On the X-ray temperature of hot gas in diffuse nebulae

TL;DR

This paper investigates the origin of the consistent soft X-ray temperatures in diffuse nebulae around hot stars, attributing it to turbulent mixing layers that filter emission and produce observed temperature ranges.

Contribution

It demonstrates that turbulent mixing layers cause the observed narrow X-ray temperature range in nebulae, supported by simulations and analysis of emission coefficients and metallicity effects.

Findings

Turbulent mixing layers produce steep DEM distributions with temperature filtering.

Higher metallicity nebulae have lower X-ray temperatures and higher luminosities.

The second temperature component in spectral fits is from hot stellar wind, while the principal is from nebular gas.

Abstract

X-ray emitting diffuse nebulae around hot stars are observed to have soft-band temperatures in the narrow range [1-3]$\times10^{6}$ K, independent of the stellar wind parameters and the evolutionary stage of the central star. We discuss the origin of this X-ray temperature for planetary nebulae (PNe), Wolf-Rayet nebulae (WR) and interstellar wind bubbles around hot young stars in our Galaxy and the Magellanic Clouds. We calculate the differential emission measure (DEM) distributions as a function of temperature from previously published simulations and combine these with the X-ray emission coefficient for the 0.3-2.0 keV band to estimate the X-ray temperatures. We find that all simulated nebulae have DEM distributions with steep negative slopes, which is due to turbulent mixing at the interface between the hot shocked stellar wind and the warm photoionised gas. Sharply peaked emission coefficients act as temperature filters and emphasize the contribution of gas with temperatures close to the peak position, which coincides with the observed X-ray temperatures for the chemical abundance sets we consider. Higher metallicity nebulae have lower temperature and higher luminosity X-ray emission. We show that the second temperature component found from spectral fitting to X-ray observations of WR nebulae is due to a significant contribution from the hot shocked stellar wind, while the lower temperature principal component is dominated by nebular gas. We suggest that turbulent mixing layers are the origin of the soft X-ray emission in the majority of diffuse nebulae.