Hyperfine splitting of [Al vi] 3.66 μm and the Al isotopic ratio in NGC 6302

Abstract

The core of planetary nebula NGC 6302 is filled with high-excitation photoionized gas at low expansion velocities. It represents a unique astrophysical situation in which to search for hyperfine structure (HFS) in coronal emission lines from highly ionized species. HFS is otherwise blended by thermal or velocity broadening. Spectra containing [Al VI] 3.66 µm 3 P2 ← 3 P1, obtained with Phoenix on Gemini South at resolving powers of up to 75 000, resolve the line into five hyperfine components separated by 20‐60 km s −1 as a result of the coupling of the I = 5/2 nuclear spin of 27 Al with the total electronic angular momentum J. The isotope 26 Al has a different nuclear spin of I = 5, and a different HFS, which allows us to place a 3σ upper limit on the 26 Al/ 27 Al abundance ratio of 1/33. We measure the HFS magnetic dipole coupling constants for [Al VI], and provide the first estimates of the electric quadrupole HFS coupling constants obtained through astronomical observations of an atomic transition. Ke yw ords: atomic data ‐ atomic processes ‐ line: identification ‐ line: profiles ‐ ISM: abundances ‐ planetary nebulae: individual: NGC 6302.