[Paper Review] Hot Jupiter engulfment by a red giant in 3D hydrodynamics

Mike Y. M. Lau, Matteo Cantiello+4 more|arXiv (Cornell University)|Oct 28, 2022

Stellar, planetary, and galactic studies4 citations

TL;DR

This study presents 3D hydrodynamical simulations of a hot Jupiter (1 $M_{\rm{J}}$) engulfed by a 1 $M_{\odot}$ red giant, modeling the planet as a polytropic gas sphere. It finds that ~90% of the planet's mass is ablated in the convective envelope, enhancing surface lithium by ~0.1 dex, with modest spin-up (~1 km s$^{-1}$) and potential for a transient powered by recombination or drag heating.

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 solar mass, 4 solar radii 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), 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.

Motivation & Objective

• To simulate the full 3D hydrodynamics of hot Jupiter engulfment by a red giant, resolving both stellar and planetary structures simultaneously.
• To quantify mass ablation, angular momentum transfer, and chemical enrichment during planetary engulfment.
• To assess the potential for observable signatures such as surface Li enhancement, stellar spin-up, and luminous transients.
• To evaluate the robustness of results with respect to numerical resolution and heating effects.
• To explore the conditions under which planetary engulfment could explain Li-rich and rapidly rotating giant stars.

Proposed method

• 3D hydrodynamical simulations using the smoothed particle hydrodynamics (SPH) code are performed to model the interaction between a 1 $M_{\odot}$ red giant and a 1 $M_{\rm{J}}$ hot Jupiter.
• The hot Jupiter is modeled as a polytropic gas sphere with a polytropic index $n = 3$, allowing self-consistent treatment of internal structure and ablation.
• The simulations resolve the stellar envelope and planetary interior globally, capturing ram pressure, tidal forces, and convective mixing throughout the engulfment process.
• The host star is initialized using a MESA stellar evolution model with a 4 $R_{\odot}$ radius and a convective envelope extending to ~0.8 $R_{\odot}$.
• The simulation tracks mass loss, angular momentum deposition, and energy dissipation, with diagnostics on surface enrichment and rotational velocity.
• Resolution tests are conducted to assess convergence of ablation and heating, particularly during the grazing phase.

Experimental results

Research questions

• RQ1 To what extent is the hot Jupiter ablated during its passage through the red giant's convective envelope?
• RQ2 What is the resulting surface enrichment in lithium and other elements due to planetary material mixing?
• RQ3 How much angular momentum is transferred to the star, and what is the resulting spin-up of the stellar envelope?
• RQ4 Can the energy dissipated via drag heating or recombination produce a detectable optical transient?
• RQ5 How robust are the results to numerical resolution, particularly in the early grazing phase?

Key findings

• Approximately 90% of the hot Jupiter's mass is ablated in the convective envelope, with the majority of the material mixing into the outer layers.
• The surface lithium abundance is enhanced by approximately 0.1 dex, though this is not statistically significant given intrinsic variations in open clusters.
• The star experiences modest spin-up, with the bulk of the envelope rotating at ~1 km s$^{-1}$, consistent with angular momentum conservation.
• The maximum induced surface rotational velocity scales linearly with the mass of the engulfed object, reaching ~10 km s$^{-1}$ for a 10 $M_{\rm{J}}$ companion.
• Drag heating near the surface could exceed the unperturbed stellar luminosity, potentially powering a luminous transient.
• A small amount of unbound ejecta (~10$^{-5}$ $M_{\odot}$) could power a recombination-driven transient with luminosity ~10 times the pre-engulfment level for several days.