Astronomers have discovered a record-breaking binary system, ZTF J1239+8347, a record-breaking case of two ‘failed stars’ (brown dwarfs) in a 57-minute orbital death spiral that may have resulted in the formation of a new star. The results were published on arXiv in a study led by Caltech’s Samuel Whitebook. The brown dwarfs are so close to each other that the more massive dwarf (primary) is ‘feeding’ the other (secondary) by transferring mass from the secondary to the primary. The accumulation of material falling onto the primary causes a hot spot, which scientists believe will eventually build up enough mass to ignite nuclear fusion and give these ‘failed stars’ a second chance to form a lower mass M-dwarf star.
New cosmic speed record of 57 minutes
ZTF J1239+8347 is a binary system of two brown dwarfs that set a new record for the smallest orbit among known isolated brown dwarf binary systems, smaller than previously expected. The two brown dwarfs orbit each other at such a high speed, completing a ‘year’ every 57.4 minutes, which is the fastest orbital period ever recorded in an isolated brown dwarf binary system. To help illustrate how dense this system is, the total separation of the two objects is so small that the entire system preparing to merge would fit comfortably within the distance between the Earth and the Moon.Caltech researchers discovered the high-speed motion of two brown dwarfs by observing the system’s pulsating glow once every hour, which is caused by the presence of a hot, glowing cloud of gas around the larger brown dwarf caused by the impact of fast-moving gas from the smaller brown dwarf colliding with the larger brown dwarf.
How a ‘vampire’ dines on dwarf fusion
To evolve from a brown dwarf into a true star, it must exceed a certain mass that was not reached during its formation within the molecular cloud. In this binary system the primary brown dwarf is acting like a ‘vampire’ by using its incredible drag to remove outer hydrogen layers from its companion through Roche lobe overflow (or loss of material from the Roche lobe). For an object to generate heat from the sustainable power of nuclear fusion, it would have to reach about 75–80 times the mass of Jupiter. Therefore, the current mass of these brown dwarfs is slightly below this limit; However, the continued accumulation of hydrogen during this interaction will continually increase the primary mass, eventually pushing it towards the critical ‘ignition’ point.A study published in The Astrophysical Journal Letters predicts that this merger will be relatively calm compared to the violent supernovae produced by colliding white dwarfs, and it is expected that this merger will result in the production of a lower mass red dwarf star and provide these ‘fail stars’ a second chance to become stars rather than the brown dwarfs that first formed.
The future of ZTF J1239+8347
The continued stability of the current mass transfer process is dependent on two factors – orbital physics and nuclear ignition. An evolutionary perspective on this issue can be found in ‘The Astrophysical Journal Letters’, which states that if the primary brown dwarf surpasses the 80-Jupiter mass threshold, it will officially ignite into a new star – a main sequence M-dwarf – permanently discarding its ‘failed star’ label. This would allow the brown dwarf to effectively reset its evolutionary clock and continue shining for trillions of years (assuming it is able to sustain nuclear fusion processes for that long). Depending on gravitational wave emissions (ripples in spacetime created by two massive, compact objects), the current state of mass transfer could cause both of these massive objects to lose orbital energy, and therefore spiral inwards towards each other – the ‘death spiral’ of mass transfer.
Second chance to shine: How mergers create new stars from ‘failures’
Before this discovery, brown dwarfs were viewed as cosmic dead ends; They will gradually cool down and dissolve into nothingness. The Caltech team’s findings show that ‘stellar failure’ is not irreversible, as a star can be created later in its life cycle, through interactions in the binary system. Additionally, their success indicates that the Zwicky Transient Facility (ZTF) can detect ultra-short-period binaries, suggesting that countless thousands of these ‘vampire’ systems may exist within the galaxy. This research significantly changes the estimated timeline of the universe by showing that even the dimest objects can eventually re-ignite.
