The CARMENES search for exoplanets around M dwarfs: Benchmarking the impact of activity in high-precision radial velocity measurements

TL;DR

This study benchmarks the impact of stellar activity on high-precision radial velocity measurements of active M dwarfs, demonstrating that regular observations and activity correction techniques significantly improve exoplanet detection sensitivity.

Contribution

It introduces a low-resolution Doppler imaging technique to quantify activity-induced RV variations and shows how removing these effects enhances planet detection around active M dwarfs.

Findings

Strong correlation between centre-of-light and chromatic index.

Removing spot-induced RV improves planet-mass sensitivity by at least three.

Regular-cadence observations are most effective for activity mitigation.

Abstract

Current exoplanet surveys using the radial velocity (RV) technique are targeting M dwarfs because any habitable zone terrestrial-mass planets will induce a high RV and orbit on shorter periods than for more massive stars. One of the main caveats is that M dwarfs show a wide range of activity levels from inactive to very active, which can induce an asymmetry in the line profiles and, consequently, a spurious RV measurement. We aim to benchmark the impact of stellar activity on high-precision RV measurements using regular-cadence CARMENES visible and near-infrared observations of the active M3.5 dwarf EV Lac. We used the newly developed technique of low-resolution Doppler imaging to determine the centre-of-light, or spot-induced RV component, for eight observational epochs. We confirm a statistically significant and strong correlation between the independently measured centre-of-light and the chromatic index, which is a measure of the amplitude variation with wavelength of the RVs. We also find circular closed-loop relations of several activity indices with RV for a subset of data that covers only several rotation periods. We also investigate the implications of large phase gaps in the periodograms of activity indicators. Finally, by removing the spot-induced RV component we improve the planet-mass sensitivity by a factor of at least three. We conclude that for active M stars, a regular-cadence observing strategy is the most efficient way to identify and eliminate sources of correlated noise.