The influence of convective blueshift on radial velocities of F, G, and K stars

F F Bauer,S V Jeffers,B Beeck,A Reiners

doi:10.1051/0004-6361/201731227

Abstract

Context.Apparent radial velocity (RV) signals induced by stellar surface features such as spots and plages can result in a false planet detection or hide the presence of an orbiting planet. Our ability to detect rocky exoplanets is currently limited by our understanding of such stellar signals.Aims.We model RV variations caused by active regions on the stellar surface of typical exoplanet-hosting stars of spectral type F, G, and K. We aim to understand how the stellar magnetic field strength, convective blueshift, and spot temperatures can influence RV signals caused by active regions.Methods.We use magneto-hydrodynamic (MHD) simulations for stars with spectral types F3V, a G2V, and a K5V. We quantify the impact of the magnetic field strength inside active regions on the RV measurement using the magnetic and non-magnetic FeI lines at 6165 Å and 6173 Å. We also quantify the impact of spot temperature and convective blueshift on the measured RV values.Results.Increasing the magnetic field strength increases the efficiency to suppress convection in active regions which results in an asymmetry between red- and blueshifted parts of the RV curves. A stronger suppression of convection also leads to an observed increase in RV amplitude for stronger magnetic fields. The MHD simulations predict convective motions to be faster in hotter stars. The suppression of faster convection leads to a stronger RV amplitude increase in hotter stars when the magnetic field is increased. While suppression of convection increases the asymmetry in RV curves,c a decreasing spot temperature counteracts this effect. When using observed temperatures for dark spots in our simulations we find that convective blueshift effects are negligible.

Highlights

For about two decades the radial velocity (RV) method has been successfully used to detect extrasolar planets
In this work we investigate the influence that the line profiles in active regions have on the resulting RV curves
Plages are observed to cover a larger surface area on the Sun, this setup enables us to investigate a variety of parameters and effects and allows us to compare our results obtained with different model setups

Summary

Introduction

For about two decades the radial velocity (RV) method has been successfully used to detect extrasolar planets. Since the first planet detection by Mayor & Queloz (1995), the RV technique has undergone major improvements (Latham et al 1998; Fischer et al 2016). Current instruments like HARPS (Mayor et al 2003) can reach a precision of a few m/s in the optical. ESPRESSO (D’Odorico et al 2007) aims to achieve an unprecedented precision of 20 cm/s at optical wavelengths. With this generation of RV machines, instrumental noise will no longer be the limiting factor in detecting exoplanets similar to Earth. Astrophysical noise sources such as stellar activity will become a concern and hinder planet detections (Fischer et al 2016)

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