The hydrodynamics of planetary engulfment

Stars expand by up to a couple orders of magnitude during the post-main-sequence. In many planetary systems, this expansion will lead to the engulfment of nearby planets and/or brown dwarfs (hereafter substellar bodies, SBs). The engulfment of an SB could explain observations of evolved stars with unusually high surface abundances of the ⁷Li lithium isotope and/or unusually high rotation rates. Similarly, engulfment could explain the presence of SBs in close orbits around stellar remnants.

A complete understanding of engulfment remains elusive, partly because it involves a wide range of spatial and temporal scales. Giant stars can be around a thousand times larger than Jupiter. This disparity of scales motivates an approach that isolates the processes at each scale, both for computational feasibility and to thoroughly understand of each of the processes, gradually building towards an understanding of engulfment as a whole.

I will present the first step in this approach: “local” simulations that model the region of the star in the vicinity of an engulfed SB. We use these simulations to qualitatively characterize the morphology of the flows around engulfed SBs, and quantitatively determine the drag forces acting on them. We then use these drag forces to integrate the equation of motion of the companion and understand its dynamics inside the star.

We found that engulfment can increase the luminosity of a 1M⊙ star by up to a few orders of magnitude. The time it takes for the star to return to its original luminosity is up to a few thousand years when the star has evolved to ≈10R⊙ and up to a few decades at the tip of the red giant branch. These results suggest engulfment is energetically significant throughout the post-main-sequence. Early in the red giant branch, small SBs will be destroyed in the convective zone, yielding potential enhancements in stellar surface lithium abundances. SBs cannot eject the envelope of a 1M⊙ star before it evolves to ≈10R⊙, if the orbit of the SB is the only energy source contributing to the ejection. In contrast, SBs as small as ≈10MJup can eject the envelope at the tip of the red giant branch.

These findings complement observational advances by missions such as Kepler and TESS which, together with models of long-term planetary system evolution, show that SB engulfment is common. These extreme events can help us understand the late-stage evolution of planetary systems. The numerical framework we introduced here can be used to study the dynamics of engulfment using inexpensive simulations that capture the physics of the flow at the scale of an engulfed SB.