The test case of HD26965: difficulties disentangling weak Doppler signals from stellar activity (1801.03970v1)

Abstract: We report the discovery of a radial velocity signal that can be interpreted as a planetary-mass candidate orbiting the K dwarf HD26965, with an orbital period of 42.364$\pm$0.015 days, or alternatively, as the presence of residual, uncorrected rotational activity in the data. Observations include data from HIRES, PFS, CHIRON, and HARPS, where 1,111 measurements were made over 16 years. Our best solution for HD26965 $b$ is consistent with a super-Earth that has a minimum mass of 6.92$\pm$0.79 M$_{\oplus}$ orbiting at a distance of 0.215$\pm$0.008 AU from its host star. We have analyzed the correlation between spectral activity indicators and the radial velocities from each instrument, showing moderate correlations that we include in our model. From this analysis, we recover a $\sim$38 day signal, which matches some literature values of the stellar rotation period. However, from independent Mt. Wilson HK data for this star, we find evidence for a significant 42 day signal after subtraction of longer period magnetic cycles, casting doubt on the planetary hypothesis for this period. Although our statistical model strongly suggests that the 42-day signal is Doppler in origin, we conclude that the residual effects of stellar rotation are difficult to fully model and remove from this dataset, highlighting the difficulties to disentangle small planetary signals and photospheric noise, particularly when the orbital periods are close to the rotation period of the star. This study serves as an excellent test case for future works that aim to detect small planets orbiting `Sun-like' stars using radial velocity measurements.

• The paper identifies a 42.364-day radial velocity signal that may indicate a super-Earth exoplanet amid complex stellar activity.
• It employs a robust analysis using Generalized Lomb-Scargle periodograms and Bayesian models on 16 years of multi-instrument data.
• The study underscores the need for refined noise models and additional spectral indicators to reliably separate stellar and planetary signals.

Analysis of Doppler Signals and Stellar Activity in HD26965

The paper "The test case of HD26965: difficulties disentangling weak Doppler signals from stellar activity" presents an in-depth exploration of radial velocity measurements for the K dwarf star HD26965. The paper aims to discern potential planetary signals from intrinsic stellar activity, utilizing observational data collected over 16 years from multiple high-resolution spectrographs, including HIRES, PFS, CHIRON, and HARPS.

Research Context and Methodology

The development of precision radial velocity techniques has significantly contributed to the discovery and characterization of exoplanets orbiting nearby stars. In the search for low-mass planets, the challenge arises from the need to differentiate between genuine planetary signals and stellar activity-induced noise, especially when these signals have similar periodicities. HD26965, classified as a K0.5V star, provides a test case due to its quiescent nature and proximity to the Sun.

The authors carry out a comprehensive analysis using both traditional Generalized Lomb-Scargle (GLS) periodograms and Bayesian statistical models to investigate potential periodicities within the radial velocity data. They employ a robust statistical framework to enhance signal detection, incorporating factors such as red noise accounts for stellar activity correlations with radial velocities.

Key Findings and Analysis

The research identifies a radial velocity signal with a period of approximately 42.364 days, which could indicate the presence of a super-Earth-mass exoplanet (HD26965 b) with a minimum mass of 6.92 $M_{\oplus}$ orbiting at 0.215 AU from the host star. Notably, this signal's origin is complicated by the presence of moderate correlations with stellar activity indicators.

Subsequent analysis involved studying Ca II H and K indices from the Mount Wilson Observatory data to ascertain any long-term magnetic cycles. After accounting for longer-term periodicities in the HK activity measurements, a possible 42-day signal was identified, closely matching the radial velocity signal, which aligns intriguingly with other suggested rotational period values from previous studies.

A major contribution of this work is its emphasis on the need for model refinements in separating planetary signals from stellar activity noise. By comparing data subsets and testing for signal variability over time, they underscore the importance of independent critical assessments of radial velocity signals in the search for exoplanets.

Implications and Future Work

The implications of this paper are significant for the methodology of exoplanet detection, particularly for signals close to the stellar rotation period. The findings suggest the necessity of improved noise models to account for subtle stellar activity effects, which is crucial in validating low-mass exoplanet candidates.

The paper advocates for the expansion of observable parameters, such as the pursuit of additional spectral activity indicators or advanced machine learning models that might better differentiate complex stellar activity patterns from planetary signals.

Looking forward, the research sets a foundation for similar investigations in other Sun-like stars, potentially refining detection techniques and contributing to an accurate census of close-in super-Earths. Future efforts should focus on the development of more sophisticated, non-linear models to effectively disentangle small amplitudes planetary signals from the background noise associated with stellar activity. Additionally, leveraging space-based photometric observations alongside ground-based spectroscopic data could further enhance detection capabilities and solidify signal origins.