Brown dwarf in low-mass binary system caused companion star's premature death

Scientists view binary systems as crucial to their understanding of stellar life cycles.
By Laurel Kornfeld | Nov 03, 2017
The brown dwarf companion of a normal Sun-like star caused the latter's premature death by siphoning material from the latter, which was on its way to evolving into a red giant, according to a group of Brazilian astronomers.

Located in the constellation Perseus, the rare low-mass binary star system is now composed of the brown dwarf and a white dwarf, the stellar remnant of the Sun-like star.

White dwarfs are stellar remnants of Sun-like stars with 0.5 to eight solar masses, which end their lives by becoming red giants before completely running out of fuel and expelling their outer materials, leaving hot cores.

Brown dwarfs are an intermediate class of objects with masses greater than those of the heaviest gas giant planets but less than those of the lowest-mass normal stars.

Some scientists think brown dwarfs fuse deuterium, an isotope of hydrogen.

Observed between 2005 and 2013 by scientists using the Pico dos Dias Observatory in Brazopolis, Minas Gerais in Brazil and the William Herschel telescope in the Canary Islands, this particular system is the lowest mass binary ever seen by astronomers.

The white dwarf has a mass between two and three tenths that of our Sun, along with a temperature of 28,500 degrees Kelvin, while the brown dwarf has between 34 and 46 times the mass of Jupiter.

According to the researchers, the precursor star of the white dwarf rapidly burned through its fuel, producing a helium core. As it ran out of fuel, it expanded in the process of becoming a red giant, and its radius may have swelled to 150 million kilometers.

Because of the high energy levels generated by this activity, the star began to gravitationally interact with its brown dwarf companion, briefly transferring mass to it in a violent process.

This led to the brown dwarf becoming engulfed by the atmosphere of its companion and orbiting within that atmosphere, in the process losing orbital angular momentum.

Eventually, so much energy was transferred that the gravitational force connecting the brown dwarf to the core of its companion was exceeded, resulting in a huge release of matter from the system to the point that the only part of the regular star left behind was its exposed helium core.

With so much of its mass gone, the star's remnant was unable to burn more helium and was no longer self-luminous, meaning it was dead and had become a white dwarf.

Leonardo Andrade de Almeida of the University of Sao Paulo's Institute for Astronomy, Geophysics, and Atmospheric Sciences (IAG-USP), lead author of a paper on the study published in Monthly Notices of the Royal Astronomical Society, said that although the brown dwarf acquired some of its companion's mass, it was not enough to transform it into a new star.




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