P / 2012 F5: an asteroid explosive

A team of astronomers from the Jagiellonian University in Krakow has carried out a detailed study of P / 2012 F5, the most promising of a new class of asteroids.
A team led by astronomers at the Jagiellonian University in Krakow, Poland, has recently used the WM Keck Observatory in Hawaii to observe a peculiar class of “active asteroids”, which spontaneously emit dust and which they are tormenting scientists for years. The team was able to measure the speed of rotation of one of these objects, suggesting that the asteroid has rotated so quickly than it exploded, releasing dust and rock fragments along a trail behind him. The results were published in the journal Astrophysical Journal Letters.



In 2010 it was discovered a new type of active asteroid that expels dust in a kind of explosion, for no apparent reason. Unlike the hundreds of thousands of asteroids in the main belt of our solar system, which move along their orbits without major shocks, active asteroids were discovered a few years ago and, in a manner that mimics the comets, with the formation of a tail due to sublimation of the ice.
The scientists assessed mainly two hypotheses. One considers that the explosion is the result of a high speed collision with another smaller object. The second hypothesis explains these explosives episodes as the consequence of a “rotational perturbation”, the expulsion of dust and debris would then be due to large centrifugal forces produced by the own gravity of the object, which would result in the breakage. Breakage due to the rotation is what we expected at the end of the so-called Yarkovsky-O’Keefe-Radzievskii-Paddack effect: a slow evolution of the rate of rotation due to asymmetric emission of heat.
To date, astronomers have identified four objects suspected of expulsion. These four asteroids are very small, with diameters of about a mile or less, and this makes them incredibly weak if observed from a typical distance of about 300 million km. Despite previous attempts, the small size of these objects has prevented scientists to determine some of the main features that could prove or disprove the theories proposed.
Until August last year, when the team of scientists led by Michal Drahus the Jagiellonian University was awarded the observing time at the Keck Observatory.
“When we pointed the Keck II object known as P / 2012 F5, we hoped to measure its rotation speed and check if it presented fragments of considerable size. And the data showed everything that we expected, ” said Drahus.
The team found at least four fragments of the object, which was known to have had an episode of expulsion in 2011. They also measured a very short period of rotation (3.24 hours), fast enough to cause the explosion of the object.



“This is really great because the fast rotation is one of the mechanisms postulated for the expulsion of dust and the onset of fragmentation for some active asteroids and comets, but until now we have not been able to fully test this hypothesis because we did not know how quickly revolved fragmented objects,” explained Drahus.
Astronomers have calculated the rotation period of the object by measuring small periodic fluctuations in brightness. These variations occur because the core, of irregular shape, rotates about its axis and reflects different amounts of sunlight during a cycle of rotation.
“This is a well-established technique, but its application on objects particularly weak is a challenge,” said Waclaw Waniak of the Jagiellonian University, who helped process the data. “The main difficulty is that the brightness should be measured every few minutes, so we cannot afford long exposures, normally required for objects so weak. We therefore need an extensive area of collection on Keck II, which is able to capture an abundant quantity of photons in a very short time. “

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