Planet and star synergy at high spectral resolution. A rationale for the characterisation of exoplanet atmospheres. I. The Infrared

Context. Spectroscopy of exoplanet atmospheres at high resolving powers is rapidly gaining popularity to measure the presence of atomic and molecular species. While this technique is robust against contaminant absorption in the Earth's atmosphere, the non stationary stellar spectrum creates a non-negligible source of noise that can alter or even prevent detection. Aims. We aim at using three-dimensional stellar simulations to directly remove the signature of the star from observations, and prior to cross correlation with templates for the planet's atmosphere Methods. We compute synthetic spectra from 3D simulations of stellar convection and we couple them with an analytical model reproducing the correct geometry of a transiting exoplanet. We apply the method to HD 189733, and analyse transmission and emission spectroscopy of its hosted exoplanet. In addition, we analyse emission spectroscopy of the non transiting exoplanet 51 Pegasi b. Results. We find a significant improvement in the planet detectability when removing the stellar spectrum with our method. We show that the method is superior to a simple parametrisation of the stellar line profile or to the use of one-dimensional stellar models. This is due to the intrinsic treatment of convection, which allows us to correctly reproduce asymmetric spectral lines, and model center-to-limb variation and Rossiter-McLaughlin effect. In the case of 51 Pegasi b, we succeed at confirming a previous tentative detection of the planet's K-band spectrum due to the improved suppression of stellar residuals. Future high-resolution observations will benefit from the synergy with stellar spectroscopy, and can be used to test the correct modelling of physical processes in stellar atmospheres. We highlight key improvements in modelling techniques and knowledge of opacity sources to extend this work to shorter wavelengths and later-type stars.