@misc{TodtPenaZuehlkeetal.2011,
  author    = {Todt, Helge Tobias and Pe{\~n}a, Miriam and Z{\"u}hlke, Julia and Oskinova, Lida and Hamann, Wolf-Rainer and Gr{\"a}fener, G{\"o}tz},
  title     = {Weak emission line central stars of planetary nebulae},
  series = {Planetary Nebulae: an Eye to the Future},
  journal   = {Planetary Nebulae: an Eye to the Future},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-413775},
  pages     = {2},
  year      = {2011},
  abstract  = {To understand the evolution and morphology of planetary nebulae, a detailed knowledge of their central stars is required. Central stars that exhibit emission lines in their spectra, indicating stellar mass-loss allow to study the evolution of planetary nebulae in action. Emission line central stars constitute about 10 \% of all central stars. Half of them are practically hydrogen-free Wolf-Rayet type central stars of the carbon sequence, [WC], that show strong emission lines of carbon and oxygen in their spectra. In this contribution we address the weak emission-lines central stars (wels). These stars are poorly analyzed and their hydrogen content is mostly unknown. We obtained optical spectra, that include the important Balmer lines of hydrogen, for four weak emission line central stars. We present the results of our analysis, provide spectral classification and discuss possible explanations for their formation and evolution.},
  language  = {en}
}
@misc{SandinSteffenJacobetal.2011,
  author    = {Sandin, Christer and Steffen, Matthias and Jacob, Ralf and Sch{\"o}nberner, Detlef and R{\"u}hling, Ute and Hamann, Wolf-Rainer and Todt, Helge Tobias},
  title     = {The role of heat conduction to the formation of [WC]-type planetary nebulae},
  series = {Proceedings of the International Astronomical Union},
  journal   = {Proceedings of the International Astronomical Union},
  number    = {582},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-41370},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-413702},
  pages     = {494 -- 495},
  year      = {2011},
  abstract  = {X-ray observations of young Planetary Nebul{\ae} (PNe) have revealed diffuse emission in extended regions around both H-rich and H-deficient central stars. In order to also repro-duce physical properties of H-deficient objects, we have, at first, extended our time-dependent radiation-hydrodynamic models with heat conduction for such conditions. Here we present some of the important physical concepts, which determine how and when a hot wind-blown bubble forms. In this study we have had to consider the, largely unknown, evolution of the CSPN, the slow (AGB) wind, the fast hot-CSPN wind, and the chemical composition. The main conclusion of our work is that heat conduction is needed to explain X-ray properties of wind-blown bubbles also in H-deficient objects.},
  language  = {en}
}
@misc{GuerreroChuHamannetal.2011,
  author    = {Guerrero, Mart{\´i}n A. and Chu, You-Hua and Hamann, Wolf-Rainer and Oskinova, Lida and Sch{\"o}nberner, Detlef and Todt, Helge Tobias and Steffen, Matthias and Ruiz, Nieves and Gruendl, Robert A. and Blair, William P.},
  title     = {Ablation and wind mass-loading in the born-again planetary nebula A 30},
  series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  journal   = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe},
  number    = {563},
  issn      = {1866-8372},
  doi       = {10.25932/publishup-41234},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-412341},
  pages     = {2},
  year      = {2011},
  abstract  = {We present XMM-Newton and Chandra observations of the born-again planetary nebula A 30. These X-ray observations reveal a bright unresolved source at the position of the central star whose X-ray luminosity exceeds by far the model expectations for photospheric emission and for shocks within the stellar wind. We suggest that a "born-again hot bubble" may be responsible for this X-ray emission. Diffuse X-ray emission associated with the petal-like features and one of the H-poor knots seen in the optical is also found. The weakened emission of carbon lines in the spectrum of the diffuse emission can be interpreted as the dilution of stellar wind by mass-loading or as the detection of material ejected during a very late thermal pulse.},
  language  = {en}
}