Granulation in the photospheres of FGK-type stars induces variability in
absorption lines, complicating exoplanet detection via radial velocities and
characterisation via transmission spectroscopy. We aim to quantify the impact
of granulation on the radial velocity and bisector asymmetry of stellar
absorption lines of varying strengths and at different limb angles. We use 3D
radiation-hydrodynamic simulations from MURaM paired with MPS-ATLAS radiative
transfer calculations to synthesise time series’ for four Fe I lines at
different limb angles for a solar-type star. Our line profiles are synthesised
at an extremely high resolution (R = 2,000,000), exceeding what is possible
observationally and allowing us to capture intricate line shape variations. Nous
introduce a new method of classifying the stellar surface into three components
and use this to parameterise the line profiles. Our parameterisation method
allows us to disentangle the contributions from p-modes and granulation,
providing the unique opportunity to study the effects of granulation without
contamination from p-mode effects. We validate our method by comparing radial
velocity power spectra of our granulation time series to observations from the
LARS spectrograph. We find that we are able to replicate the granulation
component extracted from observations of the Fe I 617 nm line at the solar disk
centre. We use our granulation-isolated results to show variations in
convective blueshift and bisector asymmetry at different limb angles, finding
good agreement with empirical results. We show that weaker lines have higher
velocity contrast between granules and lanes, resulting in higher
granulation-induced velocity fluctuations. Our parameterisation provides a
computationally efficient strategy to construct new line profiles, laying the
groundwork for future improvements in mitigating stellar noise in exoplanet
studies.

Cet article explore les excursions dans le temps et leurs implications.

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