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R. Wasatonic (Villanova University), M. T. Mirtorabi (Inst. for Adv. Studies for Basic Sci., Zanjan, IRAN), E. F. Guinan (Villanova University), S. Messina (Catania Astrophy. Obs., Italy)
As a pilot program, Wing near-IR, TiO and V-band photometry is being conducted of several chromospherically active stars. The program stars are \lambda And (G8 III-IV, V ~ +3.8; Prot ~ 54d)), IM Peg (K1 IIIe, V ~ +5.8; Prot ~ 24.7d), II Peg (K0 IV; V ~ +7.0; Prot = 6.72d), and V711 Tau (G5V + K0 IV, V ~ +5.8; Prot = 2.84d). The light variations are relatively large (\Delta V ~ 0.10 to 0.25 mag) and are believed to arise from the uneven and changing distribution of large, starspots regions on differentially rotating surfaces. The photometry is being conducted with 28-cm and 36-cm telescopes located in Allentown, PA and Zanjan, Iran, respectively.
The Wing three color system consists of narrow and intermediate band interference filters centered on the TiO bandhead at 719 nm, and on two relatively line-free spectral regions at 754 nm and 1024 nm. A TiO-index is formed from the photometry; this index measures the relative strength of TiO feature. The TiO absorption strength is very sensitive to temperature for cool stars and is expected to be strong in the cooler star spot regions with T < 4200 K. TiO photometry has an advantage over conventional photometry in that it provides unambiguous measures of the fractional cool starspot coverage. As the stars rotate, the variation in the TiO index yields information about the longitudinal distribution of the starspots. Importantly, combining the TiO curves with the V-band and near-IR light curves allows the discrimination of white-light plage and cool starspot contributions. Initial results indicate that the observed V-band light variations found for \lambda And and IM Peg primarily arise from bright-spot (plage) features rather than dark starspots as is usually assumed. In the shorter period, classical RS CVn stars V711 Tau and II Peg, the continuum light curves appear to be dominated by dark spots. Models using both bright and dark spot features have been developed and being used to analyzed the light and TiO-index curves. The models account for the cool/hot spot characteristics such as fill-factor, longitude, latitude, and temperatures. We will discuss the results of the modeling for these stars and the implications for stellar dynamo theories.
This research is supported by NSF/RUI grant AST 00-71260 and an AAS Small Research Grant which we gratefully acknowledge.