The mid-infrared [S IV]/[Ne II] versus [Ne III]/[Ne II] correlation

Themid-infrared (MIR) ratio [Ne III](15.6) (mu m)/[Ne II](12.8) (mu m) is a strong diagnostic of the ionization state of emission-line objects, due to its use of only strong neon emission lines only weakly affected by extinction. However, this ratio is not available to ground-based telescopes as only a few spectroscopic windows are available in the MIR. To deal with this problem, we aimed to verify if there exists a conversion law between the ground-accessible, strong MIR line ratio [S IV]/[Ne II] and the diagnostic [Ne III]/[Ne II] ratio that can serve as a reference for future ground-based observations. We collated the [S IV](10.5) (mu m), [Ne II](12.8) (mu m), [Ne III](15.6) (mu m) and [S III](18.7) (mu m) emission-line fluxes from a wide range of sources in the rich Spitzer and Infrared Space Observatory archives, and compared the [Ne III]/[Ne II], [S IV]/[S III] and [S IV]/[Ne II] ratios. We find a strong correlation between the [S IV]/[Ne II] and [Ne III]/[Ne II] ratios, with a linear fit of log([Ne III]/[Ne II]) = 0.81 log([S IV]/[Ne II]) + 0.36, accurate to a factor of similar to 2 over four orders of magnitude in the line ratios. This demonstrates clearly the ability of ground-based infrared spectrographs to do ionization studies of nebulae.