Using Doppler imaging to model stellar activity and search for planets around Sun-like stars

ABSTRACT Doppler imaging (DI) is a well-established technique to map a physical field at a stellar surface from a time series of high-resolution spectra. In this proof-of-concept study, we aim to show that traditional DI algorithms, originally designed for rapidly rotating stars, also have the ability to model the activity of Sun-like stars, when observed with new-generation highly stable spectrographs, and search for low-mass planets around them. We used DI to retrieve the relative brightness distribution at the surface of the Sun from radial velocity (RV) observations collected by HARPS-N (High Accuracy Radial velocity Planet Searcher for the Northern hemisphere) between 2022 and 2024. The brightness maps obtained with DI have a typical angular resolution of ${\sim} 36^{\circ }$ and are a good match to low-resolution disc-resolved Dopplergrams of the Sun at epochs when the absolute, disc-integrated RV exceeds ${\sim}2$ m s−1. The RV residuals after DI correction exhibit a dispersion of about 0.6 m s−1, comparable with existing state-of-the-art activity correction techniques. Using planet injection-recovery tests, we also show that DI can be a powerful tool for blind planet searches, so long as the orbital period is larger than ${\sim}100$ d (i.e. 3–4 yr stellar rotation periods), and that it yields planetary mass estimates with an accuracy comparable to, for example, multidimensional Gaussian process regression. Finally, we highlight some limitations of traditional DI algorithms, which should be addressed to make DI a reliable alternative to state-of-the-art RV-based planet search techniques.