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Context: Chromospheric activity produces both photometric and spectroscopicvariations that can be mistaken as planets. Large spots crossing the stellardisc can produce planet-like periodic variations in the light curve of a star.These spots clearly affect the spectral line profiles and their perturbationsalter the line centroids creating a radial velocity jitter that mightcontaminate" the variations induced by a planet. Precise chromospheric activitymeasurements are needed to estimate the activity-induced noise that should beexpected for a given star. Aims: We obtain precise chromospheric activitymeasurements and projected rotational velocities for nearby (d < 25 pc) cool(spectral types F to K) stars, to estimate their expected activity-relatedjitter. As a complementary objective, we attempt to obtain relationshipsbetween fluxes in different activity indicator lines, that permit atransformation of traditional activity indicators, i.e, CaII H & K lines, toothers that hold noteworthy advantages. Methods: We used high resolution(~50000) echelle optical spectra. To determine the chromospheric emission ofthe stars in the sample, we used the spectral subtraction technique. Rotationalvelocities were determined using the cross-correlation technique. To inferactivity-related radial velocity (RV) jitter, we used empirical relationshipsbetween this jitter and the R'_HK index. Results: We measured chromosphericactivity, as given by different indicators throughout the optical spectra, andprojected rotational velocities for 371 nearby cool stars. We have builtempirical relationships among the most important chromospheric emission lines.Finally, we used the measured chromospheric activity to estimate the expectedRV jitter for the active stars in the sample.

Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter