# Using photo-ionisation models to derive carbon and oxygen gas-phase abundances in the rest UV

## TL;DR

This paper introduces a new UV-based method to accurately determine carbon and oxygen gas-phase abundances in ionised gas regions, useful for studying high-redshift star-forming galaxies.

## Contribution

A novel approach using UV emission lines and photo-ionisation models to derive nebular abundances, improving systematic studies at high redshift.

## Key findings

- Results consistent with direct electron temperature methods.
- Method effectively fixes C/O before deriving O/H and ionisation parameter.
- Applicable to high-redshift star-forming galaxy studies.

## Abstract

We present a new method to derive oxygen and carbon abundances using the ultraviolet (UV) lines emitted by the gas-phase ionised by massive stars. The method is based on the comparison of the nebular emission-line ratios with those predicted by a large grid of photo-ionisation models. Given the large dispersion in the O/H - C/O plane, our method firstly fixes C/O using ratios of appropriate emission lines and, in a second step, calculates O/H and the ionisation parameter from carbon lines in the UV. We find abundances totally consistent with those provided by the direct method when we apply this method to a sample of objects with an empirical determination of the electron temperature using optical emission lines. The proposed methodology appears as a powerful tool for systematic studies of nebular abundances in star-forming galaxies at high redshift.