ALMA image of comet factory around Oph-IRS 48
ALMA and VLT image of comet factory around Oph-IRS 48
ALMA image of dust trap/comet factory around Oph-IRS 48 (annotated)
The location of the system Oph-IRS 48 in the constellation of Ophiuchus
Videos
New observations of a “dust trap” around a young star solve long-standing planet formation mystery
Astronomers using the new Atacama Large
Millimeter/submillimeter Array (ALMA) have imaged a region around a
young star where dust particles can grow by clumping together. This is
the first time that such a dust trap has been clearly observed and
modelled. It solves a long-standing mystery about how dust particles in
discs grow to larger sizes so that they can eventually form comets,
planets and other rocky bodies. The results are published in the journal
Science on 7 June 2013.
Astronomers now know that planets around other stars are plentiful.
But they do not fully understand how they form and there are many
aspects of the formation of comets, planets and other rocky bodies that
remain a mystery. However, new observations exploiting the power of ALMA
are now answering one of the biggest questions: how do tiny grains of
dust in the disc around a young star grow bigger and bigger — to
eventually become rubble, and even boulders well beyond a metre in size?
Computer models suggest that dust grains grow when they collide and
stick together. However, when these bigger grains collide again at high
speed they are often smashed to pieces and sent back to square one. Even
when this does not happen, the models show that the larger grains would
quickly move inwards because of friction between the dust and gas and
fall onto their parent star, leaving no chance that they could grow even
further.
Somehow the dust needs a safe haven where the particles can continue growing until they are big enough to survive on their own [1]. Such “dust traps” have been proposed, but there was no observational proof of their existence up to now.
Nienke van der Marel, a PhD student at Leiden Observatory in the
Netherlands, and lead author of the article, was using ALMA along with
her co-workers, to study the disc in a system called Oph-IRS 48 [2].
They found that the star was circled by a ring of gas with a central
hole that was probably created by an unseen planet or companion star.
Earlier observations using ESO’s Very Large Telescope had already shown
that the small dust particles also formed a similar ring structure. But
the new ALMA view of where the larger millimetre-sized dust particles
were found was very different!
“At first the shape of the dust in the image came as a complete surprise to us,” says van der Marel. “Instead
of the ring we had expected to see, we found a very clear cashew-nut
shape! We had to convince ourselves that this feature was real, but the
strong signal and sharpness of the ALMA observations left no doubt about
the structure. Then we realised what we had found.”
What had been discovered was a region where bigger dust grains were
trapped and could grow much larger by colliding and sticking together.
This was a dust trap — just what the theorists were looking for.
As van der Marel explains: “It’s likely that we are looking at a
kind of comet factory as the conditions are right for the particles to
grow from millimetre to comet size. The dust is not likely to form
full-sized planets at this distance from the star. But in the near
future ALMA will be able to observe dust traps closer to their parent
stars, where the same mechanisms are at work. Such dust traps really
would be the cradles for new-born planets.”
The dust trap forms as bigger dust particles move in the direction of
regions of higher pressure. Computer modelling has shown that such a
high pressure region can originate from the motions of the gas at the
edge of a gas hole — just like the one found in this disc.
“The combination of modelling work and high quality observations of ALMA makes this a unique project”,
says Cornelis Dullemond from the Institute for Theoretical Astrophysics
in Heidelberg, Germany, who is an expert on dust evolution and disc
modelling, and a member of the team. “Around the time that these
observations were obtained, we were working on models predicting exactly
these kinds of structures: a very lucky coincidence.”
The observations were made while the ALMA array was still being constructed. They made use of the ALMA Band 9 receivers [3] — European-made devices that allow ALMA to create its so far sharpest images.
“These observations show that ALMA is capable of delivering
transformational science, even with less than half of the full array in
use,” says Ewine van Dishoeck of the Leiden Observatory, who has
been a major contributor to the ALMA project for more than 20 years. “The
incredible jump in both sensitivity and image sharpness in Band 9 gives
us the opportunity to study basic aspects of planet formation in ways
that were simply not possible before.”
