These Two Young Stars May Not Have Been Born Together After All

Almost every massive star in the galaxy has a companion, and almost everything known about those pairs comes from looking at them long after they finished forming. By then the orbit has had millions of years to shift, so a grown-up binary can no longer say whether its two stars grew up together or found each other later.

A study published on 7 September in Nature Astronomy puts the question to a pair that is not finished yet. IRAS 07299-1651 is a knot of gas and dust with two young stars at its center, both still pulling in the material that will make them. Yichen Zhang of Shanghai Jiao Tong University led the project, with first author Yao Wang doing most of the analysis.

Their team combined sharp radio images of the system from ALMA, the array of dishes on a plateau in northern Chile, with data from the Very Large Array in New Mexico and infrared pictures from the James Webb Space Telescope and the Very Large Telescope. The ALMA observations span almost eight years, according to the Instituto de Astrofísica de Andalucía, one of the institutions on the paper. Over that span, the two dots moved around each other.

That movement is the measurement everything else rests on. Catching it let the team reconstruct the orbit in three dimensions instead of as a smear on the sky: where the plane of the orbit lies, which way the stars run around it, and how each star's disk of gas and dust is angled against it.

The orbits that fit best are stretched, close to parabolic, which is the shape at the boundary between a closed loop and a single pass that never returns. The two stars are about 200 astronomical units apart, 200 times the distance from Earth to the Sun. And both disks are strongly tilted out of the orbital plane. The team is fitting a short arc of a long path, so what it reports are preferred solutions rather than a settled orbit, with no period attached.

A pair this close together was expected to have a tidier history. Star formation models predict that binaries at such small separations should come from disk fragmentation, as the institute's account of the work explains. The massive disk around one growing star becomes unstable and breaks up, and a surviving fragment grows into the companion. Stars made that way inherit the disk's rotation, so the orbit comes out close to circular, with the disks roughly lined up with it. This system has neither.

What the authors offer instead is what they call a core merger. The two stars began forming on their own, in two clumps of gas that were never bound to each other, and then passed close enough on a near-parabolic path to be caught. The orbit is the leftover of that encounter, and the disks never had a reason to line up with it. The properties they measure, the authors write, "are naturally explained by" that picture, and a core merger "may represent an important pathway" for making stretched massive binaries, not the pathway.

Tilted disks on their own would not carry that argument. Gas falling onto a young star from different directions at different times can leave its disk askew with no encounter at all, and the paper's own reference list includes a 2024 study of how fragmentation makes misaligned binaries at roughly these separations. The case rests on the combination: a near-parabolic orbit and two disks out of plane, in one system.

This system has been read before, by many of the same people. In March 2019, in the same journal, a group led by Zhang reported the first measurements of how this pair moves and came to the opposite conclusion. "The observations indicate that disk fragmentation at several hundred astronomical units may have formed the binary," they wrote. That paper is also where the pair's combined mass comes from: a minimum of 18 suns, worked out from the difference in the two stars' velocities along our line of sight.

Neither reading was ever stated as settled. The 2019 conclusion was hedged too, and it rested on a much shorter run of data: the team could see how far apart the two stars were and how fast each was moving toward or away from us, but not their motion around each other. That motion is what the new observations add, and it is the measurement that tells the two scenarios apart. The favored explanation has moved because the data got better, not because anyone was caught out.

"Our findings suggest that the two stars were not born together, but began forming independently before undergoing a close gravitational encounter," said Rubén Fedriani, an astronomer at the Instituto de Astrofísica de Andalucía in Granada who led the infrared analysis, speaking in Spanish in his institute's announcement. Fedriani also makes a claim the paper itself does not: that watching two young massive stars orbit each other while a system of this kind is still forming inside its birth cloud is something nobody had measured directly before.

The stakes are not local to one cloud. Massive binaries are where supernovae, X-ray binaries and merging black holes come from, so how a pair gets together (and the orbit it starts with) sets what it can become. Nothing here retires disk fragmentation: the paper's own opening lists it among the competing scenarios, alongside core fragmentation and capture, and this is one object.

What the work does hand other groups is a method and the material to check it. The reduced ALMA and Very Large Array images are posted openly on Zenodo, the orbit was fitted with open-source code the paper names by version, and the system will keep moving. Another team can refit the same arc today, or wait a few years and fit a longer one.