Astronomers Catch a Star Slowly Devouring a Brown Dwarf Companion

Astronomers have made the first direct observation of a star slowly stripping material from a brown dwarf companion in a process that could stretch on for billions of years, rather than ending in the rapid destruction that theorists had long assumed awaited such close-orbiting objects. The finding, led by researchers at the Massachusetts Institute of Technology, identifies two binary systems in which a brown dwarf -- an object too small to sustain hydrogen fusion like a true star but larger than a planet -- is gradually feeding its own mass onto a nearby low-mass star.

A star with an unusual appetite

The better-characterized of the two systems, cataloged as ZTF J0440+2325, lies roughly 300 light-years from Earth in the Milky Way. It consists of a small star, formally known as an M dwarf, and a brown dwarf locked in an orbit that completes in just 86.65 minutes -- a cycle so tight that the entire orbit would fit within the volume of the sun. Streams of gas pulled from the brown dwarf strike the star's surface, producing a hot spot that brightens and dims as the pair revolves, the signature that first drew attention to the system.

MIT physicist Kevin Burdge noticed the unusual, triangular shape of that brightness pattern several years ago in data from the Zwicky Transient Facility, a wide-field survey camera at Palomar Observatory in California. The light curve initially looked like it might belong to a "black widow" system, in which a millisecond pulsar's radiation slowly destroys a stellar companion. According to MIT's account of the discovery, Burdge said simply, "We couldn't explain what this was."

Graduate student Aaron Householder and Burdge later revisited the signal with additional telescope time and found that the pulsar explanation did not fit: the star showed a small gravitational wobble inconsistent with a black widow. "When we measured that wobble, we found we were not seeing a black widow," Householder said, according to the MIT account. Instead, the pair determined they were watching something that had been predicted in theory since the 1980s but never confirmed observationally: a brown dwarf stably transferring its own mass onto a star, rather than being torn apart in one violent episode. "This is the first time we've caught a low-mass star actively accreting from another low-mass object," Householder said.

The numbers behind the discovery

The team, whose results appear in a paper published in Nature Astronomy on October 5, describes two such systems. In ZTF J0440+2325, the accreting star has a mass of about 0.106 times the sun's (roughly 111 times Jupiter's mass), while the donor brown dwarf weighs in at about 0.029 solar masses, or about 30 Jupiter masses. A second system, ZTF J1444+4820, orbits even faster, completing a lap every 67 minutes, and pairs a star of about 0.099 solar masses with a somewhat heavier brown dwarf donor of roughly 0.040 solar masses, or 41.6 Jupiter masses.

To pin down those figures, the researchers combined several lines of evidence. High-speed, simultaneous five-color photometry came from the HiPERCAM instrument on the 10.4-meter Gran Telescopio Canarias in the Canary Islands, which tracked the brightness changes produced by the hot spot in detail. Spectra from the Keck I and Keck II telescopes in Hawaii measured the stars' radial velocities -- the small back-and-forth motions caused by the orbiting companion's gravity -- which ruled out the pulsar scenario and pinned down the masses. For the second, less fully characterized system, the team added infrared spectroscopy from the SPHEREx mission and X-ray limits from the Swift spacecraft. According to MIT's release, the brown dwarf in the primary system is losing mass to its companion at a rate of roughly one hundred-thousandth of an Earth's mass per year. At that pace, the paper estimates the donor could survive, albeit shrinking, for tens of millions to several billion years before either being fully consumed or settling into a different configuration. Modeling of the systems' overall ages placed both pairs at roughly 5.2 to 5.4 billion years old, suggesting the mass transfer itself is a long-lived, stable arrangement rather than a brief transitional phase.

A different fate than sudden engulfment

The possibility of stable mass transfer from a very low-mass, fully convective donor onto a star has been discussed theoretically since papers by Saul Rappaport and collaborators in 1983 and by Lynn Hjellming and Ronald Webbink in 1987, which calculated that such transfer should remain stable as long as the donor is markedly less massive than the star it is feeding -- specifically, below about 63 percent of the accretor's mass. Both newly reported systems fall comfortably under that threshold, with mass ratios of roughly 0.27 and 0.41, consistent with the stable, gradual process the team observed rather than a runaway spiral that would end in rapid engulfment.

That distinction matters because astronomers have generally pictured close encounters between stars and much smaller companions ending abruptly. Earth itself is expected to be swallowed whole when the sun swells into a red giant billions of years from now, a scenario modeled in earlier research by astronomers Klaus-Peter Schröder and Robert Connon Smith. The MIT-led team's systems show that at least some substellar companions -- objects in the hazy zone between planets and stars -- face a slower-motion ending, losing mass gradually rather than being destroyed in a single episode. The authors also note that because neither star in either system shows signs of being an evolved, swollen giant, the two pairs likely did not form through the dramatic "common envelope" process that produces most known ultra-compact binaries; instead, they appear to have formed at wider separations and spiraled inward over time, though the authors say the exact mechanism driving that migration is not yet settled.

"Instead of swallowing the thing up, the star can gradually eat it, for billions of years," said Kevin Burdge, an assistant professor of physics at MIT and a co-author of the study.

Open questions and what comes next

Several aspects of the discovery remain unresolved. The second system, ZTF J1444+4820, still lacks the kind of phase-resolved radial-velocity measurements that confirmed the accretor's identity in the first system, so the team is seeking additional spectroscopy, ideally in the near-infrared, to pin down its component masses and temperatures with the same confidence. The physics of how the infalling material's energy is absorbed and radiated at the point of impact -- the hot spot itself -- is also only loosely modeled in the current analysis; the authors describe the modeling used so far as "qualitative" and call for more detailed work on energy transport in this kind of direct-impact accretion, a process they say is largely unexplored for such low-mass stars. Determining precisely how long the mass transfer can continue will require combining mass-transfer calculations with models of tidal forces and angular momentum loss, together with broader population studies of how many such systems might exist throughout the galaxy.

On that last point, the researchers found both systems through a systematic search of Zwicky Transient Facility data for short-period variable stars with unusually large brightness swings -- a reminder that such pairs are rare enough that only a handful are known. The team expects the Vera C. Rubin Observatory's Legacy Survey of Space and Time, now ramping up in Chile, to be particularly effective at turning up more examples, because its survey will repeatedly image the sky in six different colors, well suited to catching the color changes produced by a hot spot moving in and out of view. Finding more such systems, the authors argue, would help settle how common this slow-motion consumption really is, and how much of it is happening, largely unnoticed, throughout the galaxy. Additional coverage of the findings appeared at Space.com, and the full study is available as an open-access manuscript through MIT's institutional repository.