Cocoon-to-Butterfly Transformation in Protoplanetary Nebula: Detection of a Spherical Halo and a Barrel-shaped Torus in IRAS 06530$-$0213

Abstract

Protoplanetary nebulae (PPNs) represent a critical evolutionary bridging stage between the asymptotic giant branch (AGB) and planetary nebula (PN) stages. Their dynamical structures provide key insights into late stellar evolution. Here, we report CO and $^{13}$CO imaging observations of the carbon-rich PPN IRAS 06530$-$0213, a source exhibiting the unidentified 21 micron emission band, conducted with the Northern Extended Millimeter Array (NOEMA). The CO maps reveal a spherical halo (diameter $\sim10''$) surrounding a central barrel-shaped torus, where the torus displays an inner diameter of $\sim1.5''$ and an outer diameter of $\sim2''$. Through three-dimensional morpho-kinematic modeling with the SHAPE software, we determine that IRAS 06530$-$0213 experienced its final thermal pulse during the AGB phase $\sim6500$ years ago, transitioning into the PPN phase $\sim4500$ years ago. Our analysis indicates that the blue-shifted CO emission, a feature also detected in several other PPNs and PNs previously misattributed to the interstellar clouds, actually originates from the obscuration of the central nebula by the remnant AGB halo. These findings are expected to deepen our understanding of the dynamical structures of PPNs, as well as their pivotal transitional role in the late-stage evolution of low-to-intermediate mass stars.