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<\/p>\nVega\u2019s Puzzling Disk
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Over the weekend I learned about Joseph Haydn\u2019s Symphony No. 47, unusual in that it offers up some of its treasures in perfect symmetry. Dubbed \u2018The Palindrome,\u2019 the symphony\u2019s third movement, Minuetto e Trio<\/em>, is crafted to play identically whether attacked normally \u2013 moving forward through the score \u2013 or backwards. You can check this out for yourself in this YouTube video<\/a>, or on this non-auditory reference<\/a>.<\/p>\n The pleasure of unexpected symmetry is profound, and when seen through the eyes of our spacecraft, can be startling. Consider the storied star Vega. We see this system from our perspective at a very low inclination angle relative to its rotational axis, as if we were looking down from above the star\u2019s pole. This face-on perspective is profoundly interesting when examined through our space-borne astronomical assets. In the image below, we get two views of Vega\u2019s disk, from Hubble and then JWST.<\/p>\n Image:<\/strong> The disk around Vega as seen by Hubble (left) and Webb (right). Hubble detects reflected light from dust the size of smoke particles largely in a halo on the periphery of the 160-billion-kilometer-wide disk. Webb resolves the glow of warm dust in a disk halo, at 37 billion kilometers out. The outer disk (analogous to the solar system\u2019s Kuiper Belt) extends from 11 billion kilometers to 24 billion kilometers. The inner disk extends from the inner edge of the outer disk down to close proximity to the star. Credit: NASA, ESA, CSA, STScI, S. Wolff (University of Arizona), K. Su (University of Arizona), A. G\u00e1sp\u00e1r (University of Arizona).<\/p>\n We don\u2019t, of course, have the perfect symmetry of the Haydn minuet and trio here, but do notice how smooth this disk appears, and particularly take note of the lack of any embedded planets, the sort of large objects we observe in formation around stars like Beta Pictoris. This bright star, a summer object in Lyra for those of us in the northern hemisphere, was well studied in the infrared by astronomers using the Spitzer Space Telescope, and now we have the clearest view ever, the subject of two upcoming papers in The Astrophysical Journal<\/em>. Co-author Andras G\u00e1sp\u00e1r (Steward Observatory, University of Arizona, calls the Vega disk \u201cridiculously smooth.\u201d<\/p>\n The view shows layered structure, with Hubble detecting dust on the outer regions of the disk and Webb\u2019s infrared view resolving the warmer dust in the inner disk, the disk in this region being sand-sized as compared to the outer halo, whose particles are of the consistency of smoke. These are useful observations because they offer information on dust movement in circumstellar disks, and also point out the stark difference among the planetary disks thus far observed. The hot, A-class Vega at 450 million years old remains a relatively young star and we would expect interactions among debris in the disk to keep replenishing it, a process that continues in our own much older system. <\/p>\n But where are the planets in formation? Lead author Kate Su (University of Arizona), says that the lack of them forces astronomers to rethink the way they\u2019ve looked at forming planetary systems. It\u2019s possible to tease out a gap at about 60 AU, but everywhere else the disk appears smooth, which rules out planets larger than Neptune in outer orbits. Adds Su:<\/p>\n \u201cWe\u2019re seeing in detail how much variety there is among circumstellar disks, and how that variety is tied into the underlying planetary systems. We\u2019re finding a lot out about the planetary systems \u2014 even when we can\u2019t see what might be hidden planets. There\u2019s still a lot of unknowns in the planet-formation process, and I think these new observations of Vega are going to help constrain models of planet formation.\u201d<\/p>\n<\/blockquote>\n In the second of the two papers, lead author Schuyler Wolff (University of Arizona) and colleagues home in on the star Fomalhaut by way of comparison, this being a star that is a close twin to Vega in terms of luminosity, distance and age. As the paper notes, \u201c[T]he Vega disk morphology differs significantly from Fomalhaut,\u201d going on to say:<\/p>\n Vega\u2019s fellow archetypical disk, Fomalhaut, has been extensively studied in scattered light (Kalas et al. 2005; G\u00e1sp\u00e1r & Rieke 2020) and shows a narrow cold belt from 130-150 au (possibly confined by an undetected planet e.g., Boley et al. 2012) with a scattered light halo extending outwards from the belt. The parent planetesimal belt is clearly detected with ALMA (MacGregor et al. 2017), with an estimated dust mass of 0.015\u00b10.010M\u2295. Fomalhaut has also recently been the target of groundbreaking JWST observations (G\u00e1sp\u00e1r et al. 2023) showing the same narrow cold belt (albeit with a slight radial offset indicative of grain size stratification) and halo.<\/p>\n<\/blockquote>\n Indeed, Fomalhaut appears to be building a planet within its debris disk, and possibly more than one, these objects acting as \u2018shepherds\u2019 shaping the disk. Any such worlds in the Vega system are clearly much smaller, if indeed they exist. Vega\u2019s dust distribution is broad, whereas at Fomalhaut the outer debris ring is the dominant source of light available for our observation. Three nested debris belts appear at Fomalhaut.<\/p>\n Nothing better than a new astronomical puzzle, especially when it challenges our notions of planet formation. You may remember Vega\u2019s role in Heinlein\u2019s Have Spacesuit Will Travel<\/em> (1958), where its civilization acts as a kind of overseer for Earth, and perhaps the \u2018Vegan Tyranny\u2019 from James Blish\u2019s Cities in Flight<\/em> series. For that matter, Vega has an important commercial role in Asimov (the source of \u2018Vegan tobacco\u2019 in the Foundation books), appears prominently in Jack Vance\u2019s work, and is of course of great interest to Ellie Arroway in Carl Sagan\u2019s book and film Contact<\/em>. How enjoyable that it now throws a new mystery at us.<\/p>\n The papers are Su et al., \u201cImaging of the Vega Debris System using JWST\/MIRI,\u201d accepted at The Astrophysical Journal<\/em> (preprint<\/a>) and Wolff et al., \u201cDeep Search for a scattered light dust halo around Vega with the Hubble Space Telescope,\u201d accepted at The Astrophysical Journal<\/em> (preprint<\/a>).<\/p>\n<\/p>\n\n
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