Unraveling the Binarity of B-type Supergiants in the Small Magellanic Cloud

Massive stars shape the universe, driving stellar evolution, influencing galaxies, and producing some of the most dramatic cosmic events. But understanding their life cycles, especially the role of binary interactions, remains a challenge. A new study by N. Britavskiy et al. investigates the binary nature of early B-type supergiants (BSGs) in the Small Magellanic Cloud (SMC) as part of the Binarity at LOw Metallicity (BLOeM) survey. By examining 262 massive stars, the team aims to better understand how these luminous objects evolve, interact, and possibly merge with their stellar companions.

A Look at the Stars

B-type supergiants are some of the most massive stars in the universe, sitting between core hydrogen and helium-burning phases. Many start their lives as hot, bright O-type stars but later transition into blue supergiants, marking a key stage in massive star evolution. However, the exact paths these stars take are unclear, and binary interactions play a crucial role. While it was once believed that most BSGs were single stars, recent studies suggest that a significant fraction are the result of binary mergers or mass transfer between stars.

Hunting for Binaries

The researchers used spectroscopic data from the Very Large Telescope (VLT) in Chile, gathering observations across nine epochs (time intervals) over three months. They measured radial velocity changes—shifts in the light spectrum caused by a star’s motion—searching for variations larger than 20 km/s, which would indicate the presence of a binary companion. In addition, they analyzed how spectral lines varied over time to distinguish between binaries and stars that might have pulsations or other internal changes.

Key Findings

The team discovered that about 23% of the BSGs show clear spectroscopic binary signatures, meaning they likely have a companion. When including stars with more subtle signs of binarity, the number rises to 34%, and after accounting for observational limitations, the estimated true binary fraction is around 40%. Interestingly, they found a drop in the binary fraction for stars cooler than 18,000 K (later than B2 spectral type), suggesting that this could mark the end of the main sequence phase for these stars.

They also identified 41 systems with estimated orbital periods, including 17 eclipsing binaries where one star passes in front of the other from our viewpoint. Some of these systems have very short orbital periods (less than 10 days), which suggests that they may currently be undergoing mass transfer.

A Window into Stellar Evolution

One striking result is that the binary fraction of BSGs in the SMC is similar to that found in the Milky Way and the Large Magellanic Cloud, meaning that metallicity (the abundance of heavy elements) does not significantly impact binarity in this mass range. This finding supports the idea that binary formation processes are largely independent of environmental factors.

Additionally, the study highlights the importance of mergers in the BSG population. The most luminous BSGs in the sample show very few binary signatures, hinting that many of them could be merger products—single stars that formed after two stars combined.

The Bigger Picture

Understanding BSGs is crucial for piecing together the life cycles of massive stars, particularly those that may go on to explode as supernovae or form exotic objects like black holes and neutron stars. This study provides new insights into how these stars interact and evolve, especially in low-metallicity environments like the SMC.

Future observations from the BLOeM survey, including additional epochs to track long-period binaries, will further refine our understanding of these massive stars. By mapping out their binary nature, scientists are getting closer to solving the puzzle of how these cosmic giants live and die.

Source: Britavskiy