# The CARMENES search for exoplanets around M dwarfs: Stable radial-velocity variations at the rotation period of AD~Leonis -- A test case study of current limitations to treating stellar activity

**Authors:** D. Kossakowski, M. Kürster, Th. Henning, T. Trifonov, J. A. Caballero, M. Lafarga, F. F. Bauer, S. Stock, J. Kemmer, S. V. Jeffers, P. J. Amado, M. Pérez-Torres, V. J. S. Béjar, M. Cortés-Contreras, I. Ribas, A. Reiners, A. Quirrenbach, J. Aceituno, D. Baroch, C. Cifuentes  
**arXiv:** [2209.05814](https://arxiv.org/abs/2209.05814)  
**Date:** 2022-09-13  
**Summary page:** https://www.summarizepaper.com/en/paper/2209.05814v1/

## Key points

- Study focuses on disentangling signals in radial-velocity (RV) data
- Investigates AD Leo, a M3.0V star with proposed planetary companion
- New RV data from CARMENES combined with archival and recent data
- Addresses confusion surrounding the binarity of AD Leo
- Bayesian framework used to determine best model for observed data
- CARMENES RV data agrees with previously reported periodicity of 2.23 days
- Correlation between RVs and some activity indicators observed
- Mixed model composed of stable and variable component performs best for entire dataset
- Residual power at period of interest suggests inability to definitively prove or disprove presence of planet orbiting in synchronization with stellar rotation based on current tools and knowledge
- Limits planets more massive than 27 Earth masses and excludes binary companion with minimum mass of 3-6 Jupiter masses on orbital periods shorter than 14 years around AD Leo.
- Study highlights limitations in distinguishing between planetary signals and stellar activity in RV data
- Inconclusive whether signal is due to planetary companion or stellar activity, requiring further investigations and advancements in observational techniques for resolution

## Summary

This study focuses on the challenge of disentangling signals in radial-velocity (RV) data that could be attributed to either a planetary companion or stellar activity. The target of investigation is AD Leo, a relatively bright and nearby M3.0V star, where the existence of a planetary companion at the same period as the stellar rotation of 2.23 days has been proposed but also contested. To further investigate this signal, the authors introduce new RV data obtained from CARMENES, an instrument capable of observing in both optical and near-infrared wavelengths. They combine these new data with archival data from HIRES and HARPS, as well as more recent data from HARPS-N, GIANO-B, and HPF. Additionally, they address the confusion surrounding the binarity of AD Leo. The analysis considers possible correlations between RVs and various stellar activity indicators accessible with CARMENES. Different models are applied within a Bayesian framework to determine which best explains the observed data: a Keplerian model, a red-noise quasi-periodic model using a Gaussian process, or a mixed model. The results show that the CARMENES RV data agree with the previously reported periodicity of 2.23 days and exhibit chromaticity. They also correlate with some activity indicators. However, when considering the entire RV dataset it is found that a mixed model composed of a stable and variable component performs best. Furthermore, when recomputing the RVs using only spectral lines insensitive to activity there is still some residual power at the period of interest; this suggests that it is not possible to definitively prove or disprove the presence of a planet orbiting in synchronization with the stellar rotation based on their current data tools machinery and knowledge of how stellar activity affects RVs. Nevertheless they are able to rule out planets more massive than 27 Earth masses (0.084 Jupiter masses) and exclude any binary companion around AD Leo with a minimum mass of 3-6 Jupiter masses on orbital periods shorter than 14 years. In conclusion this study highlights the limitations in distinguishing between planetary signals and stellar activity in RV data while providing insights into nature of signal around AD Leo; however it remains inconclusive whether it is due to planetary companion or stellar activity requiring further investigations and advancements in observational techniques for resolution of ambiguity.