Uncertainties in (E)UV model atmosphere fluxes

TL;DR

This paper compares NLTE model atmosphere codes TMAP and TLUSTY, highlighting systematic differences in EUV flux predictions caused by level dissolution treatment and cutoff frequency choices, emphasizing the need for improved theoretical modeling.

Contribution

It identifies the impact of cutoff frequency choices on EUV flux predictions and advocates for generalizing level dissolution theory to all species for more reliable UV flux modeling.

Findings

Systematic differences in EUV fluxes due to cutoff frequency choices.

Agreement between codes below 1500 Å for stars under 30000K when cutoff is aligned.

Uncertainty in modeling Sirius B's EUV flux highlights need for improved level dissolution theory.

Abstract

During the comparison of synthetic spectra calculated with two NLTE model atmosphere codes, namely TMAP and TLUSTY, we encounter systematic differences in the EUV fluxes due to the treatment of level dissolution by pressure ionization. Systematic differences may occur due to a code-specific cutoff frequency of the H I Lyman bound-free opacity. This is the case for TMAP and TLUSTY. Both codes predict the same flux level at wavelengths lower than about 1500 A for stars with effective temperatures below about 30000K only, if the same cutoff frequency is chosen. In the case of Sirius B, we demonstrate an uncertainty in modeling the EUV flux reliably in order to challenge theoreticians to improve the theory of level dissolution. The theory of level dissolution in high-density plasmas, which is available for hydrogen only should be generalized to all species. Especially, the cutoff frequencies for the bound-free opacities should be defined in order to make predictions of UV fluxes more reliable.