# Effects of continuum fudging on non-LTE synthesis of stellar spectra. I. Effects on estimates of UV continua and Solar Spectral Irradiance variability

## TL;DR

This study examines how artificial fudge factors used in non-LTE spectral synthesis impact the accuracy of UV solar and stellar irradiance variability estimates, revealing potential underestimations and model dependencies.

## Contribution

It provides the first detailed analysis of the effects of opacity fudge factors on UV irradiance variability estimates across different stellar models.

## Key findings

- Fudge factors can underestimate UV variability by up to 20%.
- The impact of fudge factors increases with decreasing stellar metallicity.
- Uncertainties are significant for facula-dominated stars with T_eff > 4000 K.

## Abstract

Synthesis performed under non Local Thermodynamic Equilibrium (non-LTE) conditions usually overestimate stellar spectra. An approach widely adopted in the literature to reduce the excess of UV radiation consists of artificially increasing the continuum opacity by using multiplicative fudge factors, which are empirically derived to impose the synthetized spectrum to match the observed one. Although the method was initially developed to improve non-LTE synthesis of spectral lines, it has been recently employed to model solar spectral irradiance variability. Such irradiance reconstruction techniques combine spectral synthesis of different types of structures, which are performed making use of factors derived from a reference, quiet Sun model. Because the opacity scales in a complex way with plasma physical properties, the question arises whether, and to what extent, fudge factors derived using a reference model can be used to adjust the opacity of models representing different types of quiet and magnetic features. Here we investigate the effects of opacity fudging on estimates of solar and stellar irradiance variability in UV bands. We find that the use of fudge factors might underestimate the variability by 19% and up to 20% in the ranges 230-300 nm and 300-400 nm, respectively. These estimates are model dependent and should be considered as upper limits. Finally, our analysis suggests the uncertainties generated by the use of fudge factors to increase with the decrease of stellar metallicity and to be significant for stars whose variability is facula-dominated and whose effective temperature is larger than approximately 4000 K.