What Really Drains a Star’s Lithium? It’s Not Where It’s Been, But What It Is

In a follow-up to their earlier work, Dantas et al. continue exploring the mystery of lithium (Li) depletion in stars, focusing on thin-disc dwarf stars in our Milky Way. Lithium, one of the earliest elements formed after the Big Bang, is fragile: while it can be created in several astrophysical events, it is also easily destroyed inside stars. This study, the second in the "Probing the Origins" series, asks a key question: do stars lose their lithium because of how they move through the Galaxy, or because of the physical traits they were born with?

Sorting Stars by Metal and Motion

To investigate this, the authors analyzed 1,188 stars from the Gaia-ESO survey. These stars were grouped by metallicity—a measure of how many heavy elements they contain—using a method called hierarchical clustering. They were also sorted based on how their orbits had changed over time: some moved inward toward the Galactic center, others outward, and some stayed roughly in place. Past research hinted that stars that moved outward tended to show lower lithium, raising the possibility that motion (or "radial migration") might be to blame for the lithium loss.

Initial Findings: Traits of Li-Depleted Stars

However, the authors found that the situation is more nuanced. The stars that had moved outward were indeed older, cooler, and less massive—and these qualities were consistent across metallicity groups. Because these physical characteristics are known to encourage lithium destruction in stellar interiors, the team concluded that these factors, not the motion itself, are the true culprits. Stars that churned inward were generally hotter and younger, preserving more of their lithium. Interestingly, lithium depletion was nearly universal in their sample—suggesting that most of these stars had lost lithium regardless of their path through the Galaxy.

Digging Deeper: A Statistical Test of Lithium Survival

To dig deeper, the team used a statistical technique known as survival analysis. Originally designed to study things like the lifespan of patients in medical studies, this method is helpful when dealing with "censored" data—like stars with only upper limits for their lithium measurements. By modeling the probability of a star’s lithium abundance falling below a certain threshold, they were able to pinpoint which factors mattered most.

Key Results: It’s All About Temperature, Metallicity, and Age

The results were striking: a star’s effective temperature (Teff) had the largest influence on whether it retained lithium. Hotter stars were much better at holding on to their lithium. Metallicity (the amount of elements heavier than helium) was next in line, with higher metallicity stars losing lithium faster. Age also played a role—older stars tended to be more depleted—but to a lesser extent. Crucially, the direction of a star's motion had only a tiny effect, reinforcing the idea that it is the star’s internal properties, not its orbit, that determine lithium survival.

Interpreting the Patterns: Why Migration Still Matters (a Little)

Although the movement of stars doesn't directly cause lithium loss, it still plays a role in shaping what kinds of stars we observe in certain parts of the Galaxy. For example, stars that migrated outward from the inner Galaxy tend to be older and metal-rich—traits that also make them more prone to lithium depletion. So, while migration doesn’t deplete lithium, it does help bring depleted stars into regions like the solar neighborhood, where they are then observed in surveys like Gaia-ESO.

Conclusion: Internal Clocks, Not Cosmic Journeys

In summary, Dantas et al. show that lithium depletion in stars is primarily driven by what the stars are like—not where they’ve been. The apparent connection between movement and lithium levels arises because stars that migrate outward just happen to be the kind that deplete lithium more effectively: old, cool, and metal-rich. Their findings not only refine our understanding of lithium in stars but also serve as a caution: lithium levels in a star’s atmosphere may not reflect the interstellar medium from which it formed, especially if the star is cool and metal-rich, like our Sun.

Source: Dantas