Modelling Stellar Irradiances I: the transition regions of FGKM stars

TL;DR

This study advances the modeling of UV stellar irradiances in FGKM stars by incorporating charge transfer and density effects, leading to more accurate spectral line predictions and enabling stellar chemical abundance estimates.

Contribution

The paper introduces improved ionization equilibrium models that better predict UV spectral lines and allow for the first reliable stellar TR abundance estimates, with no observed FIP effect.

Findings

Significant improvement in modeling Si IV, C IV, N V lines, reducing prediction errors.

First reliable estimates of stellar chemical abundances in the transition region.

No evidence of the FIP effect in the TR of the studied stars.

Abstract

We employed advanced ionisation equilibrium models that we developed in Dufresne et. al. (2024), which include charge transfer and density effects, to model UV stellar irradiances for a sample of stars. Our sample includes $\epsilon$ Eridani (K2 V), $\alpha$ Centauri A (G2 V), Procyon (F5 V) and Proxima Centauri (M5.5 Ve). We measured line fluxes from STIS datasets and used O IV and O V as density diagnostics to find the formation pressure of ions in the transition region (TR) and adopted a simple Differential Emission Measure (DEM) modelling. Our findings indicate significant improvements in modelling spectral lines from anomalous ions such as Si IV, C IV and N V of the Li- and Na-like sequences, which produce the strongest lines in the UV. For example, the Si IV lines were under-predicted by a factor of five and now are within 40% the observed fluxes. The improved models allow us to obtain for the first time reliable estimates of some stellar chemical abundances in the TR. We compared our results with available photospheric abundances in the literature and found no evidence for the First Ionisation Potential (FIP) effect in the TR of our stellar sample. Finally, we compared our results with the solar TR that can also be described by photospheric abundances.