By Claudia Aguilera, MAS and IAUC´s postdoctoral researcher
Nowadays, when we hear discussions about lithium, it is very likely that we imagine a technological context, due to lithium being used in batteries, or in a political-economical context, given that Chile is one of the top lithium-producing countries in the world. However, it is unlikely that we think about how lithium is widely used in astrophysics to study very different problems.
Now, thanks to lithium, we have found two dying stars that have devoured some of the planets that used to orbit them.
Lithium is a scarce element in the Universe. Moreover, at the temperatures found in the interior of stars similar to the Sun, which are burning hydrogen steadily in their cores, lithium is destroyed by proton capture. Given that the exterior of Sun-like stars is colder than its interior, lithium can still exist in a thin layer close to the surface. If lithium were to move too close to the interior, proton capture would destroy it.
But stars similar to the Sun do not live forever. When they run out of fuel (hydrogen) they expand, and their temperatures decrease, so they become red giants. And the lithium that was in a thin layer around the star now dilutes into a broader layer. In consequence, we will see even less lithium in the surface of the star than before.
To summarize, red giant stars expand from 10 to 100 times their original size, they are older, on their way to death, and they have a very low lithium fraction in the surface. Unless, that is, that as the star grows, it engulfs some celestial object or external material containing a lot of lithium.
What object in the universe do we know with large lithium contents? One option is planets.
Let’s think what will happen (in a couple of billion years) when the Sun expands 10 (or 100) times its current size in the Solar System: Its surface would reach the Earth’s orbit, and even beyond. Mercury and Venus will be engulfed by the Sun, and possibly the Earth too.
Then, is it possible that the star engulfs its planets and that these, in turn, leave a visible signal in the star, like a testimony that they once existed? To answer this, we modeled in a first paper this catastrophic event to predict the signals and effects that we could observe in stars . In a second work, we studied the lithium abundances of stars in the open cluster Trumpler 20, which the Gaia-ESO Survey obtained. With this information, we find that 2 of these giants have unusually high lithium abundances, that can be explained if the stars have devoured the planets around them .
Open clusters, groups of gravitationally bound stars, are especially useful to find chemically unusual objects. This is because the stars in an open cluster are born at roughly the same time, in the same plane, so we expect that their abundances are also very similar. When we find stars in the cluster that do not look like their sisters, we suspect that something may have happened to them. The abundance of lithium in the two enriched stars coincides with the predictions of our models, which include planet engulfment.
So what is left to be done? We have to confirm that these stars have swallowed their unfortunate planets, and find more systems like these. Among other things, studying this process allows foreseeing of what will happen when its the Sun’s turn to grow, and it’s our turn to be its lunch.
But do not worry about the Sun devouring Earth… for that to happen we will have to wait 5 billion years more.
Main image: A typical open cluster, NGC 290 observed with the Hubble Space Telescope (Credit: ESA & NASA).