2022A&A...668A.174G

Context. The Ca II H&K and Hα lines are two of the most used activity diagnostics for detecting stellar activity signals in the optical regime, and for inferring possible false positives in exoplanet detection with the radial velocity method. The flux in the two lines is known to follow the solar activity cycle, and to correlate well with sunspot number and other activity diagnostics. However, for other stars, the flux in these lines is known to have a wide range of correlations, increasing the difficulty in the interpretation of the signals observed with the Hα line. Aims. In this work we investigate the effect of the Hα bandpass width on the correlation between the Ca II and Hα indices with the aim of improving the Hα index to better identify and model the signals coming from activity variability. Methods. We used a sample of 152 FGK dwarfs observed with HARPS for more than 13 yr with enough cadence to be able to detect rotational modulations and cycles in activity proxies. We calculated the Ca II and Hα activity indices using a range of bandwidths for Hα between 0.1 and 2.0 Å. We studied the correlation between the indices' time series at long and short timescales, and analysed the impact of stellar parameters, activity level, and variability on the correlations. Results. The correlation between Ca II and Hα, both at short and long timespans, is maximised when using narrow Hα bandwidths, with a maximum at 0.6 Å. For some inactive stars, as the activity level increases, the flux in the Hα line core increases, while the flux in the line wings decreases as the line becomes shallower and broader. The balance between these fluxes can cause stars to show the negative correlations observed in the literature when using a wide bandwidth on Hα. These anti-correlations may become positive correlations if using the 0.6 Å bandwidth. We demonstrate that rotationally modulated signals observed in S_Ca II_, which appear flat or noisy when using 1.6 Å on S_Hα, can become more evident if a 0.6 Å bandpass is used instead. Low activity variability appears to be a contributing factor for the cases of weak or no correlations. Conclusions. Calculating the Hα index using a bandpass of 0.6 Å maximises the correlation between Ca II and Hα, both at short and long timescales. On the other hand, the use of the broader 1.6 Å, generally used in exoplanet detection to identify stellar activity signals, degrades the signal by including the flux in the line wings. In view of these results, we strongly recommend the use of a 0.6 Å bandwidth when computing the Hα index for the identification of activity rotational modulation and magnetic cycle signals in solar-type stars.