What is happening around Fomalhaut?

Image credit: NASA/ESA

The circumstellar debris disk at Fomalhaut (α PsA, 7.7 pc, A3V) was among the first to be detected way back in 1984 by the IRAS satellite. Such debris disks are analogous to the  Solar System’s Kuiper belt and can be seen among the closest stars via their infrared excesses. Given what we know about the diversity of planetary systems, there is no reason to suspect these disks should be similar to the Solar System case. They might be more extensive or rarefied, or for that matter denser and compact. If the disk is acted upon gravitationally by large planets it may become a ring, with well-defined inner and outer edges, which might be continually replenished by loss from the inner gaseous planet. The material giving rise to the infrared excess at Fomalhaut is known to arise from collisional processes and sublimation of large bodies. Such a ring might persist around a young  (~ 400 Myr) star like Fomalhaut, not having had time to dissipate via further accretion.

Could a star rather more massive than the Sun really harbour a large planet shepherding, and perhaps replenishing, a huge disk? This is what seems to be observed in visible light by Hubble (above), using techniques to subtract the image of the central star. The finding has been recently confirmed, and the absence of a near-infrared detection limits is mass to less than ~ 2MJup , demonstrably sufficient to at least sculpt the disk. It is worth noting that Fomalhaut has a very distant (57000 AU) common proper motion companion, Gliese 879 (TW PsA), and possible that the offset of the disk from Fomalhaut arises dynamically via binary interaction. The system is imaged (left) by Herschel at 70 μm in the far-infrared, showing the smooth disk, which arises from cometary collisions occurring at a very high rate. The enormous number of collisions required (the equivalent of 2000 1 km size comets every day) seem more reasonable if we postulate disk replenishment from the planet. At even longer wavelengths, 850 μm, purely thermal emission by dust is traced, resulting in the toroidal pattern shown (right), where the highest intensities are seen where the line of sight intercepts the largest amounts of disk dust.

The observed spectral energy distribution of the planet from optical through mid-infrared can not be explained by direct or scattered radiation alone. Two models have very recently been proposed: either a large circumplanetary disk around a massive, but unseen, planet or the aftermath of a collision during the past 100 years of two Kuiper Belt-like objects of radii about 50 km. The former scenario would be supported if there indeed is some mechanism whereby the planet is still losing mass to the disk. Speculatively, Fomalhaut b could be a super-Earth sized molten lava body, continually pummelled by collisions. Further observations will be needed to elucidate the connection between the giant body dynamically inferred by the existence of the disk and the visible light source clearly seen in the HST data.

Update: Fomalhaut b has been confirmed and appears to be on a rogue, disk crossing orbit.


One Response to “What is happening around Fomalhaut?”

  1. […] of Fomalhaut, was first detected in 1984. I have looked at the putative planet of Fomalhaut in a previous post; its existence and nature remain controversial. The beta Pic planet on the other hand was first […]

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