Accounting for stellar activity signals in radial-velocity data by using Change Point Detection techniques

Active regions on the photosphere of a star have been the major obstacle for detecting Earth-like exoplanets with the radial velocity (RV) method. A commonly employed solution to addressing stellar activity is to assume a linear relationship between the RV observations and the activity indicators along the entire time series, and then remove the estimated contribution of activity from the variation in RV data (overall correction method). However, since active regions evolve on the photosphere over time, correlations between the RV observations and the activity indicators will correspondingly be anisotropic. We present an approach which recognizes the RV locations where the correlations between the RV and the activity indicators significantly change, to better account for variations in RV caused by stellar activity. The proposed approach uses a general family of statistical breakpoint methods, often referred to as Change-Point Detection (CPD) algorithms. A thorough comparison is made between the breakpoint-based approach and the overall correction method. To ensure wide representativity we use measurements from real stars having different levels of stellar activity and whose spectra have different signal-to-noise ratios. When the corrections for stellar activity are applied separately on each temporal segment identified by the breakpoint method, the corresponding residuals in the RV time series are typically much smaller if compared to those obtained with the overall correction method. Consequently the Generalized Lomb-Scargle periodogram contains a smaller number of peaks caused by active regions. The CPD algorithm is particularly effective when focusing on active stars with long time series, such as Alpha Cen B. In that case we demonstrate that the breakpoint method improves the detection limit of exoplanets on average by 74% with respect to the overall correction method.