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Session 22 - YSOs' Variability.
Display session, Monday, January 15
North Banquet Hall, Convention Center

[22.05] Rotation Periods and Variability of Orion Nebula Cluster Stars

K. L. Rhode, W. Herbst (Wesleyan U.)

For the past several years an observing program has been underway at Van Vleck Observatory to monitor variability of stars in the Orion Nebula Cluster (ONC). A large fraction of the stars in this region are pre-main sequence stars and are variable at detectable levels. Photometric data are obtained with the 0.6-meter Perkin reflector using a CCD detector and a Cousins I-band filter. Typically, six to ten ONC fields are imaged on 25 to 30 clear nights over the course of the observing season.

Results from the fourth season (1993/94) of monitoring are presented here. Preliminary analysis of the data for 241 stars has already revealed 20 with periodic variations characteristic of T Tauri stars. Half of these are newly-determined periods, while the other half confirm periods found in previous years. The total number of known rotation periods for ONC members based on Van Vleck data now stands at 85. The bimodal frequency distribution of rotation periods seen in previous years is further supported by these data; likewise, excellent agreement with the periods found in previous seasons is obtained (Choi amp; Herbst 1995 AJ, in press). In all cases, periods found for the same star in different seasons agree to better than 1%.

Many of the stars studied do not exhibit periodic variations but do display interesting forms of irregular variability. One class of such objects is seen to undergo eclipse-like events similar to those seen for UXors (Eaton amp; Herbst 1995 AJ, in press). Another class displays large-amplitude (>0.5 mag) variations on long timescales (months or years) with relatively little night-to-night scatter. Light curves for representative irregular variables are displayed. Ongoing efforts to quantify and understand the observed behaviors --- which may reflect accretion processes occurring near the star --- are discussed.

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