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

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.

10 pages, 3 figures, 5 tables

References in corpus (5)

• An XMM-Newton view of the young open cluster NGC 6231 -- II. The OB star population
• X-ray enabled MOCASSIN: a 3D code for photoionized media
• Theoretical calculations of the HI, HeI and HeII free-bound continuou emission spectra
• The effects of spatially distributed ionisation sources on the temperature structure of HII region
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Cited by in corpus (3)

• Numerical Models for the Diffuse Ionized Gas in Galaxies. II. Three-dimensional radiative transfer in inhomogeneous interstellar structures as a tool for analyzing the diffuse ionized gas
• Blue galaxies: modelling nebular He II emission in high redshift galaxies
• A Refined Sample of Lyman Excess H II Regions