13:45 〜 14:00
[PPS06-01] Size evolution of close-in super-Earths: effects of giant impacts and photoevaporation
キーワード：惑星半径、スーパーアース、巨大衝突、光蒸発
To date, transit observations report more than 3000 exoplanets, and we know their orbital architecture, i.e., their size and period distribution. The size distribution of super-Earths in close-in orbits is bimodal with a radius gap (e.g., Fulton et al., 2017). Besides, planetary sizes within systems show that adjacent planet pairs have the same size and separations (peas in a pod; e.g., Weiss et al., 2018). We have investigated whether these features are explained simultaneously. The former feature, the radius gap, is considered as a result of envelope evolution through photoevaporation (e.g., Owen & Wu 2017) or other mechanisms (e.g., Gupta & Schlichting 2019). The latter feature, peas in a pod, is not well investigated. It still remains unclear whether this feature is reproduced in the formation processes of planetary systems. We consider planetary growth via giant impacts along with the envelope loss via both giant impacts and photoevaporation to study the size evolution of planets. We have developed the N-body simulation code where planetary sizes are derived by the summation of the core size and envelope size using the scaling law (Valencia et al. 2006) and analytical expression (Owen & Wu 2017). We also consider the envelope loss in giant impacts based on recent SPH simulation results (Kegerreis et al. 2020). We have performed simulations by changing initial envelope fractions and surface density profiles of protoplanets based on observations (Dai et al., 2020).
Our simulations show that impacts are not head-on but grazing encounters, which do not cause significant envelope loss. However, high-velocity collisions sometimes cause whole envelope loss events. As a result, some bare rocky planets are formed. These bare rocky planets are located at inner orbits and have larger masses, typically, since they tend to experience a larger number of collisions. Interestingly, these massive planets barely lose their whole envelope via photoevaporation, especially when the surface density profiles of protoplanets are shallow, i.e., inner protoplanets are massive. We also found that there are several planets located in the outer region where the initial envelopes can be retained. Small inner planets lose their envelopes via photoevaporation. The final size distribution of planets is bimodal due to the envelope loss via photoevaporation. The distribution of planets on the period-radius plane roughly agrees with that of observed planets. In addition, the neighboring formed planets have the same size and same separations. The combination of giant impacts and photoevaporation provides a possible explanation of the orbital architecture of observed super-Earths. However, our results produce too many large planets. This suggests that some additional envelope loss mechanisms are necessary to explain the observed size distributions of planets.
Our simulations show that impacts are not head-on but grazing encounters, which do not cause significant envelope loss. However, high-velocity collisions sometimes cause whole envelope loss events. As a result, some bare rocky planets are formed. These bare rocky planets are located at inner orbits and have larger masses, typically, since they tend to experience a larger number of collisions. Interestingly, these massive planets barely lose their whole envelope via photoevaporation, especially when the surface density profiles of protoplanets are shallow, i.e., inner protoplanets are massive. We also found that there are several planets located in the outer region where the initial envelopes can be retained. Small inner planets lose their envelopes via photoevaporation. The final size distribution of planets is bimodal due to the envelope loss via photoevaporation. The distribution of planets on the period-radius plane roughly agrees with that of observed planets. In addition, the neighboring formed planets have the same size and same separations. The combination of giant impacts and photoevaporation provides a possible explanation of the orbital architecture of observed super-Earths. However, our results produce too many large planets. This suggests that some additional envelope loss mechanisms are necessary to explain the observed size distributions of planets.