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