A new study has confirmed the existence of a strange type of supermassive star that grows by “feeding” on its expanding, dying companion star.
While astronomers have long suspected the existence of barium stars—named for their unusually high levels of the heavy element barium—formed by consuming material from a companion star, they have now finally observed this process in these “vampire” stars.
Astronomers William P. Bidelman and Philip Keena first discovered barium stars in 1951 after noticing high levels of barium in their atmospheres. All stars are primarily composed of hydrogen and helium, but they contain small amounts of heavier elements like barium.
The Hubble Telescope observes the red giant star CW Leonis in the constellation Leo. The nearby star, 58 Leonis, is believed to be a rare barium star. (Image: ESA/Hubble, NASA, Toshiya Ueta (University of Denver), Hyosun Kim (KASI)).
Barium stars operate on a different level. In addition to barium, they contain a large amount of heavy elements formed through a specific process known as the s-process.
Astronomers have known that the s-process occurs within massive stars near the end of their life cycles when neutrons collide with lighter elements like helium and hydrogen, fusing them into heavier elements such as carbon, strontium, and barium.
However, barium stars themselves are not always at the end of their life cycles, so they cannot form these elements independently.
In a paper published on arXiv on September 4, astronomers confirmed that these strange heavy stars are “cosmic leeches.”
The research team discovered two new barium stars. Importantly, in addition to measuring the s-process elements within the stars, they ruled out many nuclear processes that could explain how these stars formed.
They also confirmed for the first time that each of these stars is a member of a binary star system. In one case, they found clear evidence that the companion star is a white dwarf—the leftover remnant of a sun-like star.
Since barium stars cannot independently form heavy elements, the companion stars are the most plausible explanation. In this scenario, to obtain barium, the companion star must go through its entire life cycle. Near the end of its life, it initiates the s-process and produces large amounts of barium and other elements, which then migrate into the upper atmosphere. When the companion star expands into a red giant, it completely loses its atmosphere. A portion of that atmosphere is transferred to the remaining star, enriching it and transforming it into a barium star.
If previously astronomers could only speculate about this scenario due to a lack of direct evidence, now the barium stars in binary systems serve as the first proof of this type of star.
