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An MIT-led team catches a star 300 light-years away in the middle of a billion-year meal

A red dwarf called ZTF J0440+2325 is slowly pulling material from a brown dwarf companion, in a binary system so compact the smaller object orbits the larger every 87 minutes.

astronomy

— 4 mins read

About 300 light-years away, a small red dwarf star is eating its companion — not in a burst, but in geological slow motion. The system, catalogued as ZTF J0440+2325, was first flagged by the Zwicky Transient Facility at Palomar Observatory and followed up with an international suite of telescopes run by Caltech, the University of Hawaii, the Instituto de Astrofísica de Canarias, and the Harvard-Smithsonian Center for Astrophysics. The study, published October 5 in Nature Astronomy and led by MIT’s Kevin Burdge, describes one of the most extreme compact binaries on record: a star of roughly 85 Jupiter masses pulling material from a brown dwarf companion of roughly 25 Jupiter masses, completing a full orbit every 87 minutes.

What makes ZTF J0440+2325 unusual is less the violence of the transfer than the timescale. Most mass-transferring binaries astronomers know of are doing it fast — the donor is close enough to the donor star’s Roche lobe that material spills over on human-observable timescales. Here, the geometry is so tight that the donor is technically still under-filling its Roche lobe, and what the red dwarf is siphoning is a slow trickle of material lost to gravitational stripping and atmospheric escape. The paper’s headline estimate is that the current phase of mass transfer could persist for anywhere between hundreds of thousands and billions of years — a span so wide that the system reads less as a transient event than as a stage in a stellar life cycle the galaxy has been quietly running since the early universe.

The physics that makes the trick rate detectable is the same thing that made the system findable in the first place: at 87 minutes per orbit, ZTF J0440+2325 is bright, eclipses its companion frequently, and shows up as a variable source in time-domain surveys. The Zwicky Transient Facility caught the system’s brightness dipping on that ultra-short period, and follow-up spectroscopy across the collaborating telescopes pinned down the masses and the donor configuration. The same weekly sky that also turned up an oldest-yet fast radio burst detected by MeerKAT, originating roughly 10 billion years ago, is the kind of pipeline now routinely turning rare compact systems into recoverable targets.

For stellar physics, the slow-accretion regime is genuinely useful. A donor that is almost filling its Roche lobe, and being eroded rather than dumped over wholesale, lets theorists test models of how substellar atmospheres respond when the tidal and irradiation environment of a much more compact orbit starts to peel them apart. The 85- and 25-Jupiter-mass numbers put both objects in uncomfortable territory — the star is barely above the hydrogen-fusion threshold, and the brown dwarf never crossed it — and the long expected duration of the transfer means observers can watch the same system evolve across decades of follow-up rather than catching it once and inferring the rest.

The practical punchline is that ZTF J0440+2325 is not a one-off so much as a sign of what current surveys can find. The Zwicky pipeline was built for transients, and a binary that eclipses every 87 minutes looks, from a naive detection standpoint, a lot like one. Once the survey flagged it, the rest of the work — confirming the masses, ruling out the alternatives, modelling the mass-transfer rate — was a fairly standard multi-telescope campaign. More systems like this will surface as ZTF’s successors, including the Vera C. Rubin Observatory’s LSST, start producing their full time-domain catalogs; whether the slow-accretion phase is common, or whether ZTF J0440+2325 is an unusually well-aligned rarity, is the question the discovery now makes it possible to ask.

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