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

TL;DR

This paper investigates the transition from AGB stars to pre-planetary nebulae, focusing on whorled patterns caused by stellar companions, exemplified by CW Leonis, and introduces a binary model to explain observed features.

Contribution

It presents a new binary orbital model explaining the whorled patterns in CW Leonis and discusses the role of companions in the AGB to pPN transition.

Findings

Binary model reproduces the pattern's velocity dependence

CW Leonis shows signs of multiple companions

Enhanced light curves indicate ongoing stellar evolution

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.