# Assessing model-based carbon and oxygen abundance derivation from ultraviolet emission lines in AGNs

## TL;DR

This study adapts a code to derive chemical abundances from ultraviolet emission lines in AGNs, evaluating model assumptions and demonstrating a robust method for estimating metallicity using specific line ratios.

## Contribution

The paper introduces an adapted version of HII-CHI-Mistry-UV for AGNs and shows that C/O ratio from UV lines reliably indicates metallicity, improving abundance estimates.

## Key findings

- C/O ratio from UV lines is a robust metallicity indicator.
- Model assumptions have limited impact on C/O-based abundance estimates.
- Prior C/O determination enhances oxygen abundance accuracy.

## Abstract

We present an adapted version of the code HII-CHI-Mistry-UV (P\'erez-Montero & Amor\'in 2017) to derive chemical abundances from emission lines in the ultraviolet, for use in narrow line regions (NLR) of Active Galactic Nuclei (AGN). We evaluate different ultraviolet emission line ratios and how different assumptions about the models, including the presence of dust grains, the shape of the incident spectral energy distribution, or the thickness of the gas envelope around the central source, may affect the final estimates as a function of the set of emission lines used. We compare our results with other published recipes for deriving abundances using the same emission lines and show that deriving the carbon-to-oxygen abundance ratio using CIII] $\lambda$ 1909 \r{A} and OIII] $\lambda$ 1665 \r{A} emission lines is a robust indicator of the metal content in AGN that is nearly independent of the model assumptions, similar to the case of star-forming regions. Moreover, we show that a prior determination of C/O allows for a much more precise determination of the total oxygen abundance using carbon UV lines, as opposed to assuming an arbitrary relationship between O/H and C/O, which can lead to non-negligible discrepancies.