Tracing the Earliest Stars: A Guide to the DECam MAGIC Survey

The study by Placco and collaborators follows up on one of astronomy’s most compelling quests: finding the oldest surviving stars in the Milky Way. These objects, called extremely and ultra metal-poor (EMP and UMP) stars, contain so little heavy-element material that they act as “fossils,” preserving information about the first stars that formed after the Big Bang. In this paper, the authors report high-resolution spectroscopic observations of six such stars, all originally identified in the new DECam MAGIC Survey using a special narrowband filter sensitive to the Ca II H and K lines. The overarching goal is to test whether these photometric metallicity estimates are reliable and to learn what these rare stars reveal about conditions in the early Universe.

Target Selection and Observations

To select promising targets, the team began with a catalog of roughly 21 million stars and focused on objects whose colors indicated very low metallicity. They required the stars to be bright enough for detailed spectroscopy and to exhibit distances beyond 30 kiloparsecs, placing them in the distant Milky Way halo. With these candidates identified, the authors used the MIKE spectrograph to record high-resolution spectra that reveal absorption lines from elements such as iron, carbon, magnesium, and barium. This step allowed them to measure each star’s effective temperature, surface gravity, chemical composition, and radial velocity with high precision.

Atmospheric Parameters and Chemical Abundances

Once detailed stellar parameters were established, the authors derived abundances for up to 16 chemical elements in each star. These measurements confirmed that five targets fall into the EMP regime, while one star, J0433−5548, has an even lower metallicity, [Fe/H] = –4.12, qualifying it as an UMP star. The team also examined how well the photometric metallicities from MAGIC agreed with these spectroscopic results. For most stars the agreement was strong, differing by only about 0.07 dex. However, for the carbon-enhanced star J0433−5548, the photometric metallicity was too high because strong carbon absorption affects the narrowband CaHK measurement, mimicking the appearance of a more metal-rich star. This demonstrates that carbon enhancement must be considered when identifying the most metal-poor stars photometrically.

The Special Case of J0433−5548

J0433−5548 receives special attention because it is a CEMP-no star, showing strong carbon enhancement without corresponding increases in heavy neutron-capture elements like barium. Its chemical abundance pattern suggests it may be a true second-generation star, one whose birth cloud was enriched by only a single Population III supernova. Using theoretical models from starfit, the authors found that its best-matching progenitor likely had a mass of about 11 M⊙ and a relatively low explosion energy. This makes the star an especially valuable probe of the earliest chemical enrichment events, providing a rare window into the properties of the first stars.

Kinematics and Dynamics

Finally, the team investigated the kinematics and orbital dynamics of the six stars using Gaia measurements and their newly obtained radial velocities. All targets belong to the distant halo, but several appear tied to specific Galactic structures. One star shows characteristics consistent with the Sagittarius tidal stream, three may be connected to the Magellanic system, and two match expectations for debris from the Gaia-Sausage/Enceladus merger event. These associations help trace how the Milky Way grew over time through interactions with smaller galaxies.

Conclusions

Overall, this first high-resolution follow-up from the DECam MAGIC Survey demonstrates that narrowband CaHK photometry is a powerful tool for efficiently discovering EMP and UMP stars, especially in the remote halo, where such stars are hardest to find. By confirming new members of this rare population and revealing their chemical and dynamical histories, the study highlights how metal-poor stars continue to shape our understanding of the earliest phases of star formation and the assembly of the Milky Way.

Source: Placco