Abstract
Context.Although the new generation of radial-velocity (RV) instruments such as ESPRESSO are expected to reach the long-term precision required to find other earths, the RV measurements are contaminated by some signal from stellar activity. This makes these detections hard.Aims.Based on real observations, we here demonstrate for the first time the effect of stellar activity on the RV of individual spectral lines. Recent studies have shown that this is probably the key for mitigating this perturbing signal. By measuring the line-by-line RV of each individual spectral line in the 2010 HARPS RV measurements ofαCen B, we study their sensitivity to telluric line contamination and line profile asymmetry. After selecting lines on which we are confident to measure a real Doppler-shift, we study the different effects of the RV signal that is induced by stellar activity on spectral lines based on their physical properties.Results.We estimate that at least 89% of the lines that appear in the spectrum ofαCen B for which we measure a reliable RV are correlated with the stellar activity signal (Pearson correlation coefficientR> 0.3 at 2σ). This can be interpreted as those lines being sensitive to the inhibition of the convective blueshift observed in active regions. Because the velocity of the convective blueshift increases with physical depth inside the stellar atmosphere, we find that the effect induced by stellar activity on the RV of individual spectral lines is inversely proportional to the line depth. The stellar activity signal can be mitigated down to ~0.8–0.9 m s−1either by selecting lines that are less sensitive to activity or by using the difference between the RV of the spectral lines that are formed at different depths in the stellar atmosphere as an activity proxy.Conclusions.This paper shows for the first time that based on real observations of solar-type stars, it is possible to measure the RV effect of stellar activity on the RV of individual spectral lines. Our results are very promising and demonstrate that analysing the RV of individual spectral lines is probably one of the solutions to mitigate stellar activity signal in RV measurements down to a level enabling the detection of other earths.
Highlights
To characterise exoplanets in depth, it is crucial to measure their radius and mass, which gives their density
As discussed in the introduction, the goal of this paper is to demonstrate that the effect of the inhibition of convective blueshift in active regions can be detected in the RVi of individual spectral lines
For other stars, according to the results shown in Gray (2009), we expect that the velocity of the convective blueshift as a function of line depth is similar and only depends on a scaling factor
Summary
To characterise exoplanets in depth, it is crucial to measure their radius and mass, which gives their density. This data set has been intensively used in previous studies to analyse the effect of stellar activity on RV measurements (e.g. Dumusque 2014, 2018; Wise et al 2018; Thompson et al 2017) During these 81 days, the lpoegri(oRdHoKf) activity index shows a sinusoidal variation 35.8 days (see top panel of Fig. 1). With an estimated rotational period of 35.8 days and a stellar radius of 0.863 R (Kervella et al 2003), α Cen B is an extremely slow rotator with vrot, equatorial = 1.22 km s−1 Based on this low rotational velocity, we can infer from stellar activity simulations that the RV effect induced by the inhibition of convection inside faculae probably dominates the RV variation induced by the flux effect originating from dark spots We refer to the RV of single lines as RVi, in contrast to RVm, which corresponds to the weighted average of all RVi, using as weight the inverse square of the RV photon-noise error of each line i
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