Rare Earth Elements in the Stars: Detecting Dy, Er, Lu, and Th in Cepheids

In this ninth installment of the Cepheid Metallicity in the Leavitt Law (C-MetaLL) survey, Trentin and collaborators present a major step forward in understanding the chemical composition of Classical Cepheid stars. Cepheids are variable stars that pulsate in brightness at regular intervals, and because their brightness relates directly to their pulsation period, they serve as “standard candles” for measuring cosmic distances. The study focuses on extending our knowledge of Cepheid chemical abundances and, for the first time, reports the detection of several rare elements, Dysprosium (Dy), Erbium (Er), Lutetium (Lu), and Thorium (Th), in these stars.

Expanding the Cepheid Census

Cepheids play a central role in building the cosmic distance ladder, which astronomers use to measure the scale of the Universe. However, one source of uncertainty in this ladder is the effect of a star’s metallicity (its chemical composition) on how bright it appears. The C-MetaLL survey was created to collect high-quality spectra of hundreds of Galactic Cepheids to understand this effect. In this paper, Trentin and colleagues analyze 136 high-resolution spectra of 60 stars, observed with three world-class instruments: UVES at the Very Large Telescope in Chile, HARPS-N at the Telescopio Nazionale Galileo in Spain, and PEPSI at the Large Binocular Telescope in Arizona. Many of these stars have long pulsation periods, up to 70 days, filling an important gap in existing data.

How the Team Measured the Stars

Each spectrum was carefully processed and analyzed to determine the stars’ temperatures, surface gravities, and chemical abundances. The team used well-established methods to measure how light is absorbed by specific elements in the stars’ atmospheres. To avoid confusion from overlapping signals, they performed spectral synthesis, creating computer models of how the spectra should look for different chemical compositions and matching these to the observed data. This approach allowed the researchers to estimate abundances for up to 33 elements per star, with an average uncertainty of only about 0.1 dex.

Discovery of Rare Elements

A highlight of the study is the detection of four rare, heavy elements: Dy, Er, Lu, and Th. These are produced by the r-process, a chain of nuclear reactions that occurs during extreme astrophysical events like neutron star mergers and certain supernovae. Dysprosium, Erbium, and Lutetium are “rare earth” elements, while Thorium is a radioactive element often used in cosmic age estimates. Trentin’s team identified the spectral lines of these elements, in some cases faint or blended with nearby lines, and determined their relative abundances compared to iron. Although some measurements, particularly for Dy and Th, are less certain due to blending effects, the detections provide valuable new evidence of r-process material in young stars of our Galaxy.

Mapping the Milky Way in Metal

Beyond detecting new elements, the researchers examined how metallicity changes across the Milky Way. They confirmed a negative metallicity gradient, stars farther from the Galactic center tend to contain fewer heavy elements. The slope they found (−0.064 dex per kiloparsec) agrees well with earlier studies using both Cepheids and open star clusters. For lighter and intermediate-mass elements, abundances decline steadily with distance; however, for heavier, neutron-capture elements, the gradient flattens, suggesting different formation histories. Using models of the Milky Way’s spiral arms, the team also showed that their most distant Cepheids trace outer structures such as the Perseus and Norma-Outer arms, depending on the model adopted.

What It All Means

This work demonstrates how Cepheids, once valued mainly as distance markers, can also serve as tracers of the Galaxy’s chemical evolution. By identifying rare elements typically seen only in very old or peculiar stars, Trentin and colleagues reveal that even young, massive stars like Cepheids carry the fingerprints of past cosmic events that seeded the Milky Way with heavy elements. The study’s findings strengthen the link between stellar chemistry and Galactic structure and provide key data for refining models of how the Milky Way formed and evolved over time.

Source: Trentin