Modelling stellar transition regions of F-G stars with advanced ionisation equilibrium Evangelia Deliporanidou and Dr. Giulio Del Zanna Department of Applied Mathematics and Theoretical Physics, University of Cambridge Abstract We employed advanced ionisation equilibrium models developed by Dufresne et al. [5], which include charge transfer and density effects to study stellar irradiances in the EUV and UV for a sample of stars. Our sample includes ε Eridani (K2 V), α Centauri A (G2 V), Procyon (F5 V), and Proxima Centauri (M5.5 Ve). We remeasured the line fluxes from STIS datasets and used O IV and O V as a density diagnostic to measure the formation pressure of ions in the transition region (TR). Our findings indicate significant improvements in modelling anomalous ions, such as Si IV, C IV and N V, from Li- and Na- like sequences, which produce the strongest lines in UV. We obtained first-time reliable estimates of some stellar chemical abundances in the TR, using a Differential Emission Measure (DEM) modelling. We also obtained significant improvements in the calculated-to-observed flux ratios. These results are compared with available photospheric abundances, demonstrating the effectiveness of our approach to look for evidence of the First Ionisation Potential (FIP) effect, in the TR of our stellar sample. Finally, we compared our results with an analysis of solar TR irradiances in the EUV. New Ion Balances We call ”anomalous” the ions of the Li- and Na-like sequences, as they are under-predicted by sig- nificant factors compared to other ions, forming at similar temperatures and densities, assuming zero density ionisation equilibrium, the standard in Astrophysics. This problem was observed on the Sun and Del Zanna et al. [2] combined EUV and UV observational data and showed for the first time that the anomalous ions are also under-predicted in stellar atmospheres. Sim and Jordan [10] developed an approximate method for ϵ Eridani, which is a K2 V dwarf star, to resolve the main discrepancies by including density and charge transfer effects. In Dufresne et al. [5], we developed advanced ionisation equilibrium models that include these effects and found significant improvements for a solar plage ob- servation in the UV. The main difference between the advanced models and the coronal approximation used in the previous CHIANTI versions [1, 3], is that the anomalous ions have increased intensities by factors of 2-5. Also, they shift the ion formation towards lower temperatures. Figure 1. Silicon ion balance at a constant electron pressure of 7×1015 cm−3 K, with charge transfer included using the Fontenla 2014 Facula model atmosphere. We also show the advanced models without charge transfer and the CHIANTI v.10.1 coronal approximation model. Dufresne et al. [5] FIP Effect Our stellar sample was chosen to investigate the First Ionisation Potential (FIP) effect [6, 8] in stellar TR. This effect, first identified in the active solar corona, is characterised by an overabundance of low-FIP elements, such as Fe, Mg, and Si compared to high-FIP elements, such as H, He and O. Many studies of stellar coronae found that the FIP and Inverse FIP (IFIP) effects are prevalent among active stars and vary with stellar activity, see the review by Seli et al. [9]. Both FIP and IFIP are explained by models from Laming [6]. Method We remeasured line fluxes from Space Telescope Imaging Spectrograph (STIS) selecting the datasets with the longer exposure time, an E140H or E140M grating within a wavelength range of 1100 < λ < 1700. We used O IV and O V as a density diagnostic and assumed a constant pressure for the ions in the stellar TR. We used a pressure of 1016 cm−3 K for ϵ Eridani, Proxima Centauri and Procyon and a pressure of 1015 cm−3 K for α Centauri A. We eventually evaluated the improvements in modelling anomalous ions and derived reliable estimates of stellar abundances via a DEM modelling. We adjusted the log DEM based on the scaling fact