The mid-infrared [SIV]/[NeII] versus [NeIII]/[NeII] correlation

Abstract

The mid-infrared (MIR) ratio [NeIII]_15.6μm/[NeII]_12.8μm 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 [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.