[meteorite-list] Herschel Spots Comet Massacre Around Nearby Star

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 11 Apr 2012 10:19:39 -0700 (PDT)
Message-ID: <201204111719.q3BHJdDh027111_at_zagami.jpl.nasa.gov>

http://www.esa.int/esaSC/SEM1XBHWP0H_index_0.html

Herschel spots comet massacre around nearby star
European Space Agency
11 April 2012

ESA's Herschel Space Observatory has studied the dusty belt around the
nearby star Fomalhaut. The dust appears to be coming from collisions
that destroy up to thousands of icy comets every day.
 
Fomalhaut is a young star, just a few hundred million years old, and
twice as massive as the Sun. Its dust belt was discovered in the 1980s
by the IRAS satellite, but Herschel's new images of the belt show it in
much more detail at far-infrared wavelengths than ever before.

Bram Acke, at the University of Leuven in Belgium, and colleagues
analysed the Herschel observations and found the dust temperatures in
the belt to be between -230 and -170??C. However, because Fomalhaut is
slightly off-centre and closer to the southern side of the belt, the
southern side is warmer and brighter than the northern side.

Both the narrowness and asymmetry of the belt are thought to be due to
the gravity of a possible planet in orbit around the star, as suggested
by earlier Hubble Space Telescope images.

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 metre across.

But this created a paradox because the Hubble Space Telescope
observations suggested solid grains more than ten times larger.

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.

To resolve the paradox, Dr Acke and colleagues suggest that the dust
grains must be large fluffy aggregates, similar to dust particles
released from comets in our own Solar System.
 
These would have both the correct thermal and scattering properties.
However, this leads to another problem.

The bright starlight from Fomalhaut should blow small dust particles out
of the belt very rapidly, yet such grains appear to remain abundant there.

The only way to overcome this contradiction is to resupply the belt
through continuous collisions between larger objects in orbit around
Fomalhaut, creating new dust.

To sustain the belt, the rate of collisions must be impressive: each
day, the equivalent of either two 10 km-sized comets or 2000 1 km-sized
comets must be completely crushed into small fluffy, dust particles.

"I was really surprised," says Dr Acke, "To me this was an extremely
large number."

To keep the collision rate so high, there must be between 260 billion
and 83 trillion comets in the belt, depending on their size. 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.

"These beautiful Herschel images have provided the crucial information
needed to model the nature of the dust belt around Fomalhaut," says
G??ran Pilbratt, ESA Herschel Project Scientist.

Contact for further information
<http://www.esa.int/SPECIALS/Herschel/SEMCFDEWF0H_0.html>
 
Received on Wed 11 Apr 2012 01:19:39 PM PDT