Credit: ALMA(ESO/NAOJ/NRAO), Hyosun Kim et al.
Image 2. (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.
(Right) ALMA image of LL Pegasi. Credit: ALMA (ESO/NAOJ/NRAO) / Hyosun Kim et al.
"It's really exciting to see such a beautiful spiral-shell pattern in
the sky. Our observations of LL Pegasi binary system have revealed the
delicately 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 highly elliptical
orbit is responsible for the morphology of gas distribution surrounding
this binary system. In particular, the bifurcation of the spiral-shell
pattern that 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 recurrent
patterns that reside far from the star at distances of a few thousand
the stellar radii.
"Because LL Pegasi is 3,400 light years away from us, 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 studies," 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 a large period range. Many planetary
nebulae—stars being 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 10
years ago due to a picture of an almost-perfect spiral taken with the
NASA/ESA Hubble Space Telescope (HST). 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 time frame," says Mark
Morris (UCLA, USA), a co-author of the study.
The regularity of the pattern was quite surprising, leading to its
being considered as a binary system in a circular orbit. It is now
equally striking that this best-characterized, unambiguous, and complete
spiral is actually 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 spatio-kinematic 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.
Movie. 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.
Movie. 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 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).
