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The hydrodynamics of planetary engulfment

Presentation #132.05 in the session Extrasolar Planets: Populations I.

Published onJun 29, 2022
The hydrodynamics of planetary engulfment

In many planetary systems, the host star will expand during post-main-sequence evolution to engulf close-orbiting planets and/or brown dwarfs (hereafter substellar bodies, SBs). Substellar engulfment stands as a possible explanation to stellar remnants with close-orbiting SB, evolved stars with unusually high surface abundances of the 7Li lithium isotope, and rapidly rotating evolved stars.

A complete understanding of this process remains elusive and challenging because it involves a wide range of spatial and temporal scales. The size of the SB can be as small as one-thousandth of the size of the giant star. This disparity of scales motivates an approach that isolates the processes at each scale, both for computational feasibility and to enable a thorough understanding of each of the processes, gradually building towards an understanding of engulfment as a whole.

Here we present the results of “local” simulations—modeling only the part of the envelope in the vicinity of the SB. These simulations help us understand the local flow morphology, and to quantitatively determine the drag forces that drive orbital decay. We used these drag forces to approximately integrate the trajectory of the SB inside the envelope without hydrodynamical simulations that account for the internal structure of the star. This simplified numerical framework enables exploration of a wide parameter space, as well as population synthesis models.

We found that engulfment of an SB between one and one hundred Jupiter masses can increase the luminosity of the star by up to several orders of magnitude for between one and a few thousand years, depending on the mass of the engulfed SB and the evolutionary stage of the star. Using analytical estimates, we also found that early in the red giant branch, smaller SBs will be destroyed in the convective zone, yielding potential enhancements in stellar surface lithium abundances. Similarly, we provide estimates for the minimum mass required to eject the envelope as a function of evolutionary stage.

These findings complement observational advances by missions such as Kepler and TESS, which when coupled with models of planetary system evolution show that SB engulfment is common. These extreme systems represent a new frontier in exoplanet studies with the potential to offer critical constraints on the late-stage evolution of planetary systems. In future work, we will study how engulfment changes the internal structure of the SB, and determine the abundance signatures expected among SBs that survive engulfment. Such signatures could potentially be detected using transmission spectroscopy from JWST.

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