Active
asteroid P/2012 F5 captured by Keck II/DEIMOS in mid-2014. Top panel
shows a wide-angle view of the main nucleus and smaller fragments
embedded in a long dust trail. Bottom panel shows a close-up view with
the trail numerically removed to enhance the visibility of the
fragments. Credit: M. Drahus, W. Waniak (OAUJ) / W. M. Keck Observatory. Hi-res image
Brightness
fluctuations of the nucleus of P/2012 F5 during two consecutive
rotation cycles. Presented versus time (top panel) and versus the
nucleus rotation phase (bottom panel). Credit: M. Drahus, W. Waniak (OAUJ) / W. M. Keck Observatory. Hi-res image
Mauna Kea, Hawaii
– A team led by astronomers from the Jagiellonian University in Krakow,
Poland,
recently used the W. M. Keck Observatory in Hawaii to observe and
measure a rare class of “active asteroids” that spontaneously emit dust
and have been confounding scientists for years. The team was able to
measure
the rotational speed of one of these objects, suggesting the asteroid
spun so
fast it burst, ejecting dust and newly discovered fragments in a trail
behind
it. The findings are being published in Astrophysical Journal Letters on
March
20, 2015.
Unlike the hundreds of thousands of asteroids in the main
belt of our solar system, which move cleanly along their orbits, active asteroids
were discovered several years ago mimicking comets with their tails formed by
calm, long lasting ice sublimation.
Then in 2010 a new type of active asteroid was discovered,
which ejected dust like a shot without an obvious reason. Scientist gravitated
around two possible hypotheses. One states the explosion is a result of a
hypervelocity collision with another minor object. The second popular
explanation describes it as a consequence of “rotational disruption”, a process
of launching dust and fragments by spinning so fast, the large centrifugal
forces produced exceed the object’s own gravity, causing it to break apart.
Rotational disruption is the expected final state of what is called the YORP
effect – a slow evolution of the rotation rate due to asymmetric emission of
heat.
To date, astronomers have identified four objects suspected
of either collision- or rotation-driven activity. These four freakish asteroids
are all very small, at a kilometer or less, which makes them unimaginably faint
when viewed from a typical distance of a couple hundred million miles. Despite prior
attempts, the tiny size of the objects kept scientists from determining some of
the key characteristics that could prove or disprove the theories.
Until last August, when the team led by Michal Drahus of the
Jagiellonian University was awarded time at Keck Observatory.
“When we pointed Keck II at P/2012 F5 last August, we hoped
to measure how fast it rotated and check whether it had sizable fragments. And
the data showed us all that,” Drahus said.
The team discovered at least four fragments of the object,
previously established to have impulsively ejected dust in mid-2011. They also
measured a very short rotation period of 3.24 hours – fast enough to cause the object
impulsively explode.
“This is really cool because fast rotation
has been suspected of catapulting dust and triggering fragmentation of some
active asteroids and comets. But up until now we couldn’t fully test this
hypothesis as we didn’t know how fast fragmented objects rotate,” Drahus said.
The astronomers calculated the object’s
rotation period by measuring small periodic fluctuations in brightness. Such
oscillations occur naturally as the irregular nucleus rotates about its spin
axis and reflects different amounts of sunlight during a rotation cycle.
“This is a well-established technique but
its application on faint targets is challenging,” said Waclaw Waniak of the
Jagiellonian University who processed the Keck Observatory data. “The main
difficulty is the brightness must to be probed every few minutes so we don’t
have time for long exposures. We needed the huge collecting area of Keck II,
which captures a plentiful amount of photons in a very short time.”
The photons were then concentrated in the
telescope’s light path and sent to the DEIMOS instrument to produce the data
that allowed the scientists to determine P/2012 F5’s nature. While monitoring
brightness in the individual 3-minute exposures, scientists also compiled all
the data to produce a single ultra-deep image, which revealed the fragments.
The success wouldn’t be possible if the
selected target, P/2012 F5, were not an ideal candidate for this study. Alex R.
Gibbs discovered the object on March 22, 2012 with the Mount Lemmon 1.5 meter
reflector. It was initially classified as a comet, based solely on its “dusty”
look. But two independent teams quickly have shown all this dust was emitted in
a single pulse about a year before the discovery – something that doesn’t
happen to comets. When the dust settled in 2013, another team using the
University of Hawaii’s 2.2-meter telescope on Mauna Kea detected a star-like
nucleus and suggested a maximum size of 2 kilometers.
“We suspected that this upper limit was
close to the actual size of the object. Consequently, we chose to observe
P/2012 F5 because – despite its small size – it appeared to be the largest and
easiest to observe active asteroid suspected of rotational disruption,” said
Jessica Agarwal of the Max Planck Institute for Solar System Research who chose
P/2012 F5 as the subject.
As a result of the study, P/2012 F5 is the
first freshly fragmented object in the solar system with a well-determined spin
rate, and this spin rate turns out to be the fastest among the active
asteroids. A careful analysis made by the team shows that these two features of
the object are consistent with the “rotational disruption” scenario. But
alternative explanations, such as fragmentation due to an impact, cannot be
completely ruled out.
“There are many faster rotators among
asteroids which don’t show signs of a recent mass loss. And there are many
hypervelocity impactors straying out there and looking for targets to hit – be
it a fast or slow rotator,” Drahus said.
“We’re indebted to the Caltech Optical
Observatories for generously awarding Keck Observatory time for this program,” said
Drahus – formerly a NRAO Jansky Fellow at Caltech. “Without the huge collecting
area of Keck II’s 10-meter mirror, we wouldn’t be able to achieve our goals so
swiftly.”
The W. M. Keck Observatory operates the
largest, most scientifically productive telescopes on Earth. The two, 10-meter
optical/infrared telescopes near the summit of Mauna Kea on the Island of
Hawaii feature a suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field spectrographs and
world-leading laser guide star adaptive optics systems.
DEIMOS (the DEep Imaging and Multi-Object
Spectrograph) boasts the largest field of view (16.7 arcmin by 5 arcmin) of any
of the Keck instruments, and the largest number of pixels (64 Mpix). It is used
primarily in its multi-object mode, obtaining simultaneous spectra of up to 130
galaxies or stars. Astronomers study fields of distant galaxies with DEIMOS,
efficiently probing the most distant corners of the universe with high
sensitivity.
Keck Observatory is a private 501(c) 3
non-profit organization and a scientific partnership of the California
Institute of Technology, the University of California and NASA.
W. M. Keck Observatory
808.881.3827
sjefferson@keck.hawaii.edu
Source: W.M. Keck Observatory
