From ALMA: “ALMA adds a new dimension to a Hubble Space Telescope result”

ESO/NRAO/NAOJ ALMA Array
ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres
ALMA

02 March 2017
Dr. Hyosun Kim
ASIAA
Tel: +886-2-2366-5418
Email: hkim@asiaa.sinica.edu.tw

Dr. Sheng-Yuan Liu
ASIAA
Tel: +886-2-2366-5440
Email: syliu@asiaa.sinica.edu.tw

Nicolás Lira T.
Press Coordinator
Joint ALMA Observatory
Santiago, Chile
Tel: +56 2 24 67 65 19
Cell: +56 9 94 45 77 26
Email: nicolas.lira@alma.cl

Masaaki Hiramatsu
Education and Public Outreach Officer, NAOJ Chile
Observatory
Tokyo, Japan

Tel: +81 422 34 3630

E-mail: hiramatsu.masaaki@nao.ac.jp

Richard Hook
Public Information Officer, ESO

Garching bei München, Germany

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Email: rhook@eso.org

Charles E. Blue
Public Information Officer
National Radio Astronomy Observatory
Charlottesville, Virginia, USA
Tel: +1 434 296 0314
Cell: +1 202 236 6324
E-mail: cblue@nrao.edu

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An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have unraveled the elliptical nature of the binary orbit of the old star LL Pegasi and its companion. The figure shows the composite image of molecular gas around LL Pegasi. By comparing this gas distribution depicted in exquisite detail by ALMA with theoretical simulations, the team concluded that the bifurcation of the spiral-shell pattern is resulted from a highly elliptical binary system. Credit: ALMA (ESO/NAOJ/NRAO) / Hyosun Kim et al.

An international team of astronomers, led by Hyosun Kim in Academia Sinica Institute of Astronomy and Astrophysics (ASIAA, Taiwan), has found a way of deriving the orbital shape of binary stars that have orbital periods too long to be directly measured. This new technique was possible thanks to an observation toward the old star LL Pegasi (also known as AFGL 3068) using the state-of-the-art telescope, the Atacama Large Millimeter/submillimeter Array (ALMA).

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This remarkable picture from the Advanced Camera for Surveys [ACS] on the NASA/ESA Hubble Space Telescope shows one of the most perfect geometrical forms created in space. It captures the formation of an unusual pre-planetary nebula, known as IRAS 23166+1655, around the star LL Pegasi (also known as AFGL 3068) in the constellation of Pegasus (the Winged Horse).

NASA/ESA Hubble Telescope
NASA/ESA Hubble Telescope

NASA Hubble ACS
“NASA Hubble/ACS

“It is exciting to see such a beautiful spiral-shell pattern in the sky. Our observations have revealed the exquisitely ordered three-dimensional geometry of this spiral-shell pattern, and we have produced a very satisfying theory to account for its details,” says Hyosun Kim.

The new ALMA images reveal the detailed features of spiral-shell pattern imprinted in the gas material continuously ejected from LL Pegasi. A comparison of this observation with computer simulations led the team, for the first time, to the conclusion that a binary system with a highly elliptical orbit is responsible for its morphology of gas distribution. In particular, the bifurcation of the spiral-shell pattern, which is clearly visible in the ALMA images, is a unique characteristic of elliptical binaries. This quintessential object opens a new window on the nature of central binaries through the repetitive patterns that reside far from the star at distances of a few thousand the stellar radii.

“The exquisite sensitivity and ability of ALMA to image with high precision such complex spiral patterns were essential for this study. We are delighted to see the crisp images translated into rich results and their implications in binary research,” says Alfonso Trejo (ASIAA, Taiwan), a co-author of the study.

Binaries in elliptical orbits for stars in late stellar evolutionary phases may be ubiquitous over an extensive period range. Many planetary nebulae (stars that are in the next stage of stellar evolution) consist of nearly spherical structures in the outer part and highly-asymmetric structures in the inner part. Near-spherical patterns include those appearing like spirals, shells, and arcs, while highly non-spherical features are bipolar- or multipolar-like. The coexistence of such geometrically distinct structures is enigmatic because it hints at the simultaneous presence of both wide and close binary interactions.