Notes
[1] The cause of the dust trap, in this
case a vortex in the disc's gas', has typical life spans of hundreds of
thousand of years. Even when the dust trap ceases to work, the dust
accumulated in the trap would take millions of years to disperse
providing ample time for the dust grains to grow larger.
[2] The name is a combination of the constellation
name of the star-forming region where the system is found and the type
of source, so Oph stands for the constellation of Ophiuchus (The Serpent
Bearer), and the IRS stands for infrared source. The distance from
Earth to Oph-IRS 48 is about 400 light-years.
[3] ALMA can observe in different frequency bands.
Band 9, operating at wavelengths of about 0.4–0.5 millimeters, is the
mode that so far provides the sharpest images.
More information
The Atacama Large Millimeter/submillimeter
Array (ALMA), an international astronomy facility, is a partnership of
Europe, North America and East Asia in cooperation with the Republic of
Chile. ALMA is funded in Europe by the European Southern Observatory
(ESO), in North America by the U.S. National Science Foundation (NSF) in
cooperation with the National Research Council of Canada (NRC) and the
National Science Council of Taiwan (NSC) and in East Asia by the
National Institutes of Natural Sciences (NINS) of Japan in cooperation
with the Academia Sinica (AS) in Taiwan. ALMA construction and
operations are led on behalf of Europe by ESO, on behalf of North
America by the National Radio Astronomy Observatory (NRAO), which is
managed by Associated Universities, Inc. (AUI) and on behalf of East
Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint
ALMA Observatory (JAO) provides the unified leadership and management
of the construction, commissioning and operation of ALMA.
This research was presented in a paper “A major asymmetric dust trap
in a transition disk“, by van der Marel et al, to appear in the journal Science on 7 June 2013.
The team is composed of Nienke van der Marel (Leiden Observatory,
Leiden, the Netherlands), Ewine F. van Dishoeck (Leiden Observatory;
Max-Planck-Institut für Extraterrestrische Physik Garching, Germany
[MPE]), Simon Bruderer (MPE), Til Birnstiel (Harvard-Smithsonian Center
for Astrophysics, Cambridge, USA [CfA]), Paola Pinilla (Heidelberg
University, Heidelberg, Germany), Cornelis P. Dullemond (Heidelberg
University), Tim A. van Kempen (Leiden Observatory; Joint ALMA Offices,
Santiago, Chile), Markus Schmalzl (Leiden Observatory), Joanna M. Brown
(CfA), Gregory J. Herczeg (Kavli Institute for Astronomy and
Astrophysics, Peking University, Beijing, China), Geoffrey S. Mathews
(Leiden Observatory) and Vincent Geers (Dublin Institute for Advanced
Studies, Dublin, Ireland).
ESO is the foremost intergovernmental astronomy organisation in
Europe and the world’s most productive ground-based astronomical
observatory by far. It is supported by 15 countries: Austria, Belgium,
Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy,
the Netherlands, Portugal, Spain, Sweden, Switzerland and the United
Kingdom. ESO carries out an ambitious programme focused on the design,
construction and operation of powerful ground-based observing facilities
enabling astronomers to make important scientific discoveries. ESO also
plays a leading role in promoting and organising cooperation in
astronomical research. ESO operates three unique world-class observing
sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO
operates the Very Large Telescope, the world’s most advanced
visible-light astronomical observatory and two survey telescopes. VISTA
works in the infrared and is the world’s largest survey telescope and
the VLT Survey Telescope is the largest telescope designed to
exclusively survey the skies in visible light. ESO is the European
partner of a revolutionary astronomical telescope ALMA, the largest
astronomical project in existence. ESO is currently planning the
39-metre European Extremely Large optical/near-infrared Telescope, the
E-ELT, which will become “the world’s biggest eye on the sky”.
Links
Contacts
Nienke van der MarelLeiden Observatory
Leiden, The Netherlands
Tel: +31 71 527 8472
Cell: +31 62 268 4136
Email: nmarel@strw.leidenuniv.nl
Ewine van Dishoeck
Leiden Observatory
Leiden, The Netherlands
Tel: +31 71 527 5814
Email: ewine@strw.leidenuniv.nl
Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
