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

Abstract

The mid-infrared (MIR) ratio [Ne III] 15.6 μm/[Ne II] 12.8 μm 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 μm, [Ne II] 12.8 μm, [Ne III] 15.6 μm and [S III] 18.7 μm 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 [Ne III]/[Ne II] 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. © 2008 The Authors. Journal compilation © 2008 RAS.