{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,3,31]],"date-time":"2026-03-31T18:05:56Z","timestamp":1774980356521,"version":"3.50.1"},"reference-count":42,"publisher":"EDP Sciences","license":[{"start":{"date-parts":[[2022,12,21]],"date-time":"2022-12-21T00:00:00Z","timestamp":1671580800000},"content-version":"vor","delay-in-days":20,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["A&amp;A"],"accepted":{"date-parts":[[2022,10,11]]},"published-print":{"date-parts":[[2022,12]]},"abstract":"<jats:p><jats:italic>Context.<\/jats:italic> The Ca\u202f<jats:sc>II<\/jats:sc> H&amp;K and H\u03b1 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 H\u03b1 line.<\/jats:p>\n<jats:p><jats:italic>Aims.<\/jats:italic> In this work we investigate the effect of the H\u03b1 bandpass width on the correlation between the Ca\u202f<jats:sc>II<\/jats:sc> and H\u03b1 indices with the aim of improving the H\u03b1 index to better identify and model the signals coming from activity variability.<\/jats:p>\n<jats:p><jats:italic>Methods.<\/jats:italic> 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\u202f<jats:sc>II<\/jats:sc> and H\u03b1 activity indices using a range of bandwidths for H\u03b1 between 0.1 and 2.0 \u00c5. We studied the correlation between the indices\u2019 time series at long and short timescales, and analysed the impact of stellar parameters, activity level, and variability on the correlations.<\/jats:p>\n<jats:p><jats:italic>Results.<\/jats:italic> The correlation between Ca\u202f<jats:sc>II<\/jats:sc> and H\u03b1, both at short and long timespans, is maximised when using narrow H\u03b1 bandwidths, with a maximum at 0.6 \u00c5. For some inactive stars, as the activity level increases, the flux in the H\u03b1 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 H\u03b1. These anti-correlations may become positive correlations if using the 0.6 \u00c5 bandwidth. We demonstrate that rotationally modulated signals observed in <jats:italic>S<\/jats:italic><jats:sub>Ca\u202f<jats:sc>II<\/jats:sc><\/jats:sub>, which appear flat or noisy when using 1.6 \u00c5 on <jats:italic>S<\/jats:italic><jats:sub>H<jats:italic>\u03b1<\/jats:italic><\/jats:sub>, can become more evident if a 0.6 \u00c5 bandpass is used instead. Low activity variability appears to be a contributing factor for the cases of weak or no correlations.<\/jats:p>\n<jats:p><jats:italic>Conclusions.<\/jats:italic> Calculating the H\u03b1 index using a bandpass of 0.6 \u00c5 maximises the correlation between Ca\u202f<jats:sc>II<\/jats:sc> and H\u03b1, both at short and long timescales. On the other hand, the use of the broader 1.6 \u00c5, 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 \u00c5 bandwidth when computing the H\u03b1 index for the identification of activity rotational modulation and magnetic cycle signals in solar-type stars.<\/jats:p>","DOI":"10.1051\/0004-6361\/202244595","type":"journal-article","created":{"date-parts":[[2022,11,3]],"date-time":"2022-11-03T19:53:48Z","timestamp":1667505228000},"page":"A174","source":"Crossref","is-referenced-by-count":20,"title":["Optimising the H<i>\u03b1<\/i> index for the identification of activity signals in FGK stars"],"prefix":"10.1051","volume":"668","author":[{"given":"J.","family":"Gomes da Silva","sequence":"first","affiliation":[]},{"given":"A.","family":"Bensabat","sequence":"additional","affiliation":[]},{"given":"T.","family":"Monteiro","sequence":"additional","affiliation":[]},{"given":"N. 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