Simulations reveal hydrodynamics of planetary engulfment of expanding star

Simulations reveal hydrodynamics of planetary engulfment of expanding star

News – When our sun takes out the hydrogen fuel at its core in about 5 billion years from now, it will expand to become a red giant that engulfs the inner planets. The dynamics and possible results of planetary immersion are poorly understood, but it is considered a relatively common fate for planetary systems.

A new study using hydrodynamic simulations reveals the forces acting on a planet when it is swallowed by an expanding star. The results show that the interaction between a substellar body (a planet or brown dwarf) with the hot gas in the outer shell of a sun-like star can lead to a variety of outcomes depending on the size of the engulfed object and the stage of the star’s evolution.

The lead author Ricardo Yarza at the University of California, Santa Cruz, will present the new findings on June 13, 2022, at 240th meeting of the American Astronomical Society (AAS) and Pasadena.

“Evolved stars can be hundreds or even thousands of times larger than their planets, and this difference in scales makes it difficult to perform simulations that accurately model the physical processes that take place on each scale,” said Yarza, a doctoral student in astronomy and astrophysics at UCSC. “Instead, we simulate a small part of the star centered on the planet to understand the flow around the planet and measure the tensile forces acting on it.”

The results may help explain recent observations of planets and brown dwarfs orbiting near stellar remnants such as white dwarfs and sub-dwarfs. Previous studies have suggested that these systems may be the end result of a planetary immersion process that involves shrinking the orbital body’s orbit and expelling the star’s outer layer.

“As the planet travels inside the star, traction forces transfer energy from the planet to the star, and the stellar envelope can become unbound if the transferred energy exceeds its binding energy,” Yarza explained.

According to calculations by Yarza and his colleagues, no substellar body less than about 100 times the mass of Jupiter can shoot out the shell of a sun-like star before it has expanded to about 10 times the radius of the Sun. At later stages of stellar evolution and expansion, however, the stellar envelope could be ejected by an object as small as ten times the mass of Jupiter, which would shrink its orbit by several orders of magnitude in the process.

The study also found that planetary immersion can increase the brightness of a sun-like star of several magnitudes for up to several thousand years, depending on the mass of the immersed object and the evolutionary stage of the star.

The framework provided by this study may be incorporated into future work to explore the effect of immersion on the structure of the star. “Our work can inform simulations of planetary immersion in the scale of the star by providing an accurate reference image of the physics in the scale of the planet,” said Yarza.

A large variety of planetary systems have now been described by exoplanet search programs. As these systems evolve, a significant portion are likely to undergo planetary engulfment. “We think it’s relatively common,” Yarza said.

An article about the new findings has been submitted for publication in Astrophysical journal and is available online at The essay’s senior authors are Enrico Ramirez-Ruiz, professor of astronomy and astrophysics, and Dongwook Lee, associate professor of applied mathematics, both at UC Santa Cruz.

Ramirez-Ruiz said he was impressed with Yarza’s work on this project. “There are many ingredients for success at the highest levels of astrophysics research, including creativity, taste in the selection of key questions, strength and breadth of knowledge, ability to communicate scientific results, technical mastery and independence. Ricardo stands out because his vector is great in all of these. basic dimensions, he said.

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