ALMA adds a new dimension to a Hubble Space Telescope result

2 March, 2017 / Read time: 6 minutes

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).

“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.

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.

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.

“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.

3D visualization of the molecular gas material surrounding LL Pegasi. Image of LL Pegasi, first as it appears to the Hubble Space Telescope, and then as it appears in the emission from molecules, as observed with ALMA. The numerical model appears beside the nebula, and both the model and the image are rotated to display the excellent three-dimensional agreement. Credit: Hyosun Kim et al. / I-Ta Hsieh (ASIAA)

“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.

“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

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).

Additional information

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).

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Organisation for Astronomical Research in the Southern Hemisphere (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 Ministry of Science and Technology (MOST) in Taiwan 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.