Mining for the Ancient: A New Catalog of Metal-Poor Stars from LAMOST DR10

In this study, Xiangyu Li and collaborators aim to unlock secrets of the early Universe by cataloging very metal-poor (VMP) stars—stars with iron abundances less than or equal to [Fe/H] = −2.0. These ancient objects formed from gas clouds that existed shortly after the Big Bang, before many heavier elements (or "metals") had been created by successive generations of stars. Because they preserve information about the early cosmos, VMP stars are crucial for understanding how the Milky Way formed and evolved. However, finding them is difficult because they are both rare and faint, often eluding traditional detection methods.

A New Way to Search with LAMOST

LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope) is a powerful instrument in China that has gathered over 12 million stellar spectra. Previous searches for VMP stars relied heavily on the blue part of the spectrum, where metal absorption lines are more abundant. But for very metal-poor stars, these features become too weak to detect clearly. Instead, this study focuses on the red portion of the spectrum, where three strong lines of ionized calcium—known as the Calcium Triplet (CaT)—remain visible even in stars with extremely low metallicity. By taking advantage of this high signal-to-noise region, the authors improved both the reliability and depth of their search.

Two Methods for Estimating Metallicity

The researchers used two main techniques to estimate how metal-poor each star is. The first, called Method 1, involves measuring the absolute brightness (or absolute magnitude) of a star, combining that with the strength of the CaT lines to estimate [Fe/H] using a formula from earlier work by Carrera et al. (2013). This method works best for stars whose distances are well-known. For fainter or more distant stars, the team introduced Method 2, which replaces the need for distance information with two other measurable properties: color (Gaia BP-RP) and surface gravity (log g). This innovation allowed them to include stars located farther than 6,000 light-years away.

Carefully Processing the Spectra

To prepare the data, the team corrected for each star’s motion using radial velocities, removed poor-quality spectra, and then zoomed in on the region of the spectrum around the CaT lines. They used computer algorithms to fit smooth curves to the data and measure the strength of each calcium line, even making adjustments when one of the lines was too faint to detect. This process resulted in a CaT index, which could be reliably converted into a metallicity value. Stars were flagged based on which lines could be measured, ensuring that uncertainties were accounted for.

Testing the Results Against Other Surveys

A key strength of this study lies in its rigorous validation of results. The researchers compared their metallicity estimates with those from several respected sources, including Gaia RVS, the APOGEE and GALAH surveys, and high-resolution studies such as PASTEL and SAGA. Their findings showed strong agreement: the typical difference (or offset) between estimates was only about 0.1 dex, and the scatter around this value was about 0.2 dex. These small differences suggest that both Method 1 and Method 2 are robust tools for identifying VMP stars from low-resolution data.

What the Catalog Contains

The final product is a catalog of 8,440 VMP stars, most of which are in the northern sky. The stars have metallicities ranging from [Fe/H] = −2.0 down to as low as −4.0, including 25 extremely metal-poor (EMP) stars with [Fe/H] ≤ −3.0. Impressively, over 7,000 of these stars are brighter than G ≈ 16, making them accessible to follow-up observations using larger telescopes. The catalog also includes information like distance, proper motion, and radial velocity, allowing researchers to explore the origins and motions of these stars within the Galaxy.

Looking Ahead

This catalog lays the groundwork for future studies of the Milky Way’s oldest stars. As surveys like Gaia continue to improve measurements of distance and motion, and with new releases from LAMOST and other projects on the way, the methods pioneered in this work will become even more powerful. By identifying and characterizing more VMP stars, astronomers will be better equipped to trace the chemical and dynamical history of our Galaxy all the way back to its earliest stages.

Source: Li