Activity time series of old stars from late F to early K

Abstract

Context. The effect of stellar activity on radial velocity (RV) measurements appears to be a limiting factor in detecting Earth-mass planets in the habitable zone of a star that is similar to the Sun in spectral type and activity level. It is crucial to estimate whether this conclusion remain true for other stars with current correction methods. Aims. We built realistic time series in radial velocity and chromospheric emission for old main-sequence F6-K4 stars. We studied the effect of the stellar parameters we investigate on exoplanet detectability. The stellar parameters are spectral type, activity level, rotation period, cycle period and amplitude, latitude coverage, and spot constrast, which we chose to be in ranges that are compatible with our current knowledge of stellar activity. Methods. This very large set of synthetic time series allowed us to study the effect of the parameters on the RV jitter and how the different contributions to the RV are affected in this first analysis of the data set. The RV jitter was used to provide a first-order detection limit for each time series and different temporal samplings. Results. We find that the coverage in latitude of the activity pattern and the cycle amplitudes have a strong effect on the RV jitter, as has stellar inclination. RV jitter trends with B–V and Log R′HK are similar to observations, but activity cannot be responsible for RV jitter larger than 2–3 m s−1 for very quiet stars: this observed jitter is therefore likely to be due to other causes (instrumental noise or stellar or planetary companions, e.g.). Finally, we show that based on the RV jitter that is associated with each time series and using a simple criterion, a planet with one Earth mass and a period of one to two years probably cannot be detected with current analysis techniques, except for the lower mass stars in our sample, but very many observations would be required. The effect of inclination is critical. Conclusions. The results are very important in the context of future RV follow-ups of transit detections of such planets. We conclude that a significant improvement of analysis techniques and/or observing strategies must be made to reach such low detection limits.