Title: Formation of wide-orbit gas giants near the stability limit in multi-stellar systems,
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.