Spanish researchers have observed a rare asteroids from the Gran Telescopio Canarias
(Spain) and have discovered that something happened around the 1st July
2011 causing a comet-like trail to appear: maybe internal rupture or
collision with another asteroid. To date, ten asteroids have been
located to date that at least at one moment have displayed a trail like
that of comets. They are named main-belt comets (MBC) as they have a
typical asteroidal orbit but display a trail at the same time, which
means that their dust (and possibly gas) emission activity is similar to
that of comets.
One of these objects, baptised as P/2012 F5 (Gibbs), was discovered in March 2012 from the Mount Lemmon Observatory
in Arizona (USA). In May and June of that year, Spanish astrophysicists
from the Gran Telescopio Canarias tracked it and have discovered when
the trail was born using mathematic calculations.
"Our models indicate that it was caused by an impulsive short-lived
event lasting just a few hours around the 1st July, 2011, with an
uncertainty of 20 days," as explained to SINC by Fernando Moreno,
researcher at the Astrophysics Institute of Andalusia (CSIC). In
collaboration with other colleagues from the Astrophysics Institute of
the Canary Islands and the University of La Laguna, the data have been
published in the 'The Astrophysical Journal Letters'.
The telescope images reveal "a fine and elongated dust structure that
coincides exactly with the synchrone of that day," says Moreno. For a
given observation date, a synchrone is the position in the sky of the
particles emitted from these types of objects with zero speed in an
instant of time. In this case the synchrone of the 1st July, 2011 is the
best adapted to the fine trail.
The width and varying brightness of the head to the end of the trail
allowed for the researchers to deduce the physical properties of the
particles and proportions of their different sizes.*
As for the maximum size and speed values of the liberated particles, the
team has calculated that the asteroid should have a radius of between
100 m and 150 m and the dust mass emitted should be about half a million
tonnes.
Researchers juggled two possible theories for the origin of the
P/2012 F5 trail: "It could have arisen from collision with another
asteroid or rather a rotational rupture." The second mechanism consists
of material gradually breaking free after partial fragmentation of the
asteroid.
In turn, the rapid spinning of the asteroid, "like an accelerating
carousel" causes pieces to break off. The rotation speed of small
asteroids can increase over time due to the Yarkovsky effect (YORP effect
for short). This can induce acceleration due to the thermal differences
of the different surface regions of the asteroid, eventually leading to
rupture.
Moreno indicates that, from the brightness distribution of the trail,
"we have verified that the dependence of the speed of particle ejection
on size is very weak, in accordance with what we already obtained for
the other asteroid of this group: 596 Scheila, which probably suffered a collision."
MBCs are main-belt asteroids situated at a distance of between 2 and
3.2 astronomical units, which is the average distance between the Earth
and the Sun. For some reason they become active and emit dust. For now
they have not been found to generate gas but this could be due to the
fact that they are weak at the very moment of observation.
Since the first discovery of an MBC in 1996, the 133P/Elst-Pizarro,
a total of ten have now been found. The presence of a trail in some has
lasted for a relatively long period of a few months, like in the cases
of 2006 VW139 and P/2010 R2 (La Sagra). The latter was discovered from
an observatory of the same name in Granada. Its activity could have been
due to an ice sublimation which could have released the gas, although
this has not been detected.
In other cases, however, activity developed during a short period of
time, like in the case of 596 Scheila. Its dust cloud dissipated very
quickly in a matter of hardly three or four weeks following its
detection.
There are also examples of MBCs that have shown recurrent activity,
like 133P/Elst-Pizarro and 238P, which have displayed a trail on more
than one occasion.
In the case of P/2012 F5, it is still unknown what group it belongs
to. More data will be available when it can be observed again in good
conditions next year in around July or August 2014.
The last documented MBC to date is the so-called P/2012 T1 (PANSTARRS),
which Spanish astrophysicist are also analyzing. Similar to what has
happened with exoplanets, many more main-belt comets will start
appearing in the coming years.
Reference: Fernando Moreno, Javier Licandro, Antonio Cabrera-Lavers. "A short-duration event as the cause of dust ejection from Main-Belt Comet P/2012 F5 (Gibbs)". The Astrophysical Journal Letters 761 (1), December 2012. Doi: 10.1088/2041-8205/761/1/L12.
Source: The Daily Galaxy via SINC
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