Abstract
ABSTRACT Long-term stellar activity variations can affect the detectability of long-period and Earth-analogue extrasolar planets. We have, for 54 stars, analysed the long-term trend of five activity indicators: log $R^{\prime }_\mathrm{{HK}}$, the cross-correlation function (CCF) bisector span, CCF full-width-at-half-maximum, CCF contrast, and the area of the Gaussian fit to the CCF; and studied their correlation with the RVs. The sign of the correlations appears to vary as a function of stellar spectral type, and the transition in sign signals a noteworthy change in the stellar activity properties where earlier type stars appear more plage dominated. These transitions become more clearly defined when considered as a function of the convective zone depth. Therefore, it is the convective zone depth (which can be altered by stellar metallicity) that appears to be the underlying fundamental parameter driving the observed activity correlations. In addition, for most of the stars, we find that the RVs become increasingly redshifted as activity levels increase, which can be explained by the increase in the suppression of convective blueshift. However, we also find a minority of stars where the RVs become increasingly blueshifted as activity levels increase. Finally, using the correlation found between activity indicators and RVs, we removed RV signals generated by long-term changes in stellar activity. We find that performing simple cleaning of such long-term signals enables improved planet detection at longer orbital periods.
Highlights
The latest highly wavelength-stabilised spectrographs can achieve hitherto unprecedented raR dial velocity (RV) instrumental precision
M a first example, we look at the correlations between the log RHK and the cross-correlation function (CCF) bisector span, FWHM, contrast, and the area for the Sun, and compare the results with the corthe signals, using the data from the literature as a starting point, in order to get updated parameters for known planets
In the analysis presented in this paper, we have used the strength of the Ca II H & K re-emission lines as a tracer of stellar activity levels
Summary
G as ESPRESSO, see e.g. Gonzalez Hernandez et al 2018; I Pepe et al 2020) can achieve hitherto unprecedented raR dial velocity (RV) instrumental precision. Within the conO text of exoplanet research, this should enable greater RV sensitivity to orbiting exoplanets through the study of the Doppler wobble reflex motion they induce on the host star As such RV measurements become more precise they become progressively more sensitive to the RV signals driven by stellar activity (see e.g. Lovis et al 2011; Dumusque et al 2011b; Meunier & Lagrange 2013). In the presence of plage, this convective blue-shift term is suppressed This mechanism, considered to dominate the stellar induced RVs for low-activity stars (Meunier et al 2010), is thought to be the biggest impediment to the RV detection of Earth-like planets.
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