Moderately eccentric warm Jupiters from secular interactions with exterior companions

Abstract

Recent studies have proposed that most warm Jupiters (WJs, giant planets with semimajor axes in the range of 0.1-1 au) probably form in situ, or arrive in their observed orbits through disc migration. However, both in situ formation and disc migration, in their simplest flavours, predict WJs to be in low-eccentricity orbits, in contradiction with many observedWJs that are moderately eccentric (e = 0.2-0.7). This paper examines the possibility that theWJ eccentricities are increased by secular interactions with exterior giant planet companions, following in situ formation or migration on a circular orbit. Eccentricity growth may arise from an inclined companion (through Lidov-Kozai cycles), or from an eccentric, nearly coplanar companion (through apsidal precession resonances). We quantify the necessary conditions (in terms of the eccentricity, semimajor axis and inclination) for external perturbers of various masses to increase the WJ eccentricity. We also consider the sample of eccentricWJs with detected outer companions, and for each system, identify the range of mutual inclinations needed to generate the observed eccentricity. For most systems, we find that relatively high inclinations (at least ~40°) are needed so that Lidov-Kozai cycles are induced; the observed outer companions are typically not sufficiently eccentric to generate the observedWJ eccentricity in a low-inclination configuration. The results of this paper place constraints on possibly unseen external companions to eccentricWJs. Observations that probe mutual inclinations of giant planet systems will help clarify the origin of eccentric WJs and the role of external companions.