Abstract
Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. 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.
Highlights
One of the main drivers of exoplanet research is the detection of Earth-mass planets orbiting in the habitable zones of their host stars
We have investigated the stellar activity of the very active midM dwarf EV Lac using high-precision radial velocity (RV) measurements secured with the CARMENES spectrograph
We find for the small subset of data comprising map 9 that there are closed-loop correlations with RV for the chromatic index (CRX), the CCFBisector, TiO 7050, TiO 8430, and the differential line width (dLW) that are not apparent in the full data set as the stellar activity features have significantly evolved
Summary
One of the main drivers of exoplanet research is the detection of Earth-mass planets orbiting in the habitable zones of their host stars. Low-mass M dwarf stars are currently the focus of high-precision radial velocity (RV) exoplanet surveys because the relative amplitude of the RV induced by an Earth-mass planet is larger for a lower-mass star than that of the highermass G dwarfs. One of the first exoplanet surveys at near-infared (NIR) wavelengths is the CARMENES survey (Calar Alto highResolution search for M dwarfs with Exo-earths with Nearinfrared and optical Échelle Spectrographs; Quirrenbach et al 2018). The guaranteed time observations (GTO) program comprises over 350 M dwarfs that were selected as the brightest J-band stars in each spectral type bin in addition to being observable from Calar Alto and, without companions at less than 5 arcsec.
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