A team of astronomers has determined the intrinsic fraction of blue horizontal-branch stars that exist in binary systems, a finding that helps resolve a long-standing debate about how these old, unusually hot stars form. The research reveals that a significant portion—nearly one-third—of these stars have a companion, providing a crucial piece of data for understanding the later stages of stellar evolution.
This discovery carries significant implications for astrophysics, as it suggests that there is no single, universal pathway for the creation of blue horizontal-branch stars. Instead, their origin appears to depend heavily on their environment within the galaxy. By quantifying the role of binary interactions, the study provides a much-needed anchor for theoretical models, allowing scientists to better simulate the life cycles of stars and the formation history of the Milky Way.
Probing an Enigmatic Stellar Phase
Blue horizontal-branch (BHB) stars are a special class of star in a late phase of evolution. They are old, low-mass, metal-poor stars that have already exhausted the hydrogen in their cores and are now burning helium. Unlike typical stars at this stage, which become cool and large red giants, BHB stars are surprisingly hot and blue. This unexpected temperature has made their formation a persistent puzzle. Astronomers have long used them as “standard candles” and kinematic tracers to map the structure and history of the Milky Way’s halo, but the physical mechanisms responsible for creating them have been debated.
Two main categories of theories have been proposed. One suggests that a single star can evolve into a BHB star if it experiences enhanced mass loss during its red-giant phase, shedding its outer hydrogen envelope to expose its hot helium-burning core. The other theory posits that interaction with a companion star is necessary. In a binary system, one star can strip mass from the other, or the two can merge in a way that produces the hot, compact object observed. Distinguishing between these scenarios requires knowing exactly how many BHB stars are in binary systems.
A Large-Scale Spectroscopic Analysis
To resolve this question, researchers conducted a detailed statistical analysis of a large stellar sample, leveraging data from a powerful survey instrument to detect the subtle signs of binary companionship.
Leveraging the LAMOST Survey
The foundation of the research was data from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). This telescope is exceptionally efficient, capable of capturing the spectra of thousands of stars simultaneously. From its vast database, the team identified a sample of 299 BHB stars that had been observed on multiple occasions. This repeated observation was critical for the study’s primary method of detecting binary systems.
Detecting Stellar Wobbles
The primary technique used to identify binaries was the radial velocity method. A star in orbit around a companion will be periodically pulled toward and away from the observer, causing its light to shift slightly toward the blue or red ends of the spectrum—a phenomenon known as the Doppler effect. A single, isolated star would show no such variation. By analyzing the changes in radial velocity across multiple observations for each of the 299 stars, the team could flag those exhibiting the characteristic wobble of a binary system.
From Observation to Intrinsic Truth
Initial analysis of the data revealed an observed binary fraction of approximately 18% to 21%, depending on the number of measurements available for each star. However, this raw number is known to be an underestimate. Many binary systems go undetected because their orbits are oriented in a way that produces no measurable radial velocity shift from our perspective, or their orbital periods are too long to be caught by the observation window. To account for these and other observational biases, the team employed sophisticated Monte Carlo simulations. By modeling different binary populations and comparing them to the observed data, they corrected for the missing systems to derive the true, intrinsic binary fraction. This correction nearly doubled the initial figure, yielding a final result of approximately 32%.
Two Divergent Formation Pathways
One of the most profound findings of the study is that the 32% overall fraction does not tell the whole story. The researchers found that the likelihood of a BHB star having a companion is dramatically different depending on its location and kinematic properties within the Milky Way, pointing to two distinct and dominant formation channels.
Binaries Driving Formation in the Galactic Disk
When the team isolated BHB stars that kinematically belong to the galactic disk, they found a remarkably high intrinsic binary fraction of about 51%. This result provides powerful evidence that for disk populations, binary interactions are the primary mechanism for creating BHB stars. Processes like mass transfer from an evolving giant to its companion or a common-envelope phase where two stars share an outer atmosphere are likely responsible for the majority of these objects.
Single Stars Shaping the Halo
In sharp contrast, BHB stars associated with the galactic halo exhibited a much lower intrinsic binary fraction of around 29%. While still significant, this lower number strongly suggests that in the halo, single-star evolutionary pathways play a much more substantial role. For these stars, mechanisms internal to the star, such as powerful stellar winds that lead to enhanced mass loss on the red-giant branch, are likely the dominant formation channel.
The Link Between Temperature and Binarity
The study also uncovered another critical clue: a direct correlation between a BHB star’s temperature and its binarity. The team divided its sample into “bluer” and “redder” subgroups and found a striking difference in their binary fractions. The bluer, hotter BHB stars had a high intrinsic binary fraction of about 45%. The redder, cooler BHB stars, however, had a binary fraction of only 23%.
This suggests that binary interactions are particularly effective at creating the hottest types of horizontal-branch stars. This finding helps place BHB stars in a broader context, connecting them to the even hotter and rarer extreme horizontal-branch (EHB) stars, which are understood to be formed almost exclusively through binary channels, with some studies estimating their intrinsic binary fraction to be above 75%. The new results for BHB stars fit neatly into this evolving picture, showing a clear trend where increased stellar temperature on the horizontal branch is linked to a greater likelihood of binary origins.
Refining Models of Galactic Evolution
By providing the first robust measurement of the intrinsic binary fraction for BHB stars and revealing its dependence on galactic population and temperature, this research provides definitive benchmarks for stellar astrophysics. These precise figures will be used to test and refine the computer models that simulate how stars live and die. Understanding which formation channel—single or binary—dominates in different environments allows theorists to build more accurate histories of the Milky Way’s formation, from the ancient, metal-poor halo to the more dynamic disk.
The work successfully clarifies the complex origins of these important stars. It confirms that nature uses multiple methods to get the job done, with binary interactions and single-star processes each playing a crucial role, their importance waxing or waning depending on the star’s location and temperature. This detailed understanding marks a significant step forward in solving one of the long-standing puzzles of stellar evolution.
