2008MNRAS.391L.113G

The mid-infrared (MIR) ratio [Neiii]_15.6µm/[Neii]_12.8µmis 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 [Siv]/[Neii] and the diagnostic [Neiii]/[Neii] ratio that can serve as a reference for future ground-based observations. We collated the [Siv]_10.5µm, [Neii]_12.8µm, [Neiii]_15.6µm and [Siii]_18.7µm emission-line fluxes from a wide range of sources in the rich Spitzer and Infrared Space Observatory archives, and compared the [Neiii]/[Neii], [Siv]/[Siii] and [Siv]/[Neii] ratios. We find a strong correlation between the [Siv]/[Neii] and [Neiii]/[Neii] ratios, with a linear fit of log([Neiii]/[Neii]) = 0.81log([Siv]/[Neii]) + 0.36, accurate to a factor of ∼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.