Lithium Reveals the Secrets of Young Stars express
Lithium is like a thermometer dipped into a star's interior. This light element is destroyed at temperatures around 2.5 million degrees, so its abundance on the surface tells us how deeply the star's material has been mixed.
Spectra of 71 young intermediate-mass stars (from 1.5 to 3.5 solar masses) show that 25–30% of them have noticeably less lithium than the original cloud. For low-mass stars, this happens more often, but for intermediate-mass stars it's a surprise. The cause is rapid rotation: like a figure skater pulling in their arms, the star spins up, enhancing mixing that carries lithium into the interior, where it perishes.
For intermediate-mass stars, this transition occurs about twice as fast as for low-mass stars. The star's magnetic field, which previously anchored to the disk, contracts, and the star spins up three times faster, while material from the disk falls onto it four times more actively. This is similar to a cyclist who lets go of the handlebars — the wheel picks up speed.
Perhaps the same link between lithium and rotation explains why stars with planets have less lithium: planets may have influenced the star's early rotation, which in turn affected lithium's fate. It's a chain: from the disk and planets to rotation, and from rotation to lithium.
Original abstract
as written by the authors arXiv:2604.05040
(Abridged) We contribute to our understanding of the evolution of young intermediate-mass stars by providing a comprehensive analysis of their lithium (Li) content. A sample of 71 intermediate-mass T Tauri (IMTT) and Herbig stars within the mass range 1.5 -- 3.5 M$$_{\odot}$$ was carefully selected for the analysis. Metallicities, rotational velocities, and accretion rates were obtained from spectra. The curves of growth for stars hotter than 8000 K were built to infer the Li abundances, which were interpreted considering standard models of stellar interiors and non-standard processes affecting Li depletion. Li is generally less strongly depleted in intermediate-mass stars than in their lower-mass counterparts, as expected from standard evolution models. However, Li abundances significantly below the cosmic value are observed in 25 -- 30$$\%$$ of intermediate-mass stars. It is also unexpected that the results show no significant difference between the 1.5 --2.5 M$$_{\odot}$$ and 2.5 -- 3.5 M$$_{\odot}$$ subsamples. Evidence is provided showing that disk-locking works in young intermediate-mass stars. This constitutes independent support for the hypothesis that magnetospheric accretion scenario operates in these sources. We found that disk-locking is effective for a timescale that is about twice shorter than for lower-mass stars, before magnetospheres reduce their sizes during the transition from the IMTT to the Herbig regime. This contraction of the magnetosphere can explain the increase in rotation by a factor of about 3 and in accretion by a factor of about 4 that is observed during this transition. We propose a complex scenario linking rotation, accretion, and the surface Li abundance. Finally, we tentatively suggest that the known relation between the presence of planets and Li depletion might also be present in intermediate-mass main sequence (MS) stars and might originate in the pre-MS.
Lithium is one of the three elements born in the first minutes after the Big Bang, along with hydrogen and helium. Stars mostly destroy it rather than create it.
In Liu Cixin's novel "The Three-Body Problem," aliens study stars to understand their evolution; the lithium test is from the same arsenal of astrophysical reconnaissance.