Carbon-Enhanced Dwarf Stars: Clues from the Galactic Halo

Stars that formed early in the universe carry valuable information about how galaxies, and even the ingredients of life, came to be. A special class of these ancient stars are carbon-enhanced stars, which show unusually high amounts of carbon compared to iron. Some of the most puzzling examples are dwarf carbon stars (dC stars): small, cool stars on the main sequence whose atmospheres contain more carbon than oxygen. Their existence challenges our understanding of how stars evolve, since single-star evolution cannot easily explain such strong carbon enrichment. Previous work suggested many of these stars may belong to the Galactic halo, a large, roughly spherical population of old, metal-poor stars that surround the Milky Way.

Candidate Stars and Data

Jay Farihi and collaborators assembled samples of candidate dC stars from the Sloan Digital Sky Survey (SDSS) and cross-matched them with precise measurements from Gaia Data Release 3 (DR3). They began with 1211 candidates, checked each against Gaia data for positions, brightness, and motions, and reduced the list to 1208 viable stars. From there, they used a combination of brightness and color data to refine the list to 1003 “high-confidence” dwarf carbon stars.

Distance Measurements

Measuring the distances to faint stars is tricky, especially because directly converting Gaia’s parallaxes can lead to large errors. Instead, the team used a Bayesian inference method that combines Gaia’s astrometry with SDSS photometry, while correcting for the effects of interstellar dust. This approach provided the first reliable distances for such a large sample of dC stars, most of which lie within about 2 kiloparsecs (roughly 6,500 light-years). The team showed that dC stars tend to be fainter than “normal” stars of similar color, which is consistent with them being metal-poor.

Galactic Motions and Classification

With accurate distances, the authors could reconstruct each star’s orbit in the Galaxy. Using a detailed model of the Milky Way’s gravitational potential, they determined the likelihood that each star belonged to one of the Galaxy’s main components: the thin disc, thick disc, or halo. Their results show that about 62% of the dC stars belong to the halo, 31% to the thick disc, and only about 7% to the thin disc. This strongly supports the idea that dC stars are primarily an old, metal-poor population.

Discussion and Implications

These findings agree with earlier, smaller studies that hinted at a halo connection. The results also suggest that many dC stars may be related to the broader class of carbon-enhanced metal-poor (CEMP) stars, which are often explained as the result of binary star systems where mass transfer enriched one star with carbon. At least some of the dC population may share this origin. However, the study also found a small number of thin disc stars, showing that carbon enhancement can sometimes occur even in more metal-rich environments. The next step will be spectroscopic studies to directly measure their chemical compositions.

Conclusions

This work provides the largest and most reliable catalogue of dC stars studied so far. By combining data from SDSS and Gaia, the authors demonstrate that most of these carbon-rich dwarfs are halo stars, making them an important new tool for “stellar archaeology”, using stars to piece together the history of the Milky Way. Future spectroscopic surveys will help determine their chemical make-up and origins more precisely, potentially offering insights into the earliest generations of stars.

Source: Farihi