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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