[BRIEF NOTE] On a "comet massacre" around Fomalhaut
Nancy Atkinson's Universe Today article highlights the latest discoveries made around Fomalhaut, a relatively heavy and bright star (a mass 2.1 times that of our sun's, a luminosity 18 times greater) that's located only 25.1 light years from our solar system. (For comparison, Alpha Centauri is 4.3 light years away.) Fomalhaut is a young star, estimated to be 200 million years of age plus or minus a hundred million or so years. A very young star might Fomalhaut might be reasonably expected to be in the process of forming a planetary system of its own, and indeed, as Sol Station points out as early as the 1980s observations of Fomalhaut found signs of a dense dust disk surrounding the star. 2008 observations made with the Hubble Space Telescope led to a possible sighting of an exoplanet in orbit, Fomalhaut b, a gas giant of undetermined size. (This discovery is unconfirmed, and the nature of Fomalhaut b--instrument error, perhaps, or evidence of something stranger than a planet--is currently being debated.) Atkinson pointed her readers to
There may be some frantic activity going on in the narrow, dusty disk surrounding a nearby star named Fomalhaut. Scientists have been trying to understand the makeup of the disk, and new observations by the Herschel Space Observatory reveals the disk may come from cometary collisions. But in order to create the amount of dust and debris seen around Fomalhaut, there would have to be collisions destroying thousands of icy comets every day.
“I was really surprised,” said Bram Acke, who led a team on the Herschel observations. “To me this was an extremely large number.”
[. . .]
Acke, from the University of Leuven in Belgium, and his team colleagues analyzed the Herschel observations and found the dust temperatures in the belt to be between –230 and –170 degrees C, and because Fomalhaut is slightly off-center and closer to the southern side of the belt, the southern side is warmer and brighter than the northern side.
Those observations collected starlight scattering off the grains in the belt and showed it to be very faint at Hubble’s visible wavelengths, suggesting that the dust particles are relatively large. But that appears to be incompatible with the temperature of the belt as measured by Herschel in the far-infrared.
While observations with Hubble suggested the grains in the dust disk would be relatively large, the Herschel data show that the dust in the belt has the thermal properties of small solid particles, with sizes of only a few millionths of a meter across. HST observations suggested solid grains more than ten times larger.
[. . .]
The bright starlight from Fomalhaut should blow small dust particles out of the belt very rapidly, yet such grains appear to remain abundant there.
So, the only way to explain the contradiction is to resupply the belt through continuous collisions between larger objects in orbit around Fomalhaut, creating new dust.
[. . .]
At Fomalhaut, however, to sustain the belt, the rate of collisions must be remarkable: each day, the equivalent of either two 10 km-sized comets or 2,000 1 km-sized comets must be completely crushed into small, fluffy dust particles.
In order to keep the collision rate so high, scientists say there must be between 260 billion and 83 trillion comets in the belt, depending on their size. This is not unfathomable, the team says, as our own Solar System has a similar number of comets in its Oort Cloud, which formed from objects scattered from a disc surrounding the Sun when it was as young as Fomalhaut.