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An international team of astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have unraveled the elliptical nature of the binary orbit of the old star LL Pegasi and its companion. The figure shows the composite image of molecular gas around LL Pegasi. By comparing this gas distribution depicted in exquisite detail by ALMA with theoretical simulations, the team concluded that the bifurcation of the spiral-shell pattern (indicated by a white box) is resulted from a highly elliptical binary system. Credit: ALMA (ESO/NAOJ/NRAO) / Hyosun Kim et al.

This phenomenon has been attributed to the binary stars with elliptical orbits. As indicated by the current research, the orbital parameters of central binaries can be obtained by a careful inspection of the outer recurrent patterns, which hint at the origin of the transition from the near-spherical to asymmetric structures.

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(Left) HST image of LL Pegasi publicized in 2010. Credit: ESA/NASA & R. Sahai. (Right) ALMA image of LL Pegasi. Credit: ALMA (ESO/NAOJ/NRAO) / Hyosun Kim et al.

LL Pegasi is a mass-losing giant star with a size of 200 times or more that of the Sun. Among the stellar evolutionary phases, it is currently on the asymptotic giant branch, which reflects the future of the Sun a few billion years from now. This star was spotlighted about ten years ago due to a picture of an almost-perfect spiral taken with the NASA/ESA Hubble Space Telescope (HST) [1]. The presence of a spiral surrounding an old star had never been reported before the discovery of this object.

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Visualizing the ALMA image cube of LL Pegasi. Each frame of the video shows the molecular gas material surrounding LL Pegasi for a different line-of-sight velocity. This velocity, advancing 1 km/s per frame, is given at the top-right corner. The field size is 20,000 times the distance between the Sun and the Earth. Credit: ALMA (ESO/NAOJ/NRAO) / Hyosun Kim et al.

“This unusually ordered system opens the door to understanding how the orbits of such systems evolve with time, since each winding of the spiral samples a different orbit in a different period,” says Mark Morris (UCLA, USA), a co-author of the study.

The regularity of the pattern was quite surprising, leading to being considered as a binary system in a circular orbit. It is now equally striking that this best-characterized, unambiguous, and complete spiral is influenced by an elliptical-orbit binary.

“While the HST image shows us the beautiful spiral structure, it is a 2D projection of a 3D shape, which becomes fully revealed in the ALMA data,” says Raghvendra Sahai (JPL, USA), a co-author of the study. The new ALMA images reveal the spatiokinematic information of dense molecular gas in the spiral-shell pattern, unveiling the dynamics of the mass loss from the giant star modulated by its orbital motion.

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Zooming into the old star LL Pegasi in the constellation of Pegasus. Credit: Hyosun Kim (ASIAA)

“The exquisite sensitivity and ability of ALMA to image with high precision such complex spiral patterns were essential for this study. We are delighted to see the crisp images translated into rich results and their implications in binary research,” says Alfonso Trejo (ASIAA, Taiwan), a co-author of the study.

“The interval of spiral arms yields the orbital period of LL Pegasi to be about 800 years, at which the binary motion can be barely detected even with continuous observations over several human lifetimes. Decoding the spiral-shell pattern is a clever way to trace back the history of orbital motion,” adds Sheng-Yuan Liu (ASIAA, Taiwan), a co-author of the study.

“By putting this striking spiral-shell on display, nature has left us some clear messages. Deciphering those messages to determine the dynamics of the central stars is the challenge that astronomers are facing,” remarks Hyosun Kim.

Notes

[1] The HST results are reported in Morris et al. 2006 (Proceeding of the International Astronomical Union Symposium 234, pp. 469-470) and Mauron & Huggins 2006 (Astronomy and Astrophysics 452, pp. 257-268).

This research was presented in a paper “The Large-Scale Nebular Pattern of a Superwind Binary in an Eccentric Orbit,” by Kim et al. to appear in the journal Nature Astronomy.

The team is composed of Hyosun Kim (ASIAA, Taiwan; East Asian Core Observatories Association Fellow), Alfonso Trejo (ASIAA, Taiwan), Sheng-Yuan Liu (ASIAA, Taiwan), Raghvendra Sahai (Jet Propulsion Laboratory, USA), Ronald E. Taam (ASIAA, Taiwan; Northwestern University, USA), Mark R. Morris (University of California, Los Angeles, USA), Naomi Hirano (ASIAA, Taiwan), and I-Ta Hsieh (ASIAA, Taiwan).

See the full article here .

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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 Organization for Astronomical Research in the Southern Hemisphere (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.

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