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D. B. Paulson (Astronomy Department, University of Texas ), W. D. Cochran (McDonald Observatory), A. P. Hatzes (Thüringer Landessternwarte Tautenburg)
We present results of a study of the chromospheric activity of stars in our program of searching for planetary mass companions to main-sequence stars in the Hyades star cluster. The Hyades constitute a homogeneous sample of stars which vary primarily in stellar mass. Thus, they form an ideal sample of stars for investigating the dependence of planet formation on the mass of the central star. However, the Hyades members are significantly younger than stars traditionally surveyed for planets using high precision radial velocity techniques. They have higher levels of stellar activity. Therefore, we have conducted this detailed investigation of the chromospheric activity of our Hyades target stars to determine whether the stellar activity will interfere with our ability to detect the radial velocity signature of substellar companions.
We used the Ca II K line as a diagnostic for chromospheric activity. We transform our measured activity index to the Mt. Wilson S index and correct for photospheric flux contributions by computing the R'HK chromospheric activity index. We examine the effect of chromospheric activity on the radial velocity measurements. Using the Bayesian "odds ratio", we find that there is only a significant correlation between activity and measured velocity for 4 stars in the sample of 82 in this study.
This research was supported by NASA grant NAG5-9227 and by NSF grant AST-9808980.