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Double star system flips planet-forming disk into pole position
New research led by an astronomer at the University of Warwick has found the first confirmed example of a double star system that has flipped its surrounding disc to a position that leaps over the orbital plane of those stars. The international team of astronomers used the Atacama Large Millimeter/sub-millimeter Array (ALMA) to obtain high-resolution images of the Asteroid belt-sized disc.
The overall system presents the unusual sight of a thick hoop of gas and dust circling at right angles to the binary star orbit. Until now this setup only existed in theorists' minds, but the ALMA observation proves that polar discs of this type exist, and may even be relatively common.
A circumbinary protoplanetary disk in a polar configuration (DOI: 10.1038/s41550-018-0667-x) (DX)
Nearly all young stars are initially surrounded by 'protoplanetary' disks of gas and dust, and in the case of single stars at least 30% of these disks go on to form planets. The process of protoplanetary disk formation can result in initial misalignments, where the disk orbital plane is different from the stellar equator in single-star systems, or different from the binary orbital plane in systems with two stars. A quirk of the dynamics means that initially misaligned 'circumbinary' disks—those that surround two stars—are predicted to evolve to one of two possible stable configurations: one where the disk and binary orbital planes are coplanar and one where they are perpendicular (a 'polar' configuration). Previous work has found coplanar circumbinary disks6, but no polar examples were known until now. Here, we report the first discovery of a protoplanetary circumbinary disk in the polar configuration, supporting the predictions that such disks should exist. The disk shows some characteristics that are similar to disks around single stars, and that are attributed to dust growth. Thus, the first stages of planet formation appear able to proceed in polar circumbinary disks.
(Score: 2) by Immerman on Wednesday January 16 2019, @04:55PM
Well, I too doubt it would actually rotate as a single entity, but figure that the stars must be orbiting much faster than the the disc, creating as the simplest effect a sort of subtle, complex gravitational strobe as their precessing elliptical paths brings them closer and further from each other. Consider the triangle formed by the two stars and an orbitting particle - the wider the triangle, the more the two stars gravitational influence will cancel out. And that will be different for a particle near the binary's poles than for another particle in the same ring near the binary plane, or anywhere in between. And of course there will be lots of other subtle effects as well.
So, over the course of a ring-year, every section of the ring will be tugged out-of-round, and toward the preferred plane by varying amounts, almost at random, with "winners" that get pulled toward the new plane a lot (still talking a tiny fraction of a degree), and "losers" that didn't get influenced as strongly. And the same thing will happen every subsequent ring-year - but with different winners and losers. Net effect is that the winners and loser from each year average out over the course of many years, and the ring rotates more-or-less coherently, though the outliers it will no doubt make the whole thing sort of "fuzzy". But fuzziness means inter-particle collisions that will constantly nudge the outliers back into circular paths in the common plane.
I would expect the inner rings to get realigned faster than the outer ones, so you'd get a sort of twisted "Guinan's hat" sort of shape, but you have to figure that this process probably started before the stars had even finished condensing themselves, while the protoplanetary disc was still the very fuzzy mostly-gaseous outer portions of the 3D protostellar disc. So the disc would be getting twisted into alignment while it was still very cloudlike, simultaneously with the initial flattening and circularizing of much of the disc into 2D rings. Heck, it might *still* be very cloudlike - the actual telescopic images are probably miniscule, barely enough to establish large-scale features like the disc's alignment.
The more I think about it, the more the complexity of the gravitational situation boggles my mind I could easily see the sustained strobing causing constant disruptions of the disc - but I have no idea whether that might slow down the formation of larger accumulations, or speed it up. I would suspect that interesting things might happen around the orbits that resonate with binary's frequency, but have no idea what those might be.
I did come across this rendering of what I assume is their working model of the current system: https://github.com/drgmk/hd98800_alma_c5/blob/master/figs/3d.png [github.com]