMS cluster members. Among this brighter sample, 14 of the 16 known K or M-type weak-line T Tauri stars (WTTS) were found to be periodic variables, while all five of the known classical T Tauri stars (CTTS) were found to be irregular variables. In the full sample, which includes 150 stars with I as faint as 18, we find that 40% of the 63 WTTS are detected as variables, nearly all of them periodic, while 55% of the 20 CTTS are also detected as variable, with none of them periodic. Our study suggests that 80%-90% of all WTTS in young clusters will be detected as periodic variables given sufficiently precise and extended monitoring, whereas CTTS will reveal themselves primarily or solely as irregular variables. This has clear consequences for PMS rotational studies based on photometric periods, suggesting that any such sample may be biased against stars that are currently actively accreting (i.e., CTTS). We examine the stability of the periodic light curves from season to season. All periodic stars show modulations of their amplitude, mean brightness, and light-curve shape on timescales of less than 1 yr, presumably due to changes in spot configurations and/or physical characteristics. In no case, however, can we find definitive evidence of a change in period, indicating that differential rotation is probably much less in WTTS than it is in the Sun. While some stars show a hint of what could be cyclic behavior analogous to the sunspot cycle, no clear cycles could be found. It appears that most of the variation in light-curve shape is caused by redistribution of spots on the surface rather than by an increase or decrease in the areal spot coverage. While most of the variables are of K or M spectral class, we do confirm the existence of three, low-amplitude, periodic G stars. The rotation periods of these more massive stars are short compared to the bulk of the sample; it appears that mid-K to early M (i.e., ∼0.5 M_☉) represents a minimum in mean rotation rate for extremely young stars. Among the nonperiodic stars, we report the detection of two possible UX Orionis stars as well as a pre-main-sequence star, HMW 15, which apparently undergoes an eclipse with a duration exceeding 3 yr.