Evolution From Asymptotic Giant Branch to Pre-planetary Nebula: Whorled Patterns and Stellar Companions

TL;DR

The paper investigates how binary and possibly triple-star dynamics drive the transition from AGB mass loss to pre-planetary nebula morphologies, using CW Leonis as a focal case. It develops and tests an eccentric-binary framework to explain the observed position-angle–dependent expansion of whorled patterns and explores a triple-system scenario to reconcile conflicting geometry indicators. It highlights porous envelope behavior and accretion-disk processes as plausible mechanisms shaping inner and outer structures during the AGB–pPN transition, supported by observational trends in brightness and central-star emergence. The findings underscore the importance of continued multi-epoch, multi-wavelength monitoring and multi-scale simulations to identify phase-transition candidates and reveal how companions sculpt circumstellar morphologies.

Abstract

Bipolar or multipolar lobes in pre-planetary nebulae (pPNe) often exhibit intertwined outer whorled patterns, resulting from stellar wind matter accumulation during the asymptotic giant branch (AGB) phase. These structures are likely triggered by stellar or substellar companions. We regard that CW Leonis currently stands at a critical transition moment, providing a vivid illustration of the progression from an AGB star in a binary system to a pPN. We have found that CW Leonis has shown significant enhancements in its optical and near-infrared light curves over the past two decades, with the recent Hubble Space Telescope image finally revealing the long-awaited central star. Utilizing an eccentric-orbit binary model, we can reproduce the position-angle dependence of the expansion velocity in the whorled pattern around CW Leonis, suggesting a nearly face-on orbital inclination. Its contradiction to the features in the innermost circumstellar envelope, corresponding to a nearly edge-on inclination, may imply the presence of an additional companion. Our updated theoretical framework explores the complexity of the whorled pattern. Further identifying and monitoring phase-transition candidates at the tip of the AGB will provide valuable insights into the AGB-pPN transition and the role of companions in shaping the morphological evolution of these stellar objects.

Figures (8)

• Figure 1: Comparison between the Atacama Large Millimeter/submillimeter Array (ALMA) map of AFGL 3068 (LL Pegasi) for the $^{12}$CO molecular line emission and a hydrodynamic model; a snapshot of the supplementary animation of kim17. Bifurcation is identified as evidence for an eccentric orbit of the central binary (white box).
• Figure 2: (Left) Face-on and edge-on views of the circumstellar spiral-shell pattern driven by the orbital motions of the central binary system, calculated by a particle simulation using the first version of the pinwheel code kim24. A perfectly circular orbit is assumed. In the bottom panel (for the edge-on image), the orbital axis lies along the vertical axis. (Right) Figure 2(a) of kim23. The expansion velocity profiles as a function of position angle at three different inclination angles.
• Figure 3: Same as in Figure \ref{['fig:cir']}, but for an eccentric orbit binary. Right panel is a copy of Figure 2(d) of kim23, which shows the minimum transverse wind velocity at the pericenter of the mass-losing star.
• Figure 4: (Left) Pseudo-color image of CW Leonis in the Hubble Space Telescope's gallery https://hubblesite.org/contents/news-releases/2021/news-2021-059. Image dimension is 2.4 arcmin across (about 0.2 light-years). Image credit: ESA/Hubble, NASA, Toshiya Ueta (Univ. of Denver), Hyosun Kim (KASI). (Right) Expansion velocity measured by kim21 using differential proper motion of the whorled pattern from the year of 2011 to 2016, as a function of the position angle (from north to the east).
• Figure 5: (a) Circumstellar density pattern formed in the orbital plane of a triple star system using a hydrodynamic simulation, overlaid by a black line (main spiral, hereafter) presenting the location of the Archimedean spiral that would be produced by the outer binary alone. (b) Density profile along an angle of 45 degrees with respect to the $y$ axis. Individual shaded regions (and horizontal two-headed arrows) indicate the radial extensions of the individual fine spiral ridges, which converge upon each other at the main spiral. Red and blue lines show the difference in the radial rates of the density decrease in the main and fine patterns. Refer to kim24 for details.