Stellar Fossils from the Outer Halo: Exploring the Most Metal-Poor Stars with the DECam MAGIC Survey

In a new study led by Vinicius Placco, astronomers report the discovery of six extremely metal-poor stars in the distant halo of the Milky Way, including one ultra metal-poor (UMP) star with an iron content more than 10,000 times lower than the Sun’s. These stars were identified using photometric data from the DECam MAGIC survey, which utilizes a narrow-band filter sensitive to calcium lines to estimate stellar metallicities. The study presents high-resolution spectroscopic follow-up observations of these stars to confirm their chemical compositions and analyze their origins.

Introduction and Motivation

The early universe was composed mostly of hydrogen and helium. The first generation of stars, called Population III stars, formed from this pristine material and began producing heavier elements through nuclear fusion. When these stars exploded, they seeded the next generation of stars with small amounts of metals (elements heavier than helium). These second-generation stars, particularly those with very low metallicities, act like time capsules, preserving the chemical signatures of the early universe. Carbon-Enhanced Metal-Poor (CEMP) stars, which show unusually high carbon relative to iron, are important for studying early stellar nucleosynthesis. Because these stars are rare and faint, identifying them requires innovative techniques, such as the photometric methods used by the MAGIC survey.

Target Selection and Observations

The six stars in this study were selected from over 21 million stars observed in the MAGIC survey, a Southern Hemisphere effort using the Dark Energy Camera. The team estimated photometric metallicities using a combination of narrow-band calcium measurements and broadband colors. Only stars with photometric metallicities [Fe/H] < –3.0 and located more than 30,000 light-years from Earth were considered, prioritizing red giants suitable for high-resolution spectroscopic observations. Spectra were obtained with the MIKE spectrograph on the 6.5-meter Magellan-Clay Telescope, allowing precise measurement of each star’s chemical makeup.

Stellar Parameters and Chemical Abundances

Using data from the Gaia space telescope and 2MASS survey, the authors calculated key stellar parameters like temperature, surface gravity, and microturbulent velocity. Spectral features from iron and other elements were measured to derive metallicity and elemental abundances. Sixteen elements were analyzed, ranging from light elements like carbon and magnesium to heavier elements like strontium and barium. One standout star, J0433–5548, was confirmed to have [Fe/H] = –4.12 and high carbon enrichment ([C/Fe] = +1.73), classifying it as a UMP CEMP star. This star likely formed from gas enriched by a single Population III supernova.

Photometric Accuracy and Carbon Impact

The study confirms that the MAGIC survey’s photometric metallicities are generally accurate to within 0.1 dex for stars with [Fe/H] between –3.3 and –3.0. However, J0433–5548’s metallicity was overestimated due to its strong carbon absorption features. Since carbon can interfere with the narrow-band calcium filter used in the survey, this causes stars to appear more metal-rich than they are. The authors modeled this effect and concluded that high carbon abundance significantly affects photometric metallicity estimates, especially for cooler stars.

Comparison with Literature and Progenitor Modeling

The chemical abundances of the six program stars were compared with existing data from the JINAbase database. While most stars followed known trends, aluminum abundances appeared lower, possibly due to temperature-related effects. The team also modeled the chemical signature of J0433–5548 against theoretical yields from metal-free supernovae. The best fit came from a 10.9 solar mass supernova with relatively low explosion energy, reinforcing the idea that this star formed from a single early supernova event.

Kinematics and Galactic Context

By combining chemical and kinematic data from Gaia, the team traced the possible origins of these stars. Two stars may have originated from a known ancient dwarf galaxy merger event (the Gaia-Sausage/Enceladus), while another could belong to the Sagittarius tidal stream. Some stars exhibit orbital properties suggesting they were once associated with the Magellanic system. These findings demonstrate that even the most metal-poor stars in the Galaxy can have diverse formation histories tied to different accretion events.

Conclusions

This study showcases the power of combining narrow-band photometry with high-resolution spectroscopy to uncover the chemical fingerprints of ancient stars. The discovery of new EMP and UMP stars, particularly those residing in the distant halo of the Milky Way, helps astronomers understand how the first stars enriched the early universe. It also validates photometric metallicity selection techniques for future surveys. Continued studies of stars like J0433–5548 will further illuminate the nature of Population III stars and the early stages of galactic chemical evolution.

Source: Placco