The CARMENES search for exoplanets around M dwarfs : Understanding the wavelength dependence of radial velocity measurements

TL;DR

This study analyzes over 8 years of CARMENES data to understand how stellar activity affects radial velocity measurements across different wavelengths, improving exoplanet detection around M dwarfs by mitigating stellar noise.

Contribution

The paper demonstrates that the chromatic index effectively identifies stellar activity effects on RVs, enabling significant noise reduction and enhancing exoplanet detection capabilities.

Findings

17% of stars show strong or moderate CRX-RV correlation

RVs can be improved by up to a factor of 4 after correction

CRX is a powerful tool for mitigating stellar activity in RV measurements

Abstract

Context. Current exoplanet surveys are focused on detecting small exoplanets orbiting in the liquid-water habitable zones of their host stars. Despite the recent significant advancements in instrumental developments, the current limitation in detecting these exoplanets is the intrinsic variability of the host star itself. Aims. Our aim is to use the full CARMENES guaranteed time observations (GTO) data set spanning more than 8 years of observations of over 350 stars to investigate the wavelength dependence of high-precision radial velocities (RV), as stellar activity features should exhibit a wavelength dependence while the RV variation due to an orbiting planet will be wavelength independent. Methods. We use the chromatic index (CRX) to quantify the slope of the measured RVs as a function of logarithmic wavelength. We investigate the dependence of the CRX in the full CARMENES GTO sample on 24 stellar activity indices in the visible and near-infrared channels of the CARMENES spectrograph and each star's stellar parameters. We also present an updated convective turnover time scaling for the calculation of the stellar Rossby number for M dwarfs. Results. Our results show that approximately 17\% of GTO stars show a strong or a moderate correlation between CRX and RV. We can improve the measured RVs by a factor of up to nearly 4 in rms by subtracting the RV predicted by the CRX-RV correlation from the measured RVs. Mid M dwarfs with moderate rotational velocities, moderate CRX-gradients and quasi-stable activity features have the best rms improvement factors. Conclusions. We conclude that the CRX is a powerful diagnostic in mitigation of stellar activity and the search for low mass rocky planets.