Cool Stars 14 - Submitted Abstract # 145
This version created on 05 October 2006

Long-term photometric monitoring of WTTS stars in the Orion Nebula
Cluster (ONC).


Sergio Messina, INAF-Catania Astrophysical Observatory, Catania, Italy
Singh Padmakar Parihar, Indian Institute of Astrophysics, Bangalore, India
Elisa Distefano, Catania University, Catania, Italy
Biman Jyoti Medhi, Dept. of Physics, Gauhati University, Assam, India

We present preliminary results of a three-year (2004-2006) multiband
photometric monitoring of a selected sky region within ONC carried out
with the 2-m Himalayan Chandra Telescope (HCT) of Indian Institute of
Astrophysics (IIA).  We collected data in the V and I filters for a
total of about 60  nights.  We identified about 350  stars most of
which (about 320) turned out to be variable.  Using different periodogram
techniques we detected over 100 periodic variables with a variability
amplitude in the range Imag=0.01--1 and with rotational periods in the
range 0.5-20  days.  The variability is likely due to the presence of
cool spots unevenly distributed along the stellar longitude whose
visibility is modulated by the star's rotation.  The amplitude of the
rotational modulation as well the rotational period of many stars show
significant variations from year to year giving some indication of the
presence of a long-term (cyclical) starspot activity and of surface
differential rotation.  Our long-term monitoring of a large sample of
periodic variables, most of which are WTTS cluster members, will allow
us i) to investigate the evolution of magnetic activity over different
timescales; ii) to determine an improved rotational period
distribution in ONC; iii) to better investigate the angular momentum 
evolution.

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