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

The re-emission in the cores of the Ca II H & K and H alpha 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 I-H alpha, two indices based on the Ca II H & K and H alpha lines, respectively, for a sample of 271 FGK stars using measurements obtained over a similar to 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) and log I-H alpha. 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 I-H alpha. 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 H alpha index.