Evidence of 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. We modeled 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 calculated a grid of interior structure models on which we performed retrievals for our sample of 28 sub-Saturns to derive their envelope mass fractions (fenv). For each planet, we ran 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 was then compared to outcomes and predictions of planet formation models. When our results are compared to the outcomes of a planetesimal accretion formation model, we find that we require a high atmospheric metallicity for intermediate-mass sub-Saturns to reproduce the simulated planet population. For higher planetary masses, a medium atmospheric metallicity provides the best agreement. 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 of the core-accretion model in which runaway accretion starts when Menv ≈ Mcore(fenv is ∼ 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 compared 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.