Effects of shocks in stellar atmosphere models on the emission line spectrum of surrounding Hii regions

TL;DR

This study assesses how shocks in stellar atmosphere models influence the emission line spectra of HII regions, finding that shocks significantly affect certain stellar parameters and nebular lines mainly at lower stellar temperatures.

Contribution

It introduces shock-inclusive stellar atmosphere models into photoionisation simulations, highlighting their impact on emission line diagnostics and physical parameter estimations.

Findings

Shocks cause artificial increases in temperature and ionisation parameters at T<30kK.

Shock models enhance He ii 4686 emission but do not fully explain observed lines in HII galaxies.

Effects are minimal for stars with T>30kK, reducing the need for shock models in most cases.

Abstract

Emission line studies from Hii regions in galaxies require tools for the inversion of line ratios into desired physical properties. These tools generally come in form of diagnostic ratios/diagrams that are based on grids of photoionisation models. An important input to the photoionisation models is the stellar atmosphere spectrum of the ionising sources. The current omission of shocks in the calculation of the former set of models could threaten the accuracy of the physical interpretation of emission line ratios from Hii regions. Current stellar atmosphere models that are crucial inputs to the grid of photoionisation models used to generate nebular emission line diagnostic diagrams might produce significant biases due to the omission of shocks. We investigate whether a new generation of photoionisation model grids, taking shocks into account, is required to compensate for the biases. We make use of the WM-Basic stellar atmosphere code, which can account for the extra energetic emission in the stellar spectral energy distribution produced by shocks, in conjunction with the photoionisation code MOCASSIN to determine whether shocks produce significant biases in the determination of the physical parameters of the interstellar medium and/or ionising stellar parameters. We conclude that these effects are only important for stellar sources with effective temperatures lower than 30kK and in this case they yield artificially high stellar temperatures, electron temperatures and nebular ionisation parameters. The magnitude of the effect is also obviously dependent on the strength of the shock and is likely to be unimportant for the majority of stellar sources. Nevertheless, we find our 20kK and 30kK shock models to strongly enhance the He ii 4686 nebular emission line. This result is however not strong enough to explain previously observed He ii 4686 line emission in the spectra of Hii galaxies.