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Description
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. Moreover, spots clearly affect the spectral line profiles. Such perturbations will in turn affect 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. The purpose of this paper is to obtain precise chromospheric activity measurements and projected rotational velocities for nearby (d<25pc) cool (spectral types F to K) stars, in order to estimate the activity-related jitter that should be expected for them. As a complementary objective, in this paper we aim to obtain relationships between fluxes in different activity indicator lines, which will permit a transformation of traditional activity indicators, i.e, CaII H & K lines, to other which hold noteworthy advantages. We used high resolution (~50000) echelle optical spectra. Standard data reduction was performed using the IRAF echelle package. To reveal the chromospheric emission of the stars in the sample we used the spectral subtraction technique. We measured equivalent widths of the chromospheric emission lines in the subtracted spectrum and transformed them into fluxes by applying empirical equivalent width-flux relationships. Rotational velocities were determined using the cross-correlation technique. To infer activity-related radial velocity (RV) jitter we used empirical relationships between this and the R'HK index. We have 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 have used the measured chromospheric activity to estimate the expected RV jitter for the active stars in the sample.
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