Tracing the Heavy Elements: How Neutron-Capture Chemistry Connects Stars and Planets

This study, led by Sharma and collaborators, investigates the abundances of neutron-capture elements in stars known to host planets. The authors focus on nine heavy elements, strontium (Sr), yttrium (Y), zirconium (Zr), barium (Ba), lanthanum (La), cerium (Ce), neodymium (Nd), praseodymium (Pr), and europium (Eu), that are produced through two key processes in stars: the slow neutron-capture process (s-process) and the rapid neutron-capture process (r-process). While previous research explored lighter elements in planet-hosting stars (PHSs), this work expands the picture to these heavier, less-studied building blocks.

Observations and Methods

The team studied 160 bright F-, G-, and K-type stars in the northern sky, using high-resolution spectroscopy from the 1.65-meter telescope at the Molėtai Astronomical Observatory in Lithuania. Eighty-six of these stars are dwarfs (like the Sun) and 74 are giants (more evolved). For each star, the authors used a technique called spectral synthesis to model how starlight is absorbed by different atoms, comparing these observations to solar measurements to reduce systematic errors. They also corrected for non-local thermodynamic equilibrium (NLTE) effects, which can slightly shift abundance measurements, especially for elements like Sr, Y, Ba, and Eu.

Element Abundances and Metallicity Trends

The study finds that, for most neutron-capture elements, abundance patterns in PHSs follow the general chemical evolution of the Milky Way. For light s-process elements (Sr, Y, Zr), abundance ratios relative to iron ([El/Fe]) generally increase as metallicity ([Fe/H]) decreases, though Sr and Y flatten at lower metallicities while Zr continues to rise. Heavy s-process elements (Ba, La, Ce) show mixed behavior, Ba remains mostly flat at low metallicities, but La and Ce increase and appear overabundant in PHSs compared to similar stars without planets. Mixed-process elements (Pr, Nd) and the r-process element Eu also increase as metallicity drops, with Eu enriched in metal-poor stars as expected from its production in massive stars.

Connections Between Chemistry and Planet Mass

The authors examined whether the abundance of these elements correlates with the mass of the largest planet in each system. They find that Sr, Y, and Ba show no strong correlation with planet mass, while Zr, La, Ce, Pr, Nd, and Eu often have moderate to strong positive correlations, especially in giant stars hosting massive planets. This suggests that certain heavy elements might be linked to the processes that form or sustain large planets, though the effects are subtle and can be masked by broader Galactic trends.

Age Clocks and Chemical Signatures

By comparing the ratio of Y to magnesium ([Y/Mg]), an established “chemical clock”, the team confirmed that younger, metal-rich dwarf stars tend to have higher [Y/Mg] and higher abundances of Sr, Y, and Zr. This supports earlier work suggesting that stellar age and metallicity influence both element production and planet occurrence.

Element Abundances and Condensation Temperatures

The paper also explores how abundances vary with condensation temperature, which distinguishes volatile elements (like C, N, O) from refractory ones (like Mg, Si, and the neutron-capture elements). Comparing each planet-hosting star to similar stars without planets, they find a slight tendency for PHSs to be richer in refractory elements, a possible fingerprint of planet formation. However, this pattern is weaker in older dwarf stars and stronger in younger, metal-rich systems with multiple planets.

Conclusions

Overall, the study finds that most neutron-capture element abundances in planet-hosting stars align with normal Galactic evolution, but some elements, notably Zr, La, and Ce, may be slightly enhanced in these stars. While correlations between heavy-element enrichment and planet mass exist for certain elements, the evidence remains tentative. These findings add to the growing picture that stellar chemistry holds clues to planetary system formation, but that these clues must be interpreted carefully against the backdrop of the Milky Way’s own chemical history.

Source: Sharma