Formation of wide-orbit gas giants near the stability limit in multi-stellar systems

TL;DR

This paper models the formation of wide-orbit gas giants in multi-stellar systems, showing they often end near the stability limit due to disk truncation and core scattering, with implications for observed systems like HD131399Ab.

Contribution

It extends existing models of gas giant formation to binary systems, incorporating tidal truncation effects and explaining the observed orbit of HD131399Ab.

Findings

The model reproduces the orbit of HD131399Ab with a semimajor axis of ~80 au and eccentricity ~0.35.

Post-circularization orbits tend to be near the stability limit in binary systems.

The process involves core scattering and gas accretion from a truncated disk.

Abstract

We have investigated the formation of a circumstellar wide-orbit gas giant planet in a multiple stellar system. We consider a model of orbital circularization for the core of a giant planet after it is scattered from an inner disk region by a more massive planet, which was proposed by Kikuchi et al (2014). We extend their model for single star systems to binary (multiple) star systems, by taking into account tidal truncation of the protoplanetary gas disk by a binary companion. As an example, we consider a wide-orbit gas giant in a hierarchical triple system, HD131399Ab. The best-fit orbit of the planet is that with semimajor axis $\sim 80$ au and eccentricity $\sim 0.35$. As the binary separation is $\sim 350$ au, it is very close to the stability limit, which is puzzling. With the original core location $\sim 20$-30 au, the core (planet) mass $\sim 50 M_{\rm E}$ and the disk truncation radius $\sim 150$ au, our model reproduces the best-fit orbit of HD131399Ab. We find that the orbit after the circularization is usually close to the stability limit against the perturbations from the binary companion, because the scattered core accretes gas from the truncated disk. Our conclusion can also be applied to wider or more compact binary systems if the separation is not too large and another planet with $> \sim$ 20-30 Earth masses that scattered the core existed in inner region of the system.