Surface Chemistry Shows Which Massive Stars Have Gained Mass from Binary Companions
Most stars, including massive ones, are in binary pairs. Astronomers think that about 70% of them have gained mass from their companions, but there's been no way to determine which ones have done so. Now, researchers have found a way.
Mass transfer between massive stars is a common occurrence, especially when they are born in binary systems. As these stars evolve, they expand and often undergo mass transfer or even merge with their companions. This process can have significant consequences, such as triggering supernova explosions and altering the survivors' evolution. However, the aftermath of mass transfer can be difficult to detect, as it may leave behind a single star that appears solitary and unchanged.
Researchers from the Max Planck Institutes have developed a technique to identify stars that have gained mass from their binary companions in the past. The key lies in the star's surface chemistry, specifically the abundance of certain elements near the star's surface. In a paper titled "Chemical fingerprints of binary mass transfer in massive stars," published in Nature Astronomy, researchers Harim Jin and Norbert Langer detail their findings.
The team analyzed a "comprehensive grid of detailed massive binary evolution models" and discovered patterns in the surface abundance of elements like helium, carbon, nitrogen, and oxygen. These patterns suggest a predictable relationship between a star's surface chemistry and its past experiences with mass transfer. Specifically, stars that have gained mass in this way exhibit a distinct pattern on a CNO (carbon-nitrogen-oxygen) abundance diagram.
The CNO diagram plots two ratios: nitrogen to carbon and nitrogen to oxygen. Stars that have acquired mass through binary interactions show up as distinct points on this diagram, setting them apart from stars that have not undergone mass transfer and from solitary stars. For example, the star Gamma Columbae (γ Columbae), which has puzzled astronomers for years, has been identified as a former mass gainer using this new method.
Initially thought to be the remnant core of a stripped-envelope supernova, γ Columbae's surface chemistry reveals a high nitrogen-to-carbon ratio, moderate nitrogen-to-oxygen ratio, and helium enrichment.
The researchers' method allows them to reconstruct the history of these mass-gaining stars, including the amount of material they accreted and the composition of that material. This information can help determine the masses of both stars involved in the mass transfer process and the efficiency with which material was accreted. By "reading the story" written in the stars' surface chemistry, the researchers have gained valuable insights into the hidden lives of these massive stars, shedding light on their evolutionary paths and the impact of mass transfer on their fates.
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