The Mystery of Wide Binaries in Metal-Poor Stars

Stars often come in pairs, orbiting a common center of mass. These binary systems are crucial in astronomy because they help scientists understand how stars form, evolve, and interact. This study, led by N. Lodieu, investigates the frequency of wide binaries among metal-poor stars—old stars with very low amounts of elements heavier than hydrogen and helium. Since these stars formed in the early universe, studying their binary properties helps us learn about star formation in ancient times. The researchers used data from the Gaia space telescope and other large surveys to search for widely separated companion stars.

Finding Metal-Poor Stars

To conduct this study, the team first needed a reliable sample of metal-poor stars. They selected stars with metallicities below −1.5 dex (which means they contain less than 3% of the Sun’s metal content). Using data from Gaia's latest data release, along with other catalogs, they refined their list to 610 stars. These stars mostly belong to the Milky Way's thick disk or halo, indicating they are among the oldest stars in our galaxy.

Searching for Binary Companions

The researchers searched for companion stars using several approaches. First, they looked for common proper motion pairs—stars moving together across the sky at the same speed and direction, which suggests they are gravitationally bound. They cross-matched their list with the Gaia catalog and found only nine possible wide binary systems, a strikingly low number.

To extend their search, they used data from the VISTA Hemisphere Survey (VHS) and the Wide-field Infrared Survey Explorer (WISE), which help detect faint stars that Gaia might miss. This additional search found eight more candidate companions. Even with these additions, the fraction of wide binaries remained below 3%, much lower than the binary frequency of stars with solar-like metallicities.

Investigating Close Binaries

The team also studied whether metal-poor stars have closely bound companions at separations smaller than 8 astronomical units (AU). They used Gaia’s RUWE parameter, which can indicate unresolved binaries. They estimated that at least 20% of metal-poor stars likely have close companions—similar to what is found in stars with higher metal content. This suggests that close binaries form at similar rates regardless of metallicity, while wide binaries are far less common in metal-poor environments.

Why Are Wide Binaries Rare in Metal-Poor Stars?

The results show a steep decline in binary frequency as separation increases. One possible explanation is that metal-poor environments affect how stars form. Lower metallicity gas is less efficient at cooling, which may lead to the formation of fewer wide binary systems. Additionally, dynamical interactions over billions of years may disrupt widely separated companions, breaking them apart over time.

Conclusion

This study provides strong evidence that metal-poor stars are much less likely to have wide binary companions than their metal-rich counterparts. However, the fraction of close binaries remains roughly the same. These findings suggest that the conditions in the early universe influenced how stars formed and evolved, shaping the binary populations we see today. Future high-resolution imaging studies will be needed to confirm the trends observed in this research and further refine our understanding of binary star formation in the early Milky Way.

Source: Lodieu