Hyperfine splitting of [Al VI] 3.66 mu m 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 vr] 3.66 mu m (3)P(2) <- (3)P(1), obtained with Phoenix on Gemini South at resolving powers of up to 75000, 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 3 sigma upper limit on the (26)Al/(27)Al abundance ratio of 1/33. We measure the HFS magnetic dipole coupling constants for [Al vr], and provide the first estimates of the electric quadrupole HFS coupling constants obtained through astronomical observations of an atomic transition.