Tracing the Past: The Ancient Metal-Poor C-19 Stellar Stream Extends Over 100 Degrees

The C-19 stellar stream is one of the most ancient and metal-poor structures in our Milky Way. By studying it, astronomers can learn about the earliest stages of the universe, as these stars formed shortly after the Big Bang. C-19 was first identified as an extremely metal-poor (EMP) stream through the Pristine survey, which specializes in finding low-metallicity stars. This study expands our knowledge of C-19 by searching for additional stream members across the sky. Using new data from the European Space Agency’s Gaia mission and advanced stream-searching algorithms, the researchers confirm that C-19 stretches across more than 100 degrees in the sky—far beyond what was previously known.

Identifying New Members

To find additional stars belonging to C-19, the team used two search techniques: STREAMFINDER, which looks for stars moving together in the Milky Way, and StarGO, a clustering algorithm that groups stars with similar properties. Initially, these searches revealed four new C-19 members located up to 60 degrees away from the main body of the stream. After the release of new data from Gaia (known as Data Release 3, or DR3), the researchers performed a more detailed search, this time considering low-metallicity stars that matched the stream’s motion. This led to the identification of 12 new stars with metallicities like C-19, confirming that the stream extends over 100 degrees.

Metallicity and Chemical Properties

Metallicity refers to how much of a star is made up of elements heavier than hydrogen and helium. The lower the metallicity, the older the star. The researchers measured the metal content of the new C-19 stars using high-resolution spectroscopic observations from telescopes such as the Magellan Telescope, the Very Large Telescope (VLT), and the Subaru Telescope. The results confirmed that these stars are extremely metal-poor, with iron abundances about 1,000 times lower than those of the Sun. This supports the idea that C-19 formed very early in the history of the universe.

Motion and Structure of the Stream

By analyzing the movement of C-19 stars, the team determined that the stream is “dynamically hot,” meaning the stars are moving with a wide range of velocities rather than in a tight, orderly stream. The velocity dispersion—how spread out the star speeds are—is around 11 km/s, which is higher than expected for a stream that likely originated from a globular cluster (a dense group of ancient stars). The width of the stream is about 200 parsecs (or about 650 light-years), making it wider and more extended than previous estimates.

What This Means for C-19’s Origins

The discovery of a wider and hotter C-19 stream raises new questions about its origins. Typically, stellar streams that come from globular clusters have a much lower velocity dispersion. One possible explanation is that C-19’s parent cluster was disrupted early in its life while embedded in a dark matter halo, a theoretical “cloud” of invisible mass that might have influenced its stars’ movements. Alternatively, interactions with smaller structures in the Milky Way or the presence of black holes in the original cluster could have heated up the stream, increasing its velocity dispersion. Another possibility is that unresolved binary stars (stars orbiting each other) in the stream are making the velocity dispersion appear larger than it really is.

Conclusion

This study significantly expands our understanding of the C-19 stream, showing that it is much larger, more diffuse, and dynamically complex than previously thought. With 22 confirmed members, its full extent stretches more than 100 degrees across the sky. These findings provide crucial clues about the formation of the first stellar structures in the Milky Way. Future observations will be necessary to determine whether unseen factors, such as dark matter subhalos or binary stars, have influenced the stream’s properties. By continuing to study C-19, astronomers hope to uncover more about the earliest eras of the universe and the role that ancient stellar streams played in shaping the Milky Way.

Source: Yuan