A formation pathway for giant planets in S-type discs of γ-Cephei-like compact binaries

Planet formation in close binary systems such as γ-Cephei is strongly challenged by the severe truncation of the circumprimary disc induced by the stellar companion, which drastically limits the available reservoir of gas and solids. Recent hydrodynamical studies suggest that a long-lived circumbinary disc may replenish the circumprimary disc with gas and dust, extending its lifetime and potentially enabling giant planet formation. However, the long-term evolution of such systems under the combined effects of viscous accretion and X-ray photoevaporation and their coupling with planet formation remain largely unexplored. We aim to investigate whether sustained mass inflow from a circumbinary reservoir can prolong the lifetime of circumprimary discs and facilitate gas giant planet formation in γ Cephei–like binary systems, even in the presence of strong photoevaporative winds. Using our code PLANETALP-B, we modelled the coupled evolution of gas, dust growth, and in situ planet formation through pebble and gas accretion in a γ-Cephei-like circumprimary disc, and we included X-ray photoevaporation and continuous mass injection from an external circumbinary disc. Gas inflow from the circumbinary disc can significantly extend the lifetime of the circumprimary disc, even under strong photoevaporative mass loss. When a fraction of solids is transferred, the lifetime of the circumprimary solid disc increases as well, enhancing the efficiency of planetary growth. As a result, our simulated planets were able to reach several Jupiter masses, in contrast to scenarios that neglect mass replenishment. We show that sustained mass transfer from a circumbinary disc can indeed play a key role in enabling giant planet formation in γ-Cephei–like close binaries. This mechanism provides a viable pathway to overcome the limitations of disc truncation, although its applicability to other types of binary systems remains to be tested with dedicated hydrodynamical simulations.