The Structure of the Molecular Envelope of the Ring Nebula (NGC 6720)

We present the first interferometric imaging of molecular line emission from the Ring Nebula,NGC 6720, in the form of Submillimeter Array (SMA) observations of C O J = 2 → 1 emission. The SMA ¹²CO(2–1) mapping data, with ∼3′′ spatial resolution and 2 km s⁻¹ velocity resolution, provide an unprecedentedly detailed, 3D view of the Ring’s clumpy molecular envelope. The morphology of the velocity-integrated SMA ¹²CO(2–1) image closely resembles those of near-IR H² and PAH emission in JWST/NIRCam imaging of NGC 6720, with the molecular gas forming a geometrically thin layer surrounding the ionized gas imaged by HST and JWST. A simple, geometrical model of the ¹²CO(2–1) data shows that the intrinsic structure of NGC 6720’s molecular envelope closely resembles a truncated, triaxial ellipsoid that is viewed close to pole-on, and that the dynamical age of the molecular envelope is ∼6000 yr. The SMA 12CO(2–1) data furthermore reveal that filamentary features seen projected in the Ring’s interior in JWST imaging are in fact fast-moving polar knots or bullets with radial velocities of ±45–50 km s⁻¹ relative to systemic, and that the hot progenitor star
remnant is positioned at the precise geometric center of the clumpy, ellipsoidal molecular shell. We assert that the Ring’s molecular envelope was formed via a relatively sudden, AGB-terminating mass ejection event ∼6000 yr ago, and that this ellipsoidal envelope was then punctured by fast, collimated polar outflows resulting from interactions between the progenitor and one or more companion stars. Such an evolutionary scenario may describe most molecule-rich, “Ring-like” planetary nebulae.