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

Abstract

X-ray emitting diffuse nebulae around hot stars are observed to have soft-band temperatures in the narrow range [1-3]×10 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 photoionized 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.

Investigating Particle Acceleration in the Wolf–Rayet Bubble G2.4+1.4

“…The expected Xray flux in the 0.3-10 keV energy band is of the order of 10 −13 erg s −1 cm −2 , though this value is very sensitive to uncertainties in the model such as spatial variations of the spectral index. Moreover, we note that thermal emission is also expected in the X-ray domain given the presence of shock-heated gas (Toalá et al 2015a;Toalá & Arthur 2018).…”

“…Indirect evidence of the presence of strong shocks can be gathered in X-rays, given that such shocks heat the plasma to very high temperatures. However, in stellar bubbles the turbulent mixing and thermal conduction produce gas of intermediate temperature and density, which only emits soft and diffuse X-rays as has been observed in four WR nebulae (Toalá & Arthur 2018).…”

Simulations of the Formation and X-ray Emission from Hot Bubbles in Planetary Nebulae

“…Using the extensively tested CHIANTI database version 7.1.3 [17], we calculated the emission coefficient (T) corresponding to the soft X-ray energy band (0.3-2.0 keV). Figure 2 (left) shows the emission coefficient for PNe chemical abundances in our Galaxy and in the Large and Small Magellanic Clouds (LMC and SMC, respectively) (see Appendix in [13] for details). Note that the different (T) curves show two peaks: the main one peaks around log 10 (T) = 6.1 − 6.3 depending on the abundance set.…”

“…In the middle and right panels, solid (dashed) lines correspond to simulations with (without) thermal conduction. See [13] for details.…”

“…Hydrodynamical ablation and photoevaporation of the dense clumps and filaments leads to turbulent mixing of cooler material into the hot gas producing favorable conditions for soft X-ray emission. Here, we discuss our most recent results on the X-ray emission from PNe in our Galaxy and in the Magellanic Clouds [11][12][13].…”