Characterisation of stellar activity of M dwarfs. I. Long-timescale variability in a large sample and detection of new cycles (2303.03998v1)

Abstract: M dwarfs are active stars that exhibit variability in chromospheric emission and photometry at short and long timescales, including long cycles that are related to dynamo processes. This activity also impacts the search for exoplanets because it affects the radial velocities. We analysed a large sample of 177 M dwarfs observed with HARPS (2003-2020) in order to characterise the long-term variability of these stars. We compared the variability obtained in three chromospheric activity indices (Ca II H & K, the Na D doublet, and Halpha) and with ASAS photometry. We focused on the detailed analysis of the chromospheric emission based on linear, quadratic, and sinusoidal models. We used various tools to estimate the significance of the variability and to quantify the improvement brought by the models. In addition, we analysed complementary photometric time series for the most variable stars to be able to provide a broader view of the long-term variability in M dwarfs. We find that most stars are significantly variable, even the quietest stars. Most stars in our sample (75%) exhibit a long-term variability, which manifests itself mostly through linear or quadratic variability, although the true behaviour may be more complex. We found significant variability with estimated timescales for 24 stars, and estimated the lower limit for a possible cycle period for an additional 9 stars that were not previously published. We found evidence of complex variability because more than one long-term timescale may be present for at least 12 stars, together with significant differences between the behaviour of the three activity indices. This complexity may also be the source of the discrepancies observed between previous publications. We conclude that long-term variability is present for all spectral types and activity level in M dwarfs, without a significant trend with spectral type or mean activity level.

• The paper reveals that about 75% of 177 M dwarfs show significant long-term chromospheric variability, challenging previous assumptions about stellar quietness.
• The paper employs linear, quadratic, and sinusoidal modeling on chromospheric and photometric data to distinguish varied activity indicators and cyclic patterns.
• The paper demonstrates that understanding these complex activity cycles is essential for accurately interpreting exoplanet RV signals and reducing false positives.

Characterisation of Stellar Activity in M Dwarfs: An Examination of Long-Timescale Variability

The paper "Characterisation of stellar activity of M dwarfs. I. Long-timescale variability in a large sample and detection of new cycles" presents an extensive analysis of the long-term chromospheric variability in M dwarfs. It examines the influence of stellar activity on observations, specifically in the context of exoplanet hunting through radial velocity (RV) methods, where stellar activity can mimic or obscure planetary signals.

The investigation evaluates data from 177 M dwarfs collected using the High Accuracy Radial velocity Planet Searcher (HARPS) from 2003 to 2020. The paper focuses on three key chromospheric activity indicators: the Ca II H & K lines, the Na D doublet, and H$\alpha$, supplemented by photometric data from the ASAS survey. The objective is to characterize the stellar activity variability over long timescales typically associated with dynamo processes.

Methodology and Analysis

The approach involves fitting linear, quadratic, and sinusoidal models to the chromospheric emission time series, testing each model's significance and determining the improvements offered by more complex models. The complex nature of the variability is addressed by estimating typical timescales where variability might be present. For photometry, the paper compares observed variability to chromospheric indices to provide a holistic view of stellar activity.

The results are compelling: long-term variability is evident in most stars, irrespective of their quietness or average activity level as measured by $\log R'_{HK}$. This contradicts the simplistic notion that only the most active stars exhibit significant long-term behavior. Approximately 75% of the sample displayed significant variability, often manifesting as linear or quadratic trends, and in many cases, complex behaviors suggested additional timescales of variability.

Key Findings

1. Long-Term Variability Across All Types: Long-term variability is ubiquitous among M dwarfs across the full range of spectral types and activity levels. This finding is crucial for planet detection efforts, as it highlights the need for careful analysis to differentiate between stellar activity and planetary signals in radial velocity data.
2. Diversity of Indicators: There is a significant difference in behavior among the chromospheric activity indicators. This divergence suggests that different indices may trace distinct aspects or layers of the stellar atmosphere, complicating their use as universal tracers for activity cycles.
3. Periodicity and Complex Behavior: The paper validates long-term periodicities for 24 stars, indicating a complex interplay of processes driving these cycles. However, the variability's complexity—evidenced by discrepancies across chromospheric and photometric datasets—suggests multiple influencing mechanisms, perhaps parallel dynamos or interactions between activity components like spots, plages, and faculae.
4. Implications for Exoplanet Studies: The findings emphasize the necessity for astronomers to account for stellar activity variability when interpreting RV data. Overlooking these complexities can lead to false positives or missed detections of exoplanets.

Theoretical and Practical Implications

Theoretically, the results challenge existing models of stellar dynamos, especially for fully convective stars where traditional dynamos—relying on a tachocline—are absent. The presence of cycles in these stars suggests alternative dynamo processes, possibly $\alpha^2$ dynamos or large-scale turbulent fields, may be at play.

Practically, understanding this variability enhances exoplanet detection capabilities by allowing researchers to refine analysis techniques and develop models that incorporate these star-specific activity profiles. This is particularly relevant for the growing interest in habitable zone planets around M dwarfs, which are prime targets for life-search missions.

Speculations on Future Directions

Future research may focus on high-cadence, long-duration observations that can more precisely map the interactions between various activity indices and deepen our understanding of the internal stellar processes. Simultaneous multi-wavelength observations could also uncover the diverse atmospheric layers contributing to observed variability. With advancements in instrumentation and data analysis techniques, the disentanglement of stellar activity from planetary signals will likely become more sophisticated, enhancing our ability to detect Earth-like exoplanets around these intriguing stars.

Overall, this comprehensive paper provides a valuable database and analytical framework to investigate M dwarf variability, establishing a baseline for stellar activity research and its impact on exoplanet studies.