Hyperfine splitting of [Al VI]3.66um and the Al isotopic ratio in NGC6302

Abstract

The core of planetary nebula NGC 6302 is filled with high-excitation photoionised 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 ionised species. HFS is otherwise blended by thermal or velocity broadening. Spectra containing [AlVI]~3.66um 3P2-3P1, obtained with Phoenix on Gemini-South at resolving powers of up to 75000, resolve the line into five hyperfine components separated by 20 to 60km/s due to the coupling of the I=5/2 nuclear spin of 27Al with the total electronic angular momentum J. 26Al has a different nuclear spin of I=5, and a different HFS, which allows us to place a 3sigma upper limit on the 26Al/27Al abundance ratio of 1/33. We measure the HFS magnetic-dipole coupling constants for [AlVI], and provide the first estimates of the electric-quadrupole HFS coupling constants obtained through astronomical observations of an atomic transition.