Optical and mid-infrared neon abundance determinations in star-forming regions

TL;DR

This study compares optical and mid-infrared neon abundance measurements in star-forming regions, revealing significant differences and proposing models to explain them, with implications for understanding nebular compositions.

Contribution

It provides a detailed comparison of neon ionic abundances from optical and IR lines and introduces a model explaining their discrepancies, along with a new ICF for neon.

Findings

IR-based neon abundances are on average four times higher than optical-based.

Photoionization models with radial abundance variations explain part of the discrepancy.

A consistent Ne/O ratio of approximately -0.70 is found across various metallicities.

Abstract

We employed observational spectroscopic data of star-forming regions compiled from the literature and photoionization models to analyse the neon ionic abundances obtained using both optical and mid-infrared emission-lines. Comparing Ne++/H+ ionic abundances from distinct methods, we found that, in average, the abundances obtained via IR emission-lines are higher than those obtained via optical lines by a factor of 4. Photoionization models with abundance variations along the radius of the hypothetical nebula provide a possible explanation for a large part of the difference between ionic abundances via optical and infrared emission-lines. Ionization Correction Factor (ICF) for the neon is obtained from direct determinations of ionic fractions using infrared emission-lines. A constant Ne/O ratio (logNe/O \approx -0.70) for a large range of metallicity, independently of the ICF used to compute the neon total abundance is derived.