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NAM 2008
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CONTACT DETAILS ARE LISTED AT THE END OF THIS RELEASE
ASTRONOMERS FIND EMBRYONIC PLANET
Using radio observatories in the UK and US and computer simulations, a
team of astronomers have identified the youngest forming planet yet
seen. Team leader Dr Jane Greaves of the University of St Andrews will
discuss the ‘protoplanet’ in her talk at the RAS National Astronomy
Meeting in Belfast on Wednesday 2 April.
Taking advantage of a rare opportunity to use the Very Large Array (VLA)
of radio telescopes in the US with the special addition of an extra
telescope 50 km away, the team studied the disk of gas and rocky
particles around the star HL Tau. This star is thought to be less than
100000 years old (by comparison the Sun is 4600 million years old) and
lies in the direction of the constellation of Taurus at a distance of
520 light years. The disk around HL Tau is unusually massive and bright,
which makes it an excellent place to search for signs of forming planets.
The VLA gives very sharp images of HL Tau and its surroundings. The team
studied the system using radio emission at a wavelength of 1.3 cm,
specifically chosen to search for the emission from super-large rocky
particles about the size of pebbles. The presence of these pebbles is a
clue that rocky material is beginning to clump together to form planets.
In the UK, scientists used the MERLIN array of radio telescopes centred
on Jodrell Bank in Cheshire, to study the same system at longer
wavelengths. This allowed the astronomers to confirm that the emission
is from rocks and not from other sources such as hot gas. Jodrell Bank
scientists Dr Anita Richards and Dr Tom Muxlow analysed the data.
The big surprise was that, as well as detecting super-large dust in the
disk around HL Tau, an extra bright 'clump' was seen in the image. It
confirms tentative ‘nebulosity’ reported a few years earlier at around
the same position, by a team lead by Dr Jack Welch of the
Berkeley-Illinois-Maryland Array. The new image shows the same system in
much greater detail.
Dr Greaves comments, “We see a distinct orbiting ball of gas and dust,
which is exactly how a very young protoplanet should look. In the
future, we would expect this to condense out into a gas giant planet
like a massive version of Jupiter. The protoplanet is about 14 times as
massive as Jupiter and is about twice as far from HL Tau as Neptune is
from our Sun.”
Dr Richards adds, “The new object, designated HL Tau b, is the youngest
planetary object ever seen and is just 1 percent as old as the young
planet found in orbit around the star TW Hydrae that made the news last
year. HL Tau b gives a unique view of how planets take shape, because
the VLA image also shows the parent disk material from which it formed.”
Team member Dr Ken Rice of the University of Edinburgh ran a computer
simulation to find out how such a massive protoplanet could form. His
animation shows a very similar body condensing out of a disk with
similar properties to that actually observed around HL Tau. The planet
forms because of gravitational instability in the disk, which is about
half as massive as the star itself. This allows small regions to
separate out and cool down into self-contained structures. This
instability mechanism has been controversial, but the simulated and real
data are such a good match that it seems the mechanism really does
operate in nature.
Dr Rice comments, “The simulations were as realistic as we could make
them and we were delighted that the results compare so well with the
observations.”
One intriguing property is that XZ Tau, another young star in the same
region, may have passed near HL Tau about 1600 years ago. Although not
required for planet formation, it is possible that this flyby 'tweaked'
the disk and helped it become unstable. This would be a very recent
event in astronomical terms. Whether the proto-planet formed in only the
last few hundred years, or sometime in the 100000 years since the birth
of HL Tau, the images provide a unique view of planet formation in
action, and the first picture of a protoplanet still embedded in its
birth material.
IMAGES AND MOVIE:
Images of HL Tau and still and movie from simulation :
http://www.roe.ac.uk/~wkmr/HLTau/HLTau.html
Figure 1: The false colour image is a map of the radio emission (at a
wavelength of 1.3 cm) emitted from the region around the star HL
Tau. The candidate protoplanet is marked 'b'. The bar at top left
(marked 50 AU) indicates 50 times the Earth-Sun distance on the same
scale, or about the size of the orbit of Pluto. HL Tau is located in the
centre of the image. The star is surrounded by a dusty disc tilted to
the line of sight; only the inner part is visible here but its extent is
indicated by the white ellipse. The arrows show the direction of the
jets of hot gas emitted as 'overspill' from the star growth process.
Image: VLA and Pie Town antenna
Figure 2: This is an image from the computer simulation of HL Tau and
its surrounding disk. In the model the dense clump (seen here at top
right) forms with a mass of about 8 times that of Jupiter at a distance
from the star about 75 times that from the Earth to the Sun. Image: Ken
Rice / Royal Observatory Edinburgh
Movie: Animation from the simulation showing the growth of the
protoplanet in the disc around HL Tau. The total time is about 3000
years and shows the protoplanet forming at the edge of the disc, and
then spiralling inwards to a distance about 75 times further from the
central star than the Earth is from the Sun. Movie credit : Ken Rice
FURTHER INFORMATION
Jodrell Bank Centre for Astrophysics
http://www.jb.man.ac.uk
University of St Andrews Astronomy Group
http://star-www.st-and.ac.uk/index.php
Royal Observatory Edinburgh
http://www.roe.ac.uk
RAS National Astronomy Meeting
http://nam2008.qub.ac.uk
RAS home page
http://www.ras.org.uk
Science and Technology Facilities Council
http://www.stfc.ac.uk
NOTES FOR EDITORS
The VLA is operated by the National Radio Astronomy Observatory (USA) on
behalf of United Universities, Inc.
MERLIN is operated by the University of Manchester, Jodrell Bank
Observatory, on behalf of the Science and Technology Facilities Council
(STFC).
The simulation ran on a supercomputer funded by the Scottish
Universities Physics Alliance (SUPA).
The RAS National Astronomy Meeting (NAM 2008) is hosted by Queen’s
University Belfast. It is principally sponsored by the RAS and the
Science and Technology Facilities Council (STFC). NAM 2008 is being held
together with the UK Solar Physics (UKSP) and Magnetosphere, Ionosphere
and Solar-Terrestrial (MIST) spring meetings.
CONTACTS
Dr Jane Greaves
School of Physics and Astronomy
University of St Andrews
North Haugh
St Andrews
Fife KY16 9SS
Scotland
E-mail: jsg5@st-and.ac.uk
Tel: +44 (0)1334 463199
Mobile: +44 (0)7864 741874
Dr Anita Richards
Jodrell Bank Centre for Astrophysics
Alan Turing Building
University of Manchester
Manchester M13 9PL
U.K.
E-mail: amsr@jb.man.ac.uk
Tel: +44 (0)161 275 4124
Mobile: +44 (0)7766 065049
Dr Tom Muxlow
MERLIN/VLBI National Facility
Jodrell Bank Observatory
Cheshire SK11 9DL
U.K.
E-mail: twbm@jb.man.ac.uk
Tel: +44 (0)1477 571321 (switchboard) 572607 (direct line)
Dr Ken Rice
Institute for Astronomy
University of Edinburgh
Royal Observatory
Blackford Hill
Edinburgh
Scotland EH9 3HJ
E-mail: wkmr@roe.ac.uk
Tel : +44 (0)131 668 8384
