ALMA image of the carbon monoxide snow line
Snow line distance compared to the Solar System
A frosty landmark for planet and comet formation
A snow line has been imaged in a far-off
infant planetary system for the very first time. The snow line, located
in the disc around the Sun-like star TW Hydrae, promises to tell us more
about the formation of planets and comets, the factors that decide
their composition, and the history of the Solar System. The results are
published today in Science Express.
Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA)
have taken the first ever image of the snow line in an infant planetary
system. On Earth, snow lines form at high altitudes where falling
temperatures turn the moisture in the air into snow. This line is
clearly visible on a mountain, where the snow-capped summit ends and the
rocky face begins.
The snow lines around young stars form in a similar way, in the
distant, colder reaches of the dusty discs from which planetary systems
form. Starting from the star and moving outwards, water (H2O)
is the first to freeze, forming the first snow line. Further out from
the star, as temperatures drop, more exotic molecules can freeze and
turn to snow, such as carbon dioxide (CO2), methane (CH4), and carbon monoxide
(CO). These different snows give the dust grains a sticky outer coating
and play an essential role in helping the grains to overcome their
usual tendency to break up in collisions, allowing them to become the
crucial building blocks of planets and comets. The snow also increases
how much solid matter is available and may dramatically speed up the
planetary formation process.
Each of these different snow lines — for water, carbon
dioxide, methane and carbon monoxide — may be linked to the formation of
particular kinds of planets [1].
Around a Sun-like star in a planetary system like our own, the water
snow line would correspond to a distance between the orbits of Mars and
Jupiter, and the carbon monoxide snow line would correspond to the orbit
of Neptune.
The snow line spotted by ALMA is the first glimpse of the carbon
monoxide snow line, around TW Hydrae, a young star 175 light-years away
from Earth. Astronomers believe this budding planetary system shares
many of the same characteristics of the Solar System when it was just a
few million years old.
“ALMA has given us the first real picture of a snow line around a
young star, which is extremely exciting because of what it tells us
about the very early period in the history of the Solar System,”
said Chunhua “Charlie” Qi (Harvard-Smithsonian Center for Astrophysics,
Cambridge, USA) one of the two lead authors of the paper. “We can now see previously hidden details about the frozen outer reaches of another planetary system similar to our own.”
But the presence of a carbon monoxide snow line could have greater
consequences than just the formation of planets. Carbon monoxide ice is
needed to form methanol, which is a building block of the more complex
organic molecules that are essential for life. If comets ferried these
molecules to newly forming Earth-like planets, these planets would then
be equipped with the ingredients necessary for life.
Before now, snow lines had never been imaged directly because they
always form in the relatively narrow central plane of a protoplanetary
disc, so their precise location and extent could not be determined.
Above and below the narrow region where snow lines exist, the star’s
radiation prevents ice formation. The dust and gas concentration in the
central plane is necessary to insulate the area from the radiation so
that carbon monoxide and other gases can cool and freeze.
This team of astronomers succeeded in peering inside this disc to
where the snow has formed with the help of a clever trick. Instead of
looking for the snow — as it cannot be observed directly — they searched
for a molecule known as diazenylium (N2H+),
which shines brightly in the millimetre portion of the spectrum, and so
is a perfect target for a telescope such as ALMA. The fragile molecule
is easily destroyed in the presence of carbon monoxide gas, so would
only appear in detectable amounts in regions where carbon monoxide had
become snow and could no longer destroy it. In essence, the key to
finding carbon monoxide snow lies in finding diazenylium.
ALMA's unique sensitivity and resolution has allowed the astronomers
to trace the presence and distribution of diazenylium and find a clearly
defined boundary approximately 30 astronomical units from the star (30
times the distance between the Earth and the Sun). This gives, in
effect, a negative image of the carbon monoxide snow in the disc
surrounding TW Hydrae, which can be used to see the carbon monoxide snow
line precisely where theory predicts it should be — the inner rim of
the diazenylium ring.
"For these observations we used only 26 of ALMA's eventual full
complement of 66 antennas. Indications of snow lines around other stars
are already showing up in other ALMA observations, and we are convinced
that future observations with the full array will reveal many more of
these and provide further, exciting insights into the formation and
evolution of planets. Just wait and see,” concludes Michiel Hogerheijde from Leiden Observatory, the Netherlands.
Notes
[1] For instance dry rocky planets form
on the inner side of the water snow line (nearest the star), where only
dust can exist. At the other extreme are the icy giant planets which
form beyond the carbon monoxide snow line.
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 appearing in the 18 July 2013 issue of Science Express.
The team is composed of C. Qi (Harvard-Smithsonian Center for
Astrophysics, USA), K. I. Öberg (Departments of Chemistry and Astronomy,
University of Virginia, USA), D. J. Wilner (Harvard-Smithsonian Center
for Astrophysics, USA), P. d’Alessio (Centro de Radioastronomía y
Astrofisica, Universidad Nacional Autónoma de Mexico, Mexico), E. Bergin
(Department of Astronomy, University of Michigan, USA), S. M. Andrews
(Harvard-Smithsonian Center for Astrophysics, USA), G. A. Blake
(Division of Geological and Planetary Sciences, California Institute of
Technology, USA), M. R. Hogerheijde (Leiden Observatory, Leiden
University, Netherlands) and E. F. van Dishoeck (Max Planck Institute
for Extraterrestrial Physics, Germany).
Qi and Öberg were joint lead authors of this work.
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
Chunhua QiHarvard-Smithsonian Center for Astrophysics
Cambridge, Mass., USA
Tel: +1 617 495 7087
Email: cqi@cfa.harvard.edu
Michiel Hogerheijde
Leiden Observatory
Leiden, The Netherlands
Tel: +31 6 4308 3291
Email: michiel@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
