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Planetary systems with sufficiently small orbital spacings can experienceplanetary mergers and ejections. The branching ratio of mergers vs ejectionsdepends sensitively on the treatment of planetary close encounters. Previousworks have adopted a simple "sticky-sphere" prescription, whose validity isquestionable. We apply both smoothed particle hydrodynamics and $N$-bodyintegrations to investigate the fluid effects in close encounters between gasgiants and the long-term evolution of closely-packed planetary systems.Focusing on parabolic encounters between Jupiter-like planets with $M_J$ and$2M_J$, we find that quick mergers occur when the impact parameter $r_p$ (thepericenter separation between the planets) is less than $2R_J$, and the mergerconserved 97% of the initial mass. Strong tidal effects can affect the"binary-planet" orbit when $r_p$ is between $2R_J$ and $4R_J$. We quantifythese effects using a set of fitting formulae that can be implemented in$N$-body codes. We run a suite of $N$-body simulations with and without theformulae for systems of two giant planets initially in unstable, nearlycircular coplanar orbits. The fluid (tidal) effects significantly increase thebranching ratio of planetary mergers relative to ejections by doubling theeffective collision radius. While the fluid effects do not change thedistributions of semi-major axis and eccentricity of each type of remnantplanets (mergers vs surviving planets in ejections), the overall orbitalproperties of planet scattering remnants are strongly affected due to thechange in the branching ratio. We also find that the merger products have broaddistributions of spin magnitudes and obliquities.

Giant Planet Scatterings and Collisions: Hydrodynamics, Merger-Ejection Branching Ratio, and Properties of the Remnants