The imprint of X-ray photoevaporation of planet-forming discs on the orbital distribution of giant planets

Abstract

Context. Numerical models have shown that disc dispersal via internal photoevaporation driven by the host star can successfully reproduce the observed pile-up of warm Jupiters near 1–2 au. However, since a range of different mechanisms have been proposed to cause the same feature, clear observational diagnostics of disc dispersal leaving an imprint in the observed distribution of giant planets could help in constraining the dominant mechanisms. Aims. We aim to assess the impact of disc dispersal via X-ray-driven photoevaporation (XPE) on giant planet separations in order to provide theoretical constraints on the location and size of any possible features related to this process within the observed semi-major axis distribution of giant planets. Methods. For this purpose, we perform a set of 1D planet population syntheses with varying initial conditions and correlate the gas giants’ final parking locations with the X-ray luminosities of their host stars in order to quantify observables of this process within the semi-major axis versus host star X-ray luminosity plane of these systems. Results. We find that XPE does create an under-density of gas giants near the gravitational radius, with corresponding pile-ups inside and/or outside this location. However, the size and location of these features are strongly dependent on the choice of initial conditions in our model, such as the assumed formation location of the planets. Conclusions. XPE can strongly affect the migration process of giant planets and leave potentially observable signatures within the observed orbital separations of giant planets. However, due to the simplistic approach employed in our model, which lacks a self-consistent treatment of planet formation within an evolving disc, a quantitative analysis of the final planet population orbits is not possible. Our results, however, should strongly motivate future studies to include realistic disc dispersal mechanisms in global planet population synthesis models with self-consistent planet formation modules.