Stellar signal components seen in HARPS and HARPS-N solar radial velocities^⋆

¹ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
e-mail: khaled.almoulla@unige.ch
² Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
³ European Southern Observatory, Avenida Alonso de Cordova 3107, Santiago de Chile, Chile
⁴ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

Received: 2 August 2022
Accepted: 7 November 2022

Abstract

Context. Radial velocity (RV) measurements induced by the presence of planets around late-type stars are contaminated by stellar signals that are on the order of a few meters per second in amplitude, even for the quietest stars. Those signals are induced by acoustic oscillations, convective granulation patterns, active regions corotating with the stellar surface, and magnetic activity cycles.

Aims. This study investigates the properties of all coherent stellar signals seen on the Sun on timescales up to its sidereal rotational period. By combining HARPS and HARPS-N solar data spanning several years, we are able to clearly resolve signals on timescales from minutes to several months.

Methods. We used a Markov chain Monte Carlo (MCMC) mixture model to determine the quality of the solar data based on the expected airmass–magnitude extinction law. We then fit the velocity power spectrum of the cleaned and heliocentric RVs with all known variability sources, to recreate the RV contribution of each component.

Results. After rejecting variations caused by poor weather conditions, we were able to improve the average intra-day root mean square (rms) value by a factor of ∼1.8. On sub-rotational timescales, we were able to fully recreate the observed rms of the RV variations. In order to also include rotational components and their strong alias peaks introduced by nightly sampling gaps, the alias powers were accounted for by being redistributed to the central frequencies of the rotational harmonics.

Conclusions. In order to enable a better understanding and mitigation of stellar activity sources, their respective impact on the total RV must be well measured and characterized. We were able to recreate RV components up to rotational timescales, which can be further used to analyze the impact of each individual source of stellar signals on the detectability of exoplanets orbiting very quiet solar-type stars and test the observational strategies of RV surveys.