Variability in stellar granulation and convective blueshift with spectral type and magnetic activity

Abstract

Context. The inhibition of small-scale convection in the Sun dominates the long-term radial velocity (RV) variability: it therefore has a critical effect on light exoplanet detectability using RV techniques. Aims. We here extend our previous analysis of stellar convective blueshift and its dependence on magnetic activity to a larger sample of stars in order to extend the T eff range, to study the impact of other stellar properties, and finally to improve the comparison between observed RV jitter and expected RV variations. Methods. We estimate a differential velocity shift for Fe and Ti lines of different depths and derive an absolute convective blueshift using the Sun as a reference for a sample of 360 F7-K4 stars with different properties (age, T eff , metallicity). Results. We confirm the strong variation in convective blueshift with T eff and its dependence on (as shown in the line list in Paper I) activity level. Although we do not observe a significant effect of age or cyclic activity, stars with a higher metallicity tend to have a lower convective blueshift, with a larger effect than expected from numerical simulations. Finally, we estimate that for 71% of the stars in our sample the RV and Log R HK variations are compatible with the effect of activity on convection, as observed in the solar case, while for the other stars, other sources (such as binarity or companions) must be invoked to explain the large RV variations. We also confirm a relationship between Log R HK and metallicity, which may affect discussions of the possible relationship between metallicity and exoplanets, as RV surveys are biased toward low Log R HK and possibly toward high-metallicity stars. Conclusions. We conclude that activity and metallicity strongly affect the small-scale convection levels in stars in the F7-K4 range, with a lower amplitude for the lower mass stars and a larger amplitude for low-metallicity stars.

Activity time series of old stars from late F to early K

“…Larger plages would lead to a larger Log R HK . Finally, the inhibition of the convective blueshift has been well determined in Meunier et al (2017a) and again is in good agreement for the Sun, which is more active than the quiet stars we discussed here (Log R HK below −5). In addition, it would not be compatible with the slope we observe between RV and Log R HK either, which agrees well with observations (see below).…”

“…There is a small departure for stars with either a very high mass or a very low mass (in our range of parameters), however. There is a possibility for the inhibition of the convective blueshift to depend slightly on T eff (Meunier et al 2017a), at least above 5400 K, but it does not explain the discrepancy, although the slope is increased. This comparison suggests that the trend (of the percentage of inhibition as a function of T eff ) could be higher than expected from the results of Meunier et al (2017a), but this is not the only possible explanation.…”

“…There is a possibility for the inhibition of the convective blueshift to depend slightly on T eff (Meunier et al 2017a), at least above 5400 K, but it does not explain the discrepancy, although the slope is increased. This comparison suggests that the trend (of the percentage of inhibition as a function of T eff ) could be higher than expected from the results of Meunier et al (2017a), but this is not the only possible explanation. For example, in Paper I, we considered a similar chromospheric emission for all spectral types (for a given structure size) because this is not constrained.…”

“…They depend on B−V and Log R HK . We also scaled the diffusion coefficient to the convective blueshift amplitude (Meunier et al 2017a).…”

“…All other parameters were kept to our solar values as in Borgniet et al (2015): meridional circulation (Komm et al 1993), other plage and network properties Schrijver 2001;Meunier et al 2017a), and spot properties (Martinez Pillet et al 1993;Baumann & Solanki 2005;Borgniet et al 2015). The five parameters for which we tested several laws (spot contrast, θ max , P rot , P cyc , and A cyc ) were used to produce time series that correspond to the same (B−V, Log R HK ) point.…”