Rebirth of X-ray Emission from the Born-Again Planetary Nebula A 30

M. A. Guerrero, N. Ruiz, W.-R. Hamann, Y.-H. Chu, H. Todt, D. Schoenberner, L. Oskinova, R. A. Gruendl, M. Steffen, W. P. Blair, J. A. Toala

The planetary nebula (PN) A 30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using multi-epoch HST images we have detected the angular expansion of these knots and derived an age of 850+280-150 yr. To investigate the spectral and spatial properties of the soft X-ray emission detected by ROSAT, we have obtained Chandra and XMM-Newton deep observations of A 30. The X-ray emission from A 30 can be separated into two components: a point-source at the central star and diffuse X-ray emission associated with the hydrogen-poor knots and the cloverleaf structure inside the nebular shell. To help us assess the role of the current stellar wind in powering this X-ray emission, we have determined the stellar parameters and wind properties of the central star of A 30 using a non-LTE model fit to its optical and UV spectrum. The spatial distribution and spectral properties of the diffuse X-ray emission is highly suggestive that it is generated by the post-born-again and present fast stellar winds interacting with the hydrogen-poor ejecta of the born-again event. Charge-exchange reactions between the ions of the stellar winds and neutral material of the born-again ejecta seem the most likely mechanism for the production of diffuse X-ray emission. Shock-heated plasma may also contribute to this emission, as the hydrogen-poor knots are ablated by the stellar winds, in which case efficient mass-loading of the stellar winds is needed to raise the density and damp the velocity of the stellar winds to reproduce the spectral properties of the diffuse emission. The origin of the X-ray emission from the central star of A 30 is puzzling: shocks in the present fast stellar wind and photospheric emission can be ruled out, while the development of a new, compact hot bubble confining the fast stellar wind seems implausible.

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