Mitigating stellar variability with realistic 3D simulations

Characterising and correcting stellar variability is the largest hurdle to overcome in our search for Earth-like worlds. One of the key contributors to stellar variability is granulation at the stellar surface, in which rising hot bubbles of plasma cause a net convective blueshift in observed stellar signals. The combined impact of the granules and inter-granular lanes can result in a C-shape bisector in spectral lines, and this shape can be used to quantify the effect of granulation on radial velocity shifts. It is at the moment not feasible to accurately probe granulation with current instrumental precision. Moreover, empirical methods attempting to characterise granulation can only predict a net velocity variability based on background power spectra, the accuracy of which remains to be tested. However, we can circumvent instrumental limitations with the aid of state of the art simulations. Using 3D hydro- and magneto-hydrodynamic (HD/MHD) simulations from MURAM paired with the radiative transfer codes MPS-ATLAS and NICOLE, we synthesise several stellar spectral lines. In particular, we aim to quantify the impact of the magnetic field (including Zeeman effects) on the ‘quiet’ stellar photosphere of Sun-like stars by comparing hydrodynamic and small scale dynamo simulations. We validate our simulations against observations of the Sun and compare the predicted variability to the more empirically driven methods based on stellar power spectra. Our aim is to isolate the most important aspects of the physics behind granulation. These results will feed into mitigating the effects on the radial velocity confirmation and characterisation of low mass, long period.