The young double star HK Tauri in the constellation of Taurus
Wide-field view of part of the Taurus star formation region
Composite views of HK Tauri from Hubble and ALMA
The motions of material in the discs around the young double star HK Tauri
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Videos
Astronomers using the Atacama Large
Millimeter/submillimeter Array (ALMA) have found wildly misaligned
planet-forming gas discs around the two young stars in the binary system
HK Tauri. These new ALMA observations provide the clearest picture ever
of protoplanetary discs in a double star. The new result also helps to
explain why so many exoplanets — unlike the planets in the Solar System —
came to have strange, eccentric or inclined orbits. The results will
appear in the journal Nature on 31 July 2014.
Unlike our solitary Sun, most stars form in binary pairs — two stars
that are in orbit around each other. Binary stars are very common, but
they pose a number of questions, including how and where planets form in
such complex environments.
“ALMA has now given us the best view yet of a binary star system
sporting protoplanetary discs — and we find that the discs are mutually
misaligned!” said Eric Jensen, an astronomer at Swarthmore College in Pennsylvania, USA.
The two stars in the HK Tauri system, which is located about 450 light-years from Earth in the constellation of Taurus (The
Bull), are less than five million years old and separated by about 58
billion kilometres — this is 13 times the distance of Neptune from the
Sun.
The fainter star, HK Tauri B, is surrounded by an edge-on protoplanetary disc
that blocks the starlight. Because the glare of the star is suppressed,
astronomers can easily get a good view of the disc by observing in visible light, or at near-infrared wavelengths.
The companion star, HK Tauri A, also has a disc, but in this case it
does not block out the starlight. As a result the disc cannot be seen in
visible light because its faint glow is swamped by the dazzling
brightness of the star. But it does shine brightly in
millimetre-wavelength light, which ALMA can readily detect.
Using ALMA, the team were not
only able to see the disc around HK Tauri A, but they could also measure
its rotation for the first time. This clearer picture enabled the
astronomers to calculate that the two discs are out of alignment with
each other by at least 60 degrees. So rather than being in the same
plane as the orbits of the two stars at least one of the discs must be
significantly misaligned.
“This clear misalignment has given us a remarkable look at a young binary star system,” said Rachel Akeson of the NASA Exoplanet Science Institute at the California Institute of Technology in the USA. “Although
there have been earlier observations indicating that this type of
misaligned system existed, the new ALMA observations of HK Tauri show
much more clearly what is really going on in one of these systems.”
Stars and planets form out of vast clouds of dust and gas. As
material in these clouds contracts under gravity, it begins to rotate
until most of the dust and gas falls into a flattened protoplanetary
disc swirling around a growing central protostar.
But in a binary system like HK Tauri things are much more complex.
When the orbits of the stars and the protoplanetary discs are not
roughly in the same plane any planets that may be forming can end up in
highly eccentric and tilted orbits [1].
“Our results show that the necessary conditions exist to modify
planetary orbits and that these conditions are present at the time of
planet formation, apparently due to the formation process of a binary
star system,” noted Jensen. “We can’t rule other theories out, but we can certainly rule in that a second star will do the job.”
Since ALMA can see the otherwise invisible dust and gas of
protoplanetary discs, it allowed for never-before-seen views of this
young binary system. “Because we’re seeing this in the early stages
of formation with the protoplanetary discs still in place, we can see
better how things are oriented,” explained Akeson.
Looking forward, the researchers want to determine if this type of
system is typical or not. They note that this is a remarkable individual
case, but additional surveys are needed to determine if this sort of
arrangement is common throughout our home galaxy, the Milky Way.
Jensen concludes: “Although understanding this mechanism is a big
step forward, it can’t explain all of the weird orbits of extrasolar
planets — there just aren’t enough binary companions for this to be the
whole answer. So that’s an interesting puzzle still to solve, too!”
Notes
[1] If the two stars and their discs
are not all in the same plane, the gravitational pull of one star will
perturb the other disc, making it wobble or precess, and vice versa. A
planet forming in one of these discs will also be perturbed by the other
star, which will tilt and deform its orbit.
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 entitled “Misaligned
Protoplanetary Disks in a Young Binary Star System”, by Eric Jensen and
Rachel Akeson, to appear in the 31 July 2014 issue of the journal Nature.
The team is composed of Eric L. N. Jensen (Dept. of Physics &
Astronomy, Swarthmore College, USA) and Rachel Akeson (NASA Exoplanet
Science Institute, IPAC/Caltech, Pasadena, USA).
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
- Research paper
- NRAO press release about HK Tauri results
- Image of HK Tauri from the NASA/ESA Hubble Space Telescope
- More about ALMA
- Photos of ALMA
- Videos of ALMA
- ALMA brochure
- The movie ALMA — In Search of our Cosmic Origins
- The ALMA Photo Book In Search of our Cosmic Origins – The Construction of the Atacama Large Millimeter/submillimeter Array
- More press releases with ALMA
Contacts
Eric L. N. JensenLead Scientist, Swarthmore College
Philadelphia, USA
Tel: +1 610-328-8249
Email: ejensen1@swarthmore.edu
Rachel Akeson
NASA Exoplanet Science Institute, IPAC/Caltech
Pasadena, USA
Tel: +1 626-395-1812
Email: rla@ipac.caltech.edu
Charles E. Blue
Public Information Officer, National Radio Astronomy Observatory
Charlottesville, USA
Tel: + 1 434 296 0314
Cell: +1 202 236 6324
Email: cblue@nrao.edu
Richard Hook
Public Information Officer, ESO
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
