X-ray emission from hot bubbles in nebulae around evolved stars

Abstract

This thesis presents an observational and numerical study on the X-ray emission from hot bubbles in nebulae around evolved stars. The observational part of this study consists mainly in observations obtained from the X-ray satellites X-ray Multi Mirror Mission (XMM-Newton) and Chandra X-ray Observatory (CXO). We have made use of optical, infrared, and ultraviolet observations that have complemented our results and analysis. These observations have allowed us to study the Wolf-Rayet (WR) nebulae S 308, NGC6888, NGC2359, and that around the WR star WR16. We have also studied the planetary nebulae (PNe) NGC6543 and Abell 78 (A78).

The X-ray telescopes, XMM-Newton and CXO, have allowed us to study the distribution and physical characteristics of the hot and diffuse gas in the WR nebulae S 308 and NGC6888 with exquisite detail. Even though the CXO observations do not map entirely NGC6888, we are able to estimate global parameters of the X-ray emission making use of ROSAT observations. Previous observations performed with Suzaku, ROSAT, and ASCA were hampered by a large number of point sources in the line of sight of the nebulae. S 308 was observed with XMM-Newton with four pointings. We have made use of the the ESAS tasks, the most up-to-date tools for the analysis of soft and diffuse X-ray emission. We found that in both nebulae the hot gas has a plasma temperature of 1-1.5×106 K and it is delineated by the [OIII] emission and not the Halpha as stated in previous studies. A notable difference between these two WR nebulae is that S 308 has a limb-brightened morphology in the distribution of its hot gas, while NGC6888 displays three maxima. Furthermore, we have reanalyzed the XMM-Newton observations of NGC2359. Our results show that there is an extra spatial component of the diffuse X-ray emission associated to a blowout in the nebula not reported by previous studies. Moreover, we identify several point-like sources in the line of sight of the main bubble that plagued previous analyses. Our analysis results in similar plasma temperatures as those as in S 308 and NGC6888.

We have studied the WR nebula around WR16 using archival XMM-Newton observations. Even though it was expected that diffuse X-ray emission should be detected from a spherical, non-disruptedWR nebula, by comparison with other WR bubbles, we are not able to detect such emission within this WR nebula. It is possible that hot gas exist inside the nebula, but with emissivity below detectable limits of the present generation of X-ray satellites.

The Cat's Eye PN (a.k.a. NGC6543) was also studied with XMM-Newton observations.We focused our analysis on observations from the Reflecting Grating Spectrometers (RGS1 and RGS2). These observations suffered from high background levels, still we are able to detect the OVII triplet at 22Å. The resultant spectrum seems to show that the N/O ratio is close to solar, but this result is not conclusive.

Finally, on observational grounds, we studied the born-again PN A78 with observations obtained during the realization of this thesis. This is the second born-again PN to harbor a point-like X-ray emission plus a diffuse component. We argue that such diffuse X-ray emission is the result of the complex interaction of the current fast stellar wind with the hydrogen-poor knots ejected in the born-again event.

On the other hand, this thesis has been enriched by carrying out two-dimension (2D) radiation-hydrodynamic simulations. These simulations have been used to study the formation, evolution, and X-ray emission from PNe. With this, we have shown that the wind-wind interaction during the formation of PNe creates hydrodynamical instabilities that change the dynamics and observables (optical and X-ray) from the hot bubbles in PNe. This effect has been downplayed by previous 1D (and analytical) works that have addressed the X-ray emission from PNe. As a result of such instabilities, we have shown that there is a difference in the hot bubble's size between models with and without thermal conduction. In the cases without such physical effect, the hot gas can leak through the gaps between clumps and filaments in the broken swept-up shell and this depressurises the bubble. The inclusion of thermal conduction evaporates and heats material from the clumpy shell, which expands to seal the gaps, preventing a loss in bubble pressure. The pressure in bubbles without conduction is dominated by the photoionized shell, while for bubbles with thermal conduction it is dominated by the hot, shocked wind.

We extended this work by computing the synthetic X-ray emission from our numerical results. We find that even models without thermal conduction can mix material into the hot bubble via instabilities, which raises the emissivity of the bubbles to observable values. This is contrary to 1D models which need saturated thermal conduction in order for the PN to be detected in X-rays. Furthermore, we can reproduce the temperatures as derived from X-ray studies.