Hot Jupiter engulfment by an early red giant in 3D hydrodynamics

¹ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
² OzGrav: The ARC Centre of Excellence for Gravitational Wave Discovery, Australia
³ School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia
⁴ Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY 10010, USA
⁵ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁶ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, MS-16, Cambridge, MA 02138, USA

^⋆ Corresponding author; mike.lau@h-its.org

Received: 1 September 2024
Accepted: 20 January 2025

Abstract

Hot Jupiters are gas giant planets with orbital periods of a few days and are found in 0.1–1% of Sun-like stars. They are expected to be engulfed during their host star’s radial expansion on the red giant branch, which may account for observed rapidly rotating and chemically enriched giant stars. We performed 3D hydrodynamical simulations of hot Jupiter engulfment by a 1 M_⊙, 4 R_⊙ early red giant. Our ‘global’ simulations simultaneously resolve the stellar envelope and planetary structure, modelling the hot Jupiter as a polytropic gas sphere. The hot Jupiter spirals in due to ram-pressure drag. A substantial fraction of its mass is continuously ablated in this process, although the mass-loss rate is resolution dependent. We estimate that this could enhance the surface lithium abundance by up to ≈0.1 dex. The hot Jupiter is disrupted by a combination of ram pressure and tidal forces near the base of the convective envelope, with the deepest material penetrating to the radiative zone. The star experiences modest spin-up (∼1 km s⁻¹), and engulfing a more massive companion may be required to produce a rapidly rotating giant. Drag heating near the surface and hydrogen recombination in the small amount of unbound ejecta recorded in the simulation could power an optical transient, although this needs to be confirmed by a calculation that has adequate resolution at the stellar surface.