It is a known fact that binary stars exchange mass with each other. In extreme scenarios, this mass transfer can even trigger a supernova explosion. This occurs when a white dwarf strips material from its companion.
However, astronomers have never observed a pair of brown dwarfs engaging in mass transfer between them.
Brown dwarfs occupy an ambiguous space between planets and stars. They possess more mass than gas giants but less mass than the smallest main-sequence stars—the red dwarfs. Brown dwarfs are sometimes labeled as failed stars or substellar objects because they simply lack sufficient mass to ignite and sustain hydrogen fusion like main-sequence stars do. Instead, they generate light and heat through deuterium fusion.
Astronomers are uncertain about the prevalence of brown dwarfs because they are very faint and difficult to detect. Nevertheless, estimates suggest there could be as many as 100 billion in the Milky Way. Like other stars, many of these billions of brown dwarfs form binary systems.
A new investigation, published in The Astrophysical Journal Letters, focuses on the brown dwarf binary system ZTF J1239+8347, which orbits in an unusually close proximity. The study is titled “A Mass-Transferring Brown Dwarf Binary in a 57-Minute Orbit,” with Samuel Wachter as the lead author. Wachter is affiliated with the California Institute of Technology (Caltech).
The pair of brown stars has an orbital period of 57.41 minutes. This is an extremely tight orbit, and observations from NASA’s Swift observatory, along with other facilities, indicate that the two brown dwarfs are in a stable interaction involving mass transfer. The researchers identified a hot spot on the surface of the donor brown dwarf that shifts position as the pair revolves around each other.
Two potential outcomes exist in this situation.
In one scenario, the accreting brown dwarf (BD) will continue to accumulate mass until it becomes massive enough to initiate hydrogen fusion. Following this, it will transform into a main-sequence star.
In the alternative scenario, the pair will eventually merge to form a single star. This outcome will also result in the creation of a more massive main-sequence star. In both cases, an increase in luminosity is anticipated.
“Failed stars are getting a second chance,” Wachter remarked. “Brown dwarfs lack the internal engines that stars possess, but this finding demonstrates that they can exhibit very interesting dynamical physics.”
Mass transfer between binary stars is not an arcane process. The more massive component gravitationally pulls the atmosphere of the less massive partner. Material eventually flows out of the donor’s Roche lobe and becomes part of the accretor.
“When one star’s gravity overcomes the other’s, material begins to flow from the less dense star toward the more dense one,” explains Wachter. “It’s almost like material is being siphoned through a nozzle.”
This marks the first time astrophysicists have confirmed such mass transfer in a brown dwarf binary system. In fact, the observation is so unusual that other members of the astronomical community have found it challenging to accept these findings. “These are very exotic objects,” stated co-author Thomas Prince, also from Caltech. “When we told some of our colleagues, they didn’t believe such a thing existed.”
The authors did not immediately trust their own conclusions. They explored alternative explanations for the observed phenomena. Perhaps one of the objects was not actually a brown dwarf but rather a compact object, such as a neutron star. They ruled this out because such a configuration would have resulted in brighter X-ray emissions.
Another possibility considered was a cataclysmic variable, which posits that a white dwarf is accreting matter from a secondary star—in this case, a brown dwarf. However, the optical spectrum contradicts this hypothesis, as does the presence of the hot spot. “Furthermore, in that arrangement, the hot spot could not reside on the irradiated brown dwarf, because the irradiating white dwarf would be visible continuously in the optical spectrum,” the authors elaborate.
They settled on the accreting BD binary hypothesis because it best accounted for the observed data.
This system also holds scientific value as it can serve as a test case for studying mass transfer. “ZTF J1239+8347 represents a potentially valuable tool for exploring the dynamics of stable mass transfer at the lowest detectable mass scales,” the authors assert.
The object ZTF J1239+8347 is relatively nearby, positioned only about 1,000 light-years away. This makes it a prime candidate for further observation by the James Webb Space Telescope (JWST). “Future observations of the system with the James Webb Space Telescope (JWST) could better constrain the temperature of the accretion atmosphere and potentially detect the atmosphere of the donor system,” the authors suggest. These observations would also allow for more precise measurements of the system’s mass ratio. Finer measurements of the hot spot would likewise impose constraints on the mass transfer rate.
But, true to many areas in astronomy and astrophysics, uncovering more examples of brown dwarf binaries undergoing mass transfer will be key to deeper comprehension. Fortunately, the Vera C. Rubin Observatory is expected to discover more such binary stars.
“We anticipate that the Vera C. Rubin Observatory will uncover dozens more of these objects,” Wachter concludes. “We want to find more to understand their incidence and how common they are. Our inference is that this happens more frequently than you might imagine.”
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