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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 SCa II, which appear flat or noisy when using 1.6 Å on SHα, 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. [ABSTRACT FROM AUTHOR]

The re-emission in the cores of the Ca II H & K and Ha 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?IHa, two indices based on the Ca II H & K and Ha 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?IHa. 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 Ha index. [ABSTRACT FROM AUTHOR]

Context. The search for planets orbiting metal-poor stars is of utmost importance for our understanding of planet formation models. However, no dedicated searches have been conducted so far for very low mass planets orbiting such objects. Only a few cases of low-mass planets orbiting metal-poor stars are thus known. Amongst these, HD?41248 is a metal-poor, solar-type star on the orbit of which a resonant pair of super-Earth-like planets has been announced. This detection was based on 62 radial velocity measurements obtained with the HARPS spectrograph (public data). Aims. We present a new planet search program that is using the HARPS spectrograph to search for Neptunes and super-Earths that orbit a sample of metal-poor FGK dwarfs. We then present a detailed analysis of 162 additional radial velocity measurements of HD?41248, obtained within this program, with the goal of confirming the existence of the proposed planetary system. Methods. We analysed the precise radial velocities, obtained with the HARPS spectrograph, together with several stellar activity diagnostics and line profile indicators. Results. A careful analysis shows no evidence for the planetary system. One of the signals, with a period of ~25?days, is shown to be related to the rotational period of the star, and is clearly seen in some of the activity proxies. We were unable to convincingly retrieve the remaining signal (P ~ 18?days) in the new dataset. Conclusions. We discuss possible causes for the complex (evolving) signals observed in the data of HD?41248, proposing that they might be explained by the appearance and disappearance of active regions on the surface of a star with strong differential rotation, or by a combination of the sparse data sampling and active region evolution. [ABSTRACT FROM AUTHOR]