Hot Jupiters and Cool Stars

TL;DR

This study investigates how stellar evolution affects the orbital fate of close-in planets, highlighting the roles of planetary mass, stellar mass-loss, and eccentricity in planet engulfment during the red giant phase.

Contribution

It provides new insights into the factors influencing planet engulfment, especially the impact of planetary mass and stellar tides during stellar evolution.

Findings

Planet engulfment sensitivity to planetary mass and initial separation.

Stellar tides dominate over planetary tides in evolved stars.

Eccentricity decay is minimal for distant, highly eccentric planets.

Abstract

Close-in planets are in jeopardy as their host stars evolve off the main sequence to the subgiant and red giant phases. In this paper, we explore the influences of the stellar mass (in the range 1.5--2\Mso ), mass-loss prescription, planet mass (from Neptune up to 10 Jupiter masses), and eccentricity, on the orbital evolution of planets as their parent stars evolve to become subgiants and Red Giants. We find that planet engulfment during the Red Giant Branch is not very sensitive to the stellar mass or mass-loss rates adopted in the calculations, but quite sensitive to the planetary mass. The range of initial separations for planet engulfment increases with decreasing mass-loss rates or stellar mass and increasing planetary masses. Regarding the planet's orbital eccentricity, we find that as the star evolves into the red giant phase, stellar tides start to dominate over planetary tides. As a consequence, a transient population of moderately eccentric close-in Jovian planets is created, that otherwise would have been expected to be absent from main sequence stars. We find that very eccentric and distant planets do not experience much eccentricity decay, and that planet engulfment is primarily determined by the pericenter distance and the maximum stellar radius.