The HARPS search for southern extra-solar planets. XXXI. Magnetic activity cycles in solar-type stars: statistics and impact on precise radial velocities

Abstract

Context. Searching for extrasolar planets through radial velocity measurements relies on the stability of stellar photospheres. Several phenomena are known to affect line profiles in solar-type stars, among which stellar oscillations, granulation and magnetic activity through spots, plages and activity cycles. Aims. We aim at characterizing the statistical properties of magnetic activity cycles, and studying their impact on spectroscopic measurements such as radial velocities, line bisectors and line shapes. Methods. We use data from the HARPS high-precision planet-search sample comprising 304 FGK stars followed over about 7 years. We obtain high-precision Ca II H&K chromospheric activity measurements and convert them to R ′ HK indices using an updated calibration taking into account stellar metallicity. We study R ′ HK variability as a function of time and search for possible correlations with radial velocities and line shape parameters.Results. The obtained long-term precision of ∼0.35% on S-index measurements is about 3 times better than the canonical Mt Wilson survey, which opens new possibilities to characterize stellar activity. We classify stars according to the magnitude and timescale of the Ca II H&K variability, and identify activity cycles whenever possible. We find that 39±8% of old solar-type stars in the solar neighborhood do not show any activity cycles (or only very weak ones), while 61±8% do have one. Non-cycling stars are almost only found among G dwarfs and at mean activity levels log R ′ HK < -4.95. Magnetic cycle amplitude generally decreases with decreasing activity level. A significant fraction of stars exhibit small variations in radial velocities and line shape parameters that are correlated with activity cycles. The sensitivity of radial velocities to magnetic cycles increases towards hotter stars, while late K dwarfs are almost insensitive. Conclusions. Activity cycles do induce long-period, low-amplitude radial velocity variations, at levels up to ∼25 m s −1 . Caution is therefore mandatory when searching for long-period exoplanets. However, these effects can be corrected to high precision by detrending the radial velocity data using simultaneous measurements of Ca II H&K flux and line shape parameters.

YARARA: Significant improvement of RV precision through post-processing of spectral time-series

“…We used HD10700 as a star of reference to perform pipeline performance comparison and a planetary injection test. This star is known to be a particular inactive star maybe in a state similar to the Maunder minimum for the Sun (Lovis et al 2011a). On 13 years of HARPS observations, the RV rms is observed to be 1.27 m/s, which is an impressive proof of stellar and instrumental stability.…”

Long-term stellar activity variations and their effect on radial-velocity measurements

“…In the analysis presented in this paper, we have used the strength of the Ca II H & K re-emission lines (as measured by log 𝑅 HK ) as a tracer of stellar activity levels. Since the log 𝑅 HK of each star in our sample is not a product of the ESO archive data we therefore measured these from the spectra, following the procedure outlined in Lovis et al (2011), which we summarise here.…”

“…where 𝐻, 𝐾, 𝑅 and 𝑉 represent the mean fluxes per wavelength interval for each band. This was measured for each star in our sample, and verified by comparing (where possible) with the results from Lovis et al (2011). We then used the calibration from Lovis et al (2011) to map the S-index from HARPS to the Mt Wilson S-index:…”

“…Finally, we compared the median log 𝑅 HK of individual targets in our sample to those from Lovis et al (2011) over the same timescale (where this was possible), finding a median error of less than 0.01. This small discrepancy can be explained by the removal of different outliers in our work.…”

“…a substantial fraction of a stellar activity cycle) presents a different set of problems for stellar-activity mitigation techniques (see e.g. Santos et al 2010;Lovis et al 2011). Indeed, over long-timescales the activity-cycle will drive changes in the magnetic network and this will, in turn, change both the plage coverage (Meunier 2018; as well as the underlying magnetic network variation, and may affect surface flows (Meunier & Lagrange 2020).…”