Ultraviolet and extreme-ultraviolet line ratio diagnostics for O IV

TL;DR

This paper develops theoretical ultraviolet and extreme-ultraviolet line ratios for O IV, demonstrating their effectiveness as diagnostics for electron temperature and density in various astrophysical plasmas, supported by comparisons with observational data.

Contribution

It provides new, comprehensive O IV line ratio diagnostics based on recent relativistic calculations, applicable across a wide range of plasma conditions, and encourages higher-resolution observations.

Findings

Good agreement between theory and observational data.

Line ratios are versatile for different plasma densities and temperatures.

Blended lines are identified to motivate future high-resolution studies.

Abstract

Aims: We generate theoretical ultraviolet and extreme-ultraviolet emission line ratios for O IV and show their strong versatility as electron temperature and density diagnostics for astrophysical plasmas. Methods: Recent fully relativistic calculations of radiative rates and electron impact excitation cross sections for O IV, supplemented with earlier data for A-values and proton excitation rates, are used to derive theoretical O IV line intensity ratios for a wide range of electron temperatures and densities. Results: Diagnostic line ratios involving ultraviolet or extreme-ultraviolet transitions in O IV are presented, that are applicable to a wide variety of astrophysical plasmas ranging from low density gaseous nebulae to the densest solar and stellar flares. Comparisons with observational data, where available, show good agreement between theory and experiment, providing support for the accuracy of the diagnostics. However, diagnostics are also presented involving lines that are blended in existing astronomical spectra, in the hope this might encourage further observational studies at higher spectral resolution.