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

We present a basic, fast, and flexible planet formation model, called SimAb (Simulating Abundances), to form giant planets and study their primary atmospheric composition soon after their formation. 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. We focus on the C/O ratio and the metallicity of the planetary atmosphere as an indicator of their compositions. 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 sub-solar or super-solar metallicity. Furthermore, the C/O ratio is affected by the dust grain fraction and the planetesimal fraction. 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. By using the C/O ratio and metallicity together we can put a lower and upper boundary on the initial orbital distance where super-solar metallicity planets are formed. We show that planetesimals are the main source for reaching super-solar metallicity planets. On the other hand, planets that mainly accrete dust grains will show a more solar composition. Super-solar metallicity planets that initiate their formation farther than the CO ice line have a C/O ratio closer to the solar value.