What happens when red giants consume their young?

Astrophysicists have long known that as stars like the Sun age, they reach a stage where they swell into large, puffy stars called red giants, many times their youthful diameters.

These stars can even expand enough to consume some of their orbiting planets, says Ricardo Yarza, a graduate student in astronomy and astrophysics at the University of California, Santa Cruz, US. “That’s something we know is going to happen to Mercury and Venus in our solar system,” he says.

That, obviously, isn’t good for a planet. But what happens to the star when it swallows a planet?

If the planet is small, probably not much. In our own solar system, comets often fall into the Sun, with little apparent effect. And while Mercury and Venus are substantially larger than comets, they are still tiny compared to the Sun itself.

But many planetary systems have close-in planets much larger than Mercury or Venus. And a significant number of these systems will eventually see at least one planet engulfed, Yarza told a recent meeting of the American Astronomical Society in Pasadena, California.

One effect is that the planet will impart its orbital momentum to the star as it plunges into its interior. “Think of the star as a cup of coffee and the planet as a spoon,” he says. “Once you put the spoon inside the coffee and start to stir it, you’re obviously making the coffee rotate. So, once a planet enters the star, it’s kind of stirring it from the inside.”

That, he says, might explain why some giant stars are rotating abnormally quickly: “One explanation is that they engulfed a planet,” Yarza says.

Planetary engulfment might also explain why some stars are strangely rich in lithium.

That’s odd, Yarza says, because lithium is easily consumed in a Sun-sized star’s nuclear furnace, so by the time such a star reaches the later stages in its life, it shouldn’t have much left. Unless, perhaps, it had recently consumed an object too small to have a lithium-burning nuclear furnace, such as a large planet or a brown dwarf (a very cool star barely larger than a giant planet).

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Finally, he says, some white dwarfs—the remnant stars created when red giants finally fall back in on themselves and collapse to a more normal size – have planets or brown dwarf stars in close orbits around them.

These, he says, could have been formed by material ejected from the red giant when it attempted to swallow an overly large planet. “If you stir the coffee hard enough with the spoon,” Yarza says, “some of the coffee is going to spill out.” In the case of a star, the spilled material would be material from its outer layers, which could then coalesce into a new object.

In an effort to look into the details, Yarza’s team modelled the engulfment of giant planets of various sizes. They found that it is indeed possible to make a star spin fast enough to eject its outer layers, providing material from which a new planet or brown dwarf could form.

But his team also found that engulfment of a large planet could very quickly impart a stunning amount of energy to the star – enough to briefly increase its brightness by a factor of as much as 10,000.

Does that mean we might be able to spot such a process in action by finding stars that are thousands of times brighter?

In theory, maybe. In practice, Yarza says, it might be difficult, because these increases in brightness are short-lived in astronomical terms, persisting for only a few thousand years. “I think it would be hard to detect,” he says.

Yarza’s study is available online at arxiv.org/abs/2203.11227.