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Astronomers catch a red dwarf slowly feeding on its brown dwarf companion
MIT-led astronomers say ZTF J0440+2325, 300 light-years away, is the first system seen where one low-mass object steadily feeds on another. Nobody has settled how long the feeding can last, and published estimates run from hundreds of thousands to billions of years.
The Scientist · Science desk
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What happened
- The feeding star is a red dwarf and its companion is a brown dwarf, with material appearing to flow from the brown dwarf onto the red dwarf.
- Kevin Burdge of MIT first noticed the system years ago in Zwicky Transient Facility data as a triangular brightness pattern that repeated again and again.
- Burdge and Aaron Householder, an MIT graduate student and the study's lead author, later revisited the source and measured its wobble with multiple telescopes.
- The study appeared in Nature Astronomy alongside a separate paper on a white dwarf that seems to be fed by the atmosphere of a planet orbiting it.
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Why it matters
- constraint With one system on record, the find shows a slow feeding state between two low-mass objects can exist but cannot yet show how often close pairs end up in it.
- contradiction Anyone repeating the billions-of-years figure is quoting the top of a projection that a second account of the same paper puts at millions.
- contradiction The idea that the sun may nibble at Earth goes beyond what was measured, and it conflicts with Burdge's own statement that Earth will be swallowed.
The case for what ZTF J0440+2325 is rests on a process of elimination. Burdge's first guess was a black widow binary, in which a dense, spinning neutron star slowly consumes a much smaller companion [8]. In those systems the light wobbles as the low-mass companion whips around its heavy partner [8]. "We weren't seeing that whipping back and forth here," Burdge said [9].
A missing wobble fits two partners with no heavy anchor between them. "If you have less mass in the system overall, things can gently orbit each other without whipping back and forth," he said [10]. For years that was only a hunch. "That was the idea. But we never had any proof. And this weird triangle just sat for years," Burdge said [11].
I think the elimination argument holds, at least as far as the coverage describes it. Black widows already show slow feeding [8]. The new claim in the MIT account is narrower: one low-mass object feeding on another [1].
The lifetime is less firm. MIT's account says the feeding rate suggests the process could carry on for hundreds of thousands or even billions of years [4]. Scientific American, describing the same study, says millions [5]. Read as 100,000 years against 1 billion, MIT's own range spans a factor of 10,000 [17]. Burdge quoted the top of it: "Instead of swallowing the thing up, the star can gradually eat it, for billions of years," he said [18]. MIT's figures are projected from the current feeding rate [4]. The coverage does not report that rate or the masses of the two objects.
Earth comes up in the same quote. "This is what will happen to the Earth when the sun becomes a red giant," Burdge said, describing the usual picture in which the star swallows its companion [18]. Scientific American applied the finding to the sun anyway. It wrote that Householder's research suggests the sun might one day nibble away at Earth's atmosphere [19]. The sun is a yellow dwarf [15]. The star measured here is a red dwarf [3].
Householder called the discovery "a happy accident" [13]. "We're excited to see if we can find a star eating a planet or maybe even two planets eating each other," he said [14].
What to watch
- The paper's measured masses and mass-transfer rate, which would show whether the lifetime sits nearer hundreds of thousands, millions, or billions of years.
- Further survey detections of repeating triangular light curves, or the star-planet pairs Householder wants to find, which would show whether this feeding state is common.