Stellar Sibling Kills Its Companion, Prematurely Hastening Transformation To White Dwarf

It is the ultimate fate of most stars to become white dwarfs, but having a brown dwarf companion creates a risk of ending up there faster than would otherwise occur. NASA

Nature is often said to be “red in tooth and claw”, and zoology is not a science for the faint-heated. Astronomy, on the other hand, is safe enough surely? If that is your view, we have bad news. Astronomers have come across a celestial murder scene, and the killer is a star's little sibling.

Stars that start with a mass less than 8 times that of the Sun ultimately evolve to white dwarfs. Instead of turning supernova like the heaviest stars, the star will fuse its helium to carbon and oxygen and shed its outer layers. What is left is very dense and faint as it slowly disperses its initial heat.

However, Dr Leonardo Andrade de Almeida of the University of São Paulo has found a white dwarf that reached this status through shortened path. The star, named HS 2231+2441, has a mass just 0.19-0.29 times that of the Sun, although it once would have been somewhat heavier. In Monthly Notices of the Royal Astronomical Society, Almeida presents a theory HS 2231+2441 went through the normal evolutionary process for stars until it became a red giant.

As HS 2231+2441 expanded, it started to interact with a nearby brown dwarf, which now has around 0.04 times the Sun's mass. The brown dwarf was much denser and drew material off the outer layers of the bloated HS 2231+2441, which by this point may have had a radius as large as the distance between the Earth and the Sun.

"This transfer of mass from the more massive star, the primary object, to its companion, which is the secondary object, was extremely violent and unstable, and it lasted a short time," Almeida said in a statement

Just as satellites begin a death spiral when they encounter the thin outer layers of the Earth's atmosphere, slowing them down, the brown dwarf began falling into HS 2231+2441, turning gravitational potential energy to kinetic energy in the process. Eventually, the released energy became sufficient to overcome the gravity of the system, causing HS 2231+2441 to eject its outer layers, leaving it with just an exposed helium core.

The remaining mass of HS 2231+2441 was insufficient to allow it to continue fusing helium, leaving it a premature white dwarf. Meanwhile, the brown dwarf held onto some of the material thrown off, but not enough to start its own fusion reactions. The orbital decay induced in the process left the pair with a period of just three hours.

Other low-mass white dwarfs have been discovered, all formed through a similar process of interaction with a companion. Nevertheless, as the lowest mass binary star system ever observed, and therefore the most extreme example, this pair provides an unusual opportunity to study this phenomenon.

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