Carbon Stars and Their Hidden Population: Insights from Gaia DR3
Carbon stars are a unique type of star whose atmospheres contain more carbon than oxygen. This makes them distinct from normal stars, which have more oxygen than carbon. The excess carbon forms molecules like C₂ (carbon dimer) and CN (cyanogen), which give carbon stars their characteristic reddish color.
There are different types of carbon stars, including giant stars on the asymptotic giant branch (AGB) and smaller, less luminous stars called dwarf carbon (dC) stars. The paper by Roulston et al. focuses on identifying dC stars, which are main-sequence stars that have gained carbon-rich material from a former AGB companion that has since turned into a white dwarf. While dC stars are thought to be common, their actual space density had not been accurately measured before this study.
The Search for Carbon Stars in Gaia DR3
The researchers used data from Gaia Data Release 3 (DR3), which includes spectra (light split into different wavelengths) of about 220 million stars. By analyzing how light is absorbed by molecules like C₂ and CN, they could distinguish carbon stars from other stars.
To improve accuracy, they trained a machine learning algorithm using known carbon stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). The model was designed to separate carbon-rich stars from normal stars, ensuring a high-purity sample.
Results: How Many Carbon Stars Did They Find?
By applying their algorithm to Gaia DR3, the researchers identified 43,574 candidate carbon stars. Most of these were giant stars, but a subset included dwarf carbon stars. The team confirmed their findings using spectroscopic observations at the Fred Lawrence Whipple Observatory, ensuring that the detected stars were genuinely carbon-rich.
For the dC stars, they calculated a local space density of about 1.96 × 10⁻⁶ stars per cubic parsec. This means that for every volume of space with a 50-parsec (163 light-years) radius, there is approximately one dC star. They also determined that dC stars are widely spread throughout the Milky Way, with a large scale height (distribution thickness) of about 856 parsecs.
Why Does This Matter?
This study helps answer important questions about how stars evolve. Since dC stars come from binary systems where material is transferred from an AGB companion, their presence gives clues about mass transfer, binary star evolution, and the chemical history of the galaxy. Understanding how many dC stars exist also helps refine models of stellar evolution.
Additionally, this research highlights the power of machine learning in astronomy. By training computers to recognize spectral patterns, scientists can process massive datasets like Gaia DR3 far more efficiently than by manual inspection.
Future Directions
The researchers plan to continue refining their classification methods and look for new types of variable dC stars. They also suggest that dC stars could be connected to other chemically peculiar stars like barium stars and carbon-enhanced metal-poor (CEMP) stars, providing further insights into stellar evolution.
Source: Roulston
