ALMA’s most detailed image of a protoplanetary disc
The star TW Hydrae is a popular target of study for astronomers because of its proximity to Earth (only about 175 light-years away) and its status as an infant star (about 10 million years old). It also has a face-on orientation as seen from Earth. This gives astronomers a rare, undistorted view of the complete protoplanetary disc around the star.
"Previous studies with optical and radio telescopes confirm that TW Hydrae hosts a prominent disc with features that strongly suggest planets are beginning to coalesce," said Sean Andrews with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA and lead author on a paper published today in the Astrophysical Journal Letters. "The new ALMA images show the disc in unprecedented detail, revealing a series of concentric dusty bright rings and dark gaps, including intriguing features that may indicate that a planet with an Earth-like orbit is forming there."
Other pronounced gaps that show up in the new images are located three billion and six billion kilometres from the central star, similar to the average distances from the Sun to Uranus and Pluto in the Solar System. They too are likely to be the results of particles that came together to form planets, which then swept their orbits clear of dust and gas and shepherded the remaining material into well-defined bands.
For the new TW Hydrae observations, astronomers imaged the faint radio emission from millimetre-sized dust grains in the disc, revealing details on the order of the distance between the Earth and the Sun (about 150 million kilometres). These detailed observations were made possible with ALMA 's high-resolution, long-baseline configuration. When ALMA's dishes are at their maximum separation, up to 15 kilometres apart, the telescope is able to resolve finer details. "This is the highest spatial resolution image ever of a protoplanetary disc from ALMA, and that won't be easily beaten in the future!" said Andrews .
"TW Hydrae is quite special. It is the nearest known protoplanetary disc to Earth and it may closely resemble the Solar System when it was only 10 million years old," adds co-author David Wilner, also with the Harvard-Smithsonian Center for Astrophysics.
Earlier ALMA observations of another system, HL Tauri, show that even younger protoplanetary discs — a mere 1 million years old — can display similar signatures of planet formation. By studying the older TW Hydrae disc, astronomers hope to better understand the evolution of our own planet and the prospects for similar systems throughout the Milky Way.
The astronomers now want to find out how common these kinds of features are in discs around other young stars and how they might change with time or environment.
 The angular resolution of the images of HL Tauri was similar to these new observations, but as TW Hydrae is much closer to Earth, finer details can be seen.
This research was presented in a paper "Ringed Substructure and a Gap at 1 AU in the Nearest Protoplanetary Disk", by S.M. Andrews et al., appearing in the Astrophysical Journal Letters.
The team is composed of Sean M. Andrews (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA), David J. Wilner (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA) , Zhaohuan Zhu (Princeton University, Princeton, New Jersey, USA), Tilman Birnstiel (Max-Planck-Institut für Astronomie, Heidelberg, Germany), John M. Carpenter (Joint ALMA Observatory, Santiago, Chile), Laura M. Peréz (Max-Planck-Institut für Radioastronomie, Bonn, Germany), Xue-Ning Bai (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA), Karin I. Öberg (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA), A. Meredith Hughes (Wesleyan University, Van Vleck Observatory, Middletown, USA), Andrea Isella (Rice University, Houston, Texas, USA) and Luca Ricci (Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA).
The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of ESO, the U.S. National Science Foundation (NSF) and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).
ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
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 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. 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 a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become "the world's biggest eye on the sky".
* Research paper – http://www.eso.org/public/archives/releases/sciencepapers/eso1611/eso1611a.pdf
* Photos of ALMA – http://www.eso.org/public/images/archive/search/?adv=&subject_name=Atacama%20Large%20Millimeter/submillimeter%20Array
* Other press releases featuring ALMA – http://www.eso.org/public/news/archive/search/?adv=&facility=36
Sean M. Andrews
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