Models and observations indicate that both stars and planets form as a cloud of material collapses into a disk. If the process proceeds in an orderly manner, then the planets will all form from the same disk and thus orbit in the same plane. And—because material from the same disk will fall into the star, bringing its momentum with it—the star will rotate with its equator along the same plane. That should lead to a tidy system with the equator of the star lined up with the plane of any planets orbiting it.
Except when it doesn't. Anything that upsets the even inflow of material—from clumping in disk to a passing star—can upset this process. We've seen the results: planet-forming disks and planetary orbits that don't line up with a star's equator.
Now, researchers are reporting a complex, four-star system where a planet-forming disk is lined up perpendicular to the stars so that it orbits over their poles.
Practice vs. theory
To a certain extent, this sort of system had been predicted. Calculations had suggested that binary star systems should have complex interactions with any disks that surround them. In some cases, this will strictly enforce orderliness, as the gravitational pull will force a disk to line up with the equators of the stars. But if the disk starts out far enough away from this plane, the gravitational interactions will flip the plane of the disk perpendicular, so it goes above and below the two stars' poles.
Although we've had calculations suggesting this was a probability, we hadn't actually observed it in any binary star systems. That prompted an international team of astronomers to take a careful look at the star system HD 98800.
Even without planets, HD 98800 is rather complex, consisting of two pairs of binary stars. This has led to some truly atrocious nomenclature, with one binary termed "A" and its two stars called "a" and "b," while the second binary is named "B" and grouped with "a" and "b" stars. Consequently, the paper is filled with discussion of the BaBb binary.
In any case, the two binary systems have stars that orbit their mutual centers of gravity at a distance about the same as the Earth's distance from the Sun. The two binary systems are separated by about 54 times that distance, or about 1.8 times the Sun-Neptune distance. All four stars in the system appear to be young, and a disk of material was found to be orbiting the BaBb pair back in the 1980s. The inner edge of this disk is set by the two stars it orbits, while the outer edge is limited by the second set of stars farther out.
But the properties of the disk—its composition and orientation relative to the two stars—wasn't clear based on these initial observations. So the researchers turned to the ALMA telescope array. The hardware there both allowed them to refine the orbits of the stars in the system and to track the material in the surrounding disk.
ALMA for the data-poor
ALMA is sensitive to wavelengths that are emitted by carbon monoxide, which is a common material in planet-forming disks. This allowed the researchers to make a high-resolution map of the disk, including its orbital velocity, which causes different regions of the disk to emit red- and blue-shifted light depending on whether they're moving toward or away from us. They then built various models of the disk based on the ALMA data.
The models indicate a number of things. To begin with, they suggest that the disk can't just be solid material, as this would be rapidly ejected from the orbit. This implies there's gas around, allowing friction and gravitational attraction to keep the disk intact. This means we've found a planet-forming disk and not a remnant debris disk that had failed to coalesce into planets.
The second clue to come from the models is that there are two possible orientations of the disk relative to the binary stars, and one of them is perpendicular to the plane of the stars' orbit. In other words, the disk loops over and under the poles of the stars. Given that this was already something predicted by orbital calculations, the researchers suspect that this is what we're looking at.
Finally, the authors simulated a disk that's a bit out of alignment from a true vertical, and they found that gravitational interactions would torque it back into a vertical alignment in less than 1,000 years. This indicates the present configuration is a stable one that would form naturally from any disk that started out anywhere close to this alignment.
How would a disk form in an off-kilter orientation? Conveniently, the additional binary in this four-star system offers a possible explanation. If it was a relatively late arrival—meaning the two binary systems formed separately and only became gravitationally bound afterwards—then its arrival could have thrown off the orientation of the disk. Alternatively, the disk could have been thrown off center by the same processes that cause this to happen to solo stars: uneven distribution of material in the cloud of material that gives birth to the systems.
One final tidbit came out of the Alma observations: the material in the disk appears to have condensed into dust particles, a key early step that enables planet formation. So, at some point in the future, there may be planets orbiting the system graced with two suns and two very bright, very nearby stars.
Nature Astronomy, 2019. DOI: 10.1038/s41550-018-0667-x (About DOIs).
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