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Session 40 - The Environment of Stars: From Protostars to the Main Sequence.
Display session, Tuesday, June 11
Great Hall,

[40.05] Long term Variations of Stars in the Orion Nebula Cluster: Evolution of Spots and a Search for FUors

A. J. B. Steinhauer, W. Herbst (Wesleyan U.), A. Henden (USRA/USNO)

Since 1991, portions of the Orion Nebula Cluster, including the central Trapezium cluster, have been monitored in the I band of the Cousins system with the 0.6 m Perkin telescope at Van Vleck Observatory. In this paper we present results from the 1994/95 observing season as well as BVRI photometry obtained with the 1.0 m telescope at the USNO in 1996. Rotation periods were determined for 76 stars during that observing season, of which 54 represent new determinations and the others confirmations. This brings the total number of stars with known rotation periods in the ONC from our work to about 140. Of particular interest here are the long term variations shown by both periodic and large amplitude irregular variables. Seven stars which were found to be periodic at every epoch are isolated and their light curves compared. While the periods remain constant from season to season to within the accuracy of determination (about 1%), the amplitudes, shapes and mean light levels vary. This supports the ``spot" or ``zone" interpretation of the variations and provides constraints on the nature of the spots, including whether they are hotter or cooler than the photospheres. Preliminary analysis suggests that hot spots may dominate in the long period stars (P > 4 days) even among the low amplitude variables, while cool spots may dominate in the shorter period stars. We have also looked at the longer term irregular variables searching for possible FUors. Some interesting light curves are found and displayed, but none that look like FU Ori.

Program listing for Tuesday