A Full Characterisation of Stars and Planets through High Resolution Spectroscopy and 3D Simulations.

The study of exoplanets atmospheres is one of the most intriguing challenges in the exoplanet field nowadays and the High Resolution Spectroscopy (HRS) has recently emerged as one of the leading methods for detecting atomic and molecular species in their atmospheres (e.g., Birkby, 2018). While this technique is particularly robust against contaminant absorption in the Earth’s atmosphere, the non-stationary stellar spectrum, in the form of either Doppler shift or distortion of the line profile during planetary transits, creates a non-negligible source of noise that can alter or even prevent detection. In the last years, it has become computationally possible to simulate the stellar surface convection that, in the end, allows to correctly reproduce asymmetric and blue-shifted spectral lines due to the granulation pattern of the stellar disk, which is a very important source of uncertainties (Chiavassa & Brogi, 2019). In the context of HRS and on the planet hand side, only recently multidimensional models have been used to detect the weak planet signal in the spectrum (e.g., Flowers et al. 2019).

However, these numerical simulations have been computed independently for star and planet so far, while acquired spectra are the result of the natural coupling at each phase along the planet orbit. A next step forward is needed: coupling stellar and planetary 3D models dynamics during the transit.

I will present the unprecedented precise synthetic spectra obtained with the upgraded 3D radiative transfer code Optim3D (Chiavassa et al. 2009). Optim3D takes as inputs the state-of-art 3D RHD stellar simulations (Stagger code, Nordlund et al. 2009, Magic et al. 2013) and the 3D Global Climate Models (SPARC/MITgcms, Showman et al. 2009, Parmentier et al. 2021) for stars and planets respectively, coupling them at any phase along the planet orbit. I will show the impact of this new approach on the detection of molecules by cross-correlating our spectra with HRS observations (e.g., Snellen et al 2010 and Brogi et al. 2016). This approach is particularly advantageous for those molecular species that are present in both the atmospheres and form in the same region of the spectrum, resulting in mixed and overlapped spectral lines (e.g. CO and H2O, crucial to constrain the C/O ratio). Moreover, the use of 3D models provides us with information about the dynamics processes at play, such as stellar convection and planetary winds.