Evidence for a gap in the envelope mass fraction of sub-Saturns

Under the core-accretion model, gas giants form via runaway accretion. This process starts when the mass of the accreted envelope becomes equal to the mass of the core. Here, we model a population of warm sub-Saturns to search for imprints of their formation history in their internal structure. Using the GAS gianT modeL for Interiors (GASTLI), we calculate a grid of interior structure models on which we perform retrievals for our sample of 28 sub-Saturns to derive their envelope mass fractions (fₑₙᵥ). For each planet, we run three different retrievals assuming low (-2.0 < log(Fe/H) < 0.5), medium ( 0.5 < log(Fe/H) < 1.4), and high (1.4 < log(Fe/H) < 1.7) atmospheric metallicity. The distribution of fₑₙᵥ in our sample is then compared to predictions of planet formation models. When compared to the outcomes of a planetesimal accretion model, we find that we require medium to high atmospheric metallicities to reproduce the simulated planet population. Additionally, we find a bimodal distribution of fₑₙᵥ in our sample with a gap that is located at different values of fₑₙᵥ for different atmospheric metallicities. For the high atmospheric metallicity case, the gap in the fₑₙᵥ distribution is located between 0.5 and 0.7, which is consistent with assumptions by the core-accretion model where runaway accretion starts when Mₑₙᵥ ≈ Mcore (fₑₙᵥ ~ 0.5). We also find a bimodal distribution of the hydrogen and helium mass fraction (fH/He) with a gap at fH/He = 0.3. The location of this gap is independent of the assumed atmospheric metallicity. Lastly, we compare the distributions of our sub-Saturns in the Neptunian savanna to a population of sub-Saturns in the Neptune desert and ridge. We find that the observed fₑₙᵥ distribution of savanna and ridge sub-Saturns is consistent with the planets coming from the same underlying population.