Evidence Says Some Blue Stars Come from Two Stars

New research shows that the properties of some blue supergiant stars can be explained by the merger of a massive star with a smaller companion. This suggests that many of the brightest and hottest stars in our galaxy are not born, but made.

One Star or Two?

The blue supergiant stage is a brief phase in the lives of hot, massive stars. Though the phase is short-lived, there seem to be a lot of blue supergiants, and despite the fact that massive stars tend to live in pairs or trios, there seem to be a lot of supergiants without a stellar partner. These curious findings could be explained by a scenario in which some blue supergiants form when a massive star swallows its companion.

Researchers have applied this line of thinking to several famous massive stars like Eta Carinae and the progenitor of the supernova SN 1987A, showing that the merger model explains these stars’ unusual properties. Now, a research team led by Athira Menon (Institute of Astrophysics of the Canary Islands and University of La Laguna) has taken the investigation a step further, using this model to explain the varied properties of a population of blue supergiant stars in a neighboring galaxy.

A Star Is Made

Menon’s team used the Modules for Experiments in Stellar Astrophysics (MESA) to model the evolution of post-merger stars and to compare the properties of blue supergiants coming from single stars to those coming from stellar mergers.

In the stellar merger scenario, blue supergiants result from the collision of a massive post-main-sequence star and its main-sequence binary companion. As the massive star expands, it donates some of its mass to its companion, which eventually becomes entangled in the extensive outer atmosphere of the larger star. Doomed by friction and tidal forces, the smaller star “dissolves” within the larger star, setting the resulting star on a new evolutionary path.

Clues from Chemical Abundances

The team found that blue supergiants formed through mergers have different surface abundances of elements like carbon and oxygen compared to supergiants arising from single stars. To compare these results to the properties of actual blue supergiants, the team amassed a sample of 59 blue supergiant stars in the Large Magellanic Cloud and divided them into three groups based on the ratios of their carbon, nitrogen, and oxygen abundances.

The first group, which had relatively little nitrogen compared to carbon and oxygen, matched the outcomes of the single-star models — these stars are likely “true” blue supergiants that evolved from single stars. The second group, with moderate N/C and N/O ratios, could be explained by either single stars or stellar mergers. The final group, which made up about 40% of the sample, had larger abundance ratios that the single-star model couldn’t reproduce.

Taking into account other factors, such as systematic offsets, Menon’s team concluded that more than half of the 59 stars in their sample came from stellar mergers. This suggests that many blue supergiant stars owe their status to a stellar merger, and the merger model is a valuable tool to understand blue supergiant populations throughout the universe.

Citation

“Evidence for Evolved Stellar Binary Mergers in Observed B-type Blue Supergiants,” Athira Menon et al 2024 ApJL 963 L42. doi:10.3847/2041-8213/ad2074