[Paper Review] Long-term magnetic activity of a sample of M-dwarf stars from the HARPS program II. Activity and radial velocity

J. Gomes da Silva, N. C. Santos+6 more|arXiv (Cornell University)|Feb 7, 2012

Stellar, planetary, and galactic studies43 references64 citations

TL;DR

This study investigates long-term magnetic activity and radial velocity (RV) variations in 27 M-dwarf stars using high-precision HARPS data over a median baseline of 5.9 years. It finds that 36% of stars with long-term activity variability exhibit RV variations up to ~5 m s⁻¹ due to magnetic cycles, highlighting the need to account for activity-induced signals when searching for low-mass exoplanets via radial velocity methods.

ABSTRACT

Due to their low mass and luminosity, M dwarfs are ideal targets if one hopes to find low-mass planets similar to Earth by using the radial velocity (RV) method. However, stellar magnetic cycles could add noise or even mimic the RV signal of a long-period companion. Following our previous work that studied the correlation between activity cycles and long-term RV variations for K dwarfs we now expand that research to the lower-end of the main sequence. Our objective is to detect any correlations between long-term activity variations and the observed RV of a sample of M dwarfs. We used a sample of 27 M-dwarfs with a median observational timespan of 5.9 years. The cross-correlation function (CCF) with its parameters RV, bisector inverse slope (BIS), full-width-at-half- maximum (FWHM) and contrast have been computed from the HARPS spectrum. The activity index have been derived using the Na I D doublet. These parameters were compared with the activity level of the stars to search for correlations. We detected RV variations up to ~5 m/s that we can attribute to activity cycle effects. However, only 36% of the stars with long-term activity variability appear to have their RV affected by magnetic cycles, on the typical timescale of ~6 years. Therefore, we suggest a careful analysis of activity data when searching for extrasolar planets using long-timespan RV data.

Motivation & Objective

• To investigate correlations between long-term magnetic activity and radial velocity (RV) variations in M-dwarf stars.
• To assess the extent to which stellar magnetic cycles can mimic or obscure low-amplitude planetary signals in RV data.
• To evaluate the reliability of CCF parameters (BIS, FWHM, contrast) and Na I D activity index as proxies for long-term activity.
• To determine whether activity-induced RV variations in M dwarfs are systematic or sporadic across the sample.
• To provide guidance for exoplanet hunters on correcting for activity-related RV noise in long-timespan observations.

Proposed method

• Acquired high-precision radial velocity measurements using the HARPS spectrograph on the ESO 3.6-m telescope.
• Computed cross-correlation function (CCF) parameters: radial velocity (RV), bisector inverse slope (BIS), full-width-at-half-maximum (FWHM), and contrast from each spectrum.
• Derived long-term activity levels using the Na I D doublet index to track magnetic cycle variations.
• Performed statistical correlation analysis between RV and activity index, and between RV and CCF parameters over the 5.9-year median baseline.
• Used sinusoidal fitting to model periodic activity cycles where possible, especially for stars with well-sampled data.
• Evaluated the significance of correlations using statistical thresholds and assessed the impact of observational sampling and data span on detectability.

Experimental results

Research questions

• RQ1 To what extent do long-term magnetic activity cycles in M-dwarfs induce radial velocity variations detectable at the 1–5 m s⁻¹ level?
• RQ2 What fraction of M-dwarfs with long-term activity variability show a significant correlation between activity and RV variations?
• RQ3 How reliable are CCF parameters (BIS, FWHM, contrast) in tracing long-term activity in M dwarfs compared to the Na I D index?
• RQ4 Are the correlations between activity and RV consistent across different stellar types, particularly in late-type M dwarfs compared to earlier K dwarfs?
• RQ5 Can activity-induced RV variations mimic or mask the signals of long-period low-mass planets in long-baseline RV surveys?

Key findings

• 36% of the 14 M-dwarfs showing long-term activity variability exhibited a significant correlation between activity and radial velocity, with peak-to-peak RV variations reaching up to ~5 m s⁻¹.
• The median long-term RV rms induced by activity was 1.5 m s⁻¹, lower than predicted for the Sun but consistent with extrapolations from earlier studies on late-type stars.
• Only Gl 433 showed a well-fitted sinusoidal activity cycle with a minimum period of 4.6 years, and a similar RV period of 4.8 years was detected.
• A significant correlation was found between activity and FWHM in 43% of variable stars, indicating that CCF width is a reliable proxy for long-term activity in M dwarfs.
• Anti-correlations between activity and BIS were observed in 21% of variable stars, but these were not statistically significant, contrasting with findings in early-K dwarfs.
• Contrast showed a weak negative correlation with activity in only 14% of cases, indicating limited utility as an activity diagnostic for long-term variability.