New Extremely R-Process-Enhanced Star Detected in the Milky Way's Thin Disk

A team of astronomers from the Chinese Academy of Sciences (CAS) and other institutions has made a groundbreaking discovery using the Gran Telescopio Canarias (GTC). They have identified a new extremely r-process-enhanced star, designated LAMOST J020623.21+494127.9 (J 0206+4941), in the Milky Way's thin disk. This finding, reported in a research paper published on July 16 on the pre-print server arXiv, sheds new light on the assembly history of our galaxy.

Comparison of high-resolution spectra of J 0206+4941 and a moderately r-process-enhanced (r-I) star, HD 124897 ([Eu/Fe] = +0.45, with similar stellar parameters. Credit: Xie et al., 2024.

R-process-enhanced (RPE) stars are a rare subset of old, metal-poor stars that exhibit significant enhancements in elements produced through the rapid neutron-capture process (r-process). These elements include europium, thorium, and uranium. RPE stars are primarily found in the galactic halo and dwarf satellite galaxies of the Milky Way, making them valuable laboratories for studying the r-process.

The research team, led by Xiao-Jin Xie of CAS, employed the High Optical Resolution Spectrograph (HORuS) on the GTC to observe J 0206+4941. Initially selected from the LAMOST medium-resolution survey due to its unusually strong europium lines, the star was confirmed as an extremely r-process-enhanced star through spectroscopic analysis.

The observations revealed that J 0206+4941 is a bright star belonging to the Milky Way's thin disk, with a metallicity of -0.54, an effective temperature of approximately 4,078 K, and an abundance ratio of europium to hydrogen of 0.78. The abundances of light elements in this star are comparable to those of similar stars, with a strong enhancement of europium and modest enhancement of barium.

J 0206+4941 is the most metal-rich highly r-process-enhanced star discovered to date, with an estimated age of 12.3 billion years. The origin of this star is uncertain, but the researchers suggest it may have formed in-situ in the Milky Way's thin disk, potentially enriched by a single binary neutron star merger or core-collapsed supernovae.

The discovery of J 0206+4941 and several other metal-rich RPE candidates highlights the need for further investigation using advanced instruments like HORuS. Continued research into RPE stars will provide valuable insights into the assembly history of the Milky Way and the r-process.