SimAb: A simple, fast, and flexible model to assess the effects of planet formation on the atmospheric composition of gas giants

Abstract

Context. The composition of exoplanet atmospheres provides us with vital insight into their formation scenario. Conversely, planet formation processes shape the composition of atmospheres and imprint their specific signatures. In this context, models of planet formation containing key formation processes help supply clues to how planets form. This includes constraints on the metallicity and carbon-to-oxygen ratio (C/O ratio) of the planetary atmospheres. Gas giants in particular are of great interest due to the amount of information we can obtain about their atmospheric composition from their spectra, and also due to their relative ease of observation. Aims. We present a basic, fast, and flexible planet formation model, called Simulating Abundances (SimAb), that forms giant planets and allows us to study their primary atmospheric composition soon after their formation. Methods. In SimAb we introduce parameters to simplify the assumptions about the complex physics involved in the formation of a planet. This approach allows us to trace and understand the influence of complex physical processes on the formed planets. In this study we focus on four different parameters and how they influence the composition of the planetary atmospheres: initial protoplanet mass, initial orbital distance of the protoplanet, planetesimal ratio in the disk, and dust grain fraction in the disk. Results. We focus on the C/O ratio and the metallicity of the planetary atmosphere as an indicator of their composition. We show that the initial protoplanet core mass does not influence the final composition of the planetary atmosphere in the context of our model. The initial orbital distance affects the C/O ratio due to the different C/O ratios in the gas phase and the solid phase at different orbital distances. Additionally, the initial orbital distance together with the amount of accreted planetesimals cause the planet to have subsolar or supersolar metallicity. Furthermore, the C/O ratio is affected by the dust grain fraction and the planetesimal ratio. Planets that accrete most of their heavy elements through dust grains will have a C/O ratio close to the solar C/O ratio, while planets that accrete most of their heavy elements from the planetesimals in the disk will end up with a C/O ratio closer to the C/O ratio in the solid phase of the disk. Conclusions. By using the C/O ratio and metallicity together we can put a lower and upper boundary on the initial orbital distance where supersolar metallicity planets are formed. We show that planetesimals are the main source for reaching supersolar metallicity planets. On the other hand, planets that mainly accrete dust grains will show a more solar composition. Supersolar metallicity planets that initiate their formation farther than the CO ice line have a C/O ratio closer to the solar value.