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Study sheds light on how expanding stars swallow planets

New simulations show how planetary engulfment plays out on varying sizes of stars and planets.

(CN) — Around 5 billion years from now, the sun will run out of hydrogen in its core and turn into a red giant that devours the inner planets of our solar system.

This process, called planetary engulfment, is thought to be a relatively common fate for planetary systems but is still little understood.

A new study using hydrodynamical simulations examines the forces acting on a planet when an expanding star swallows it, revealing the various possible interactions of a substellar body of a planet or brown dwarf. Depending on the size of the engulfed object and the stage of a star’s evolution, the hot gas in the outer envelope of a sun-like star can lead to a range of outcomes, researchers say.

Lead author Ricardo Yarza from the University of California, Santa Cruz, will present the new findings Monday at the 240th meeting of the American Astronomical Society in Pasadena.

“Evolved stars can be hundreds or even thousands of times larger than their planets, and this disparity of scales makes it difficult to perform simulations that accurately model the physical processes occurring at each scale,” Yarza said in a press release. “Instead, we simulate a small section of the star centered on the planet to understand the flow around the planet and measure the drag forces acting on it.”

The new research may help explain recent observations of planets and brown dwarfs as they closely orbit stellar remnants such as white dwarfs and subdwarfs.

Previous studies suggested that these systems may be the result of planetary engulfment processes that involve shrinking of the engulfed body’s orbit and ejection of the outer layers of the star.

“As the planet travels inside the star, drag 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 said.

Yarza and his team calculate that no substellar bodies smaller than about 100 times the mass of Jupiter can eject the envelope 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 10 times the mass of Jupiter, shrinking its orbit by several orders of magnitude in the process.

The study also found that planetary engulfment can increase the luminosity of a sun-like star by several orders of magnitude for up to several thousand years, depending on the mass of the engulfed object and the evolutionary stage of the star.

The team says their findings could be incorporated into future work exploring the effect of engulfment on the structure of the star.

“Our work can inform simulations of planetary engulfment at the scale of the star by providing an accurate reference picture of the physics at the scale of the planet,” Yarza said.

Many varying planetary systems have been identified by exoplanet search programs. As these systems evolve, many will likely undergo planetary engulfment, Yarza said.

“We believe it is relatively common,” he said.

A paper on these findings has been submitted to Astrophysical Journal for publication and is available to read online. Senior authors on the paper are Enrico Ramirez-Ruiz, professor of astronomy and astrophysics, and Dongwook Lee, associate professor of applied mathematics, both at UC Santa Cruz.

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