2020MNRAS.491.1369A

Many warm Jupiters (WJs) have substantial eccentricities, which are linked to their formation and migration histories. This paper explores eccentricity excitation of WJs due to planet-planet scattering, beginning with three to four planets in unstable orbits, with the innermost planet placed in the range (0.1-1) au. Such a setup is consistent with either in situ formation or arrival at sub-au orbits due to disc migration. Most previous N-body experiments have focused on 'cold' Jupiters at several au, where scattering results in planet ejections, efficiently exciting the eccentricities of surviving planets. In contrast, scattering at sub-au distances results in a mixture of collisions and ejections, and the final eccentricities of surviving planets are unclear. We conduct scattering experiments for a range of planet masses and initial spacings, including the effect of general relativistic apsidal precession, and systematically catalogue the scattering outcomes and properties of surviving planets. A comparable number of one-planet and two-planet systems are produced. Two-planet systems arise exclusively through planet-planet collisions, and tend to have low eccentricities/mutual inclinations and compact configurations. One-planet systems arise through a combination of ejections and collisions, resulting in higher eccentricities. The observed eccentricity distribution of solitary WJs (lacking detection of a giant planet companion) is consistent with roughly 60 per cent of the systems having undergone in situ scattering, and the remaining experiencing a quiescent history.