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Centro de Astrofísica da Universidade do Porto

On the long-term correlation between the flux in the Ca ii H & K and Hα lines for FGK stars

J. Gomes da Silva, N. C. Santos, I. Boisse, X. Dumusque, C. Lovis

Abstract
The re-emission in the cores of the Ca ii H & K and Hα lines are well known proxies of stellar activity. However, these activity indices probe different activity phenomena: the first is more sensitive to plage variation, while the other is more sensitive to filaments. In this paper, we study the long-term correlation between log R'HK and log IHα, two indices based on the Ca ii H & K and Hα lines, respectively, for a sample of 271 FGK stars using measurements obtained over a ~9 year time span. Because stellar activity is one of the main obstacles to the detection of low-mass and long-period planets, understanding this activity index correlation further can give us some hints about the optimal target to focus on ways to correct for these activity effects. We found a great variety of long-term correlations between log R'HK. Around 20% of our sample has a strong positive correlation between the indices while about 3% show strong negative correlation. These fractions are compatible with those found for the case of early-M dwarfs. Stars exhibiting a positive correlation have a tendency to be more active when compared to the median of the sample, while stars showing a negative correlation are more present among higher metallicity stars. There is also a tendency for the positively correlated stars to be more present among the coolest stars, a result which is probably due to the activity level effect on the correlation. Activity level and metallicity therefore seem to be playing a role on the correlation between log R'HK and log IHα. Possible explanations based on the influence of filaments for the diversity in the correlations between these indices are discussed in this paper. As a parallel result, we show a way to estimate the effective temperature of FGK dwarfs that exhibit a low activity level by using the Hα index.

Keywords
stars: activity - stars: chromospheres - stars: solar-type - planetary systems

Astronomy and Astrophysics
Volume 566, Page A66_1
June 2014

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