Activity and telluric contamination in HARPS observations of Alpha Centauri B

The Alpha Centauri system is the primary target for planet search as it is the closest star system composed of a solar twin α CenA, a K-dwarf α Cen B, and an M-dwarf Proxima Centauri, which has a confirmed planet in the temperate zone. α CenA and B weremonitored intensively with the HARPS spectrograph for over 10 yr, providing high-precision radial velocity (RV) measurements. In this work, we study the available data to better understand the stellar activity and other contaminating signals.We highlight the importance of telluric contamination and its impact on the RV measurements. Our suggested procedures lead to discarding about 5 per cent of HARPS data, providing a data set with an rms improved by a factor of 2. We compile and quantify the behaviour of 345 spectral lines with a wide range of line shapes and sensitivity to activity.