@article{ToalaGuerreroTodtetal.2015,
  author    = {Toala, Jes{\´u}s Alberto and Guerrero, Mart{\´i}n A. and Todt, Helge Tobias and Hamann, Wolf-Rainer and Chu, Y.-H. and Gruendl, R. A. and Sch{\"o}nberner, Detlef and Oskinova, Lida and Marquez-Lugo, R. A. and Fang, X. and Ramos-Larios, Gerardo},
  title     = {The born-again Planetary nebula A78: an X-RAY twin of A30},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {799},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {1},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.1088/0004-637X/799/1/67},
  pages     = {10},
  year      = {2015},
  abstract  = {We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: they harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time an NLTE code for expanding atmospheres that takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with a temperature of kT = 0.088 keV (T approximate to 1.0 x 10(6) K). We estimate X-ray luminosities in the 0.2-2.0 keV energy band of L-X,L-CSPN =(1.2 +/- 0.3) x 10(31) erg s(-1) and L-X,L-DIFF =(9.2 +/- 2.3) x 10(30) erg s(-1) for the CSPN and diffuse components, respectively.},
  language  = {en}
}
@article{SteffenHubrigTodtetal.2014,
  author    = {Steffen, M. and Hubrig, Swetlana and Todt, Helge Tobias and Schoeller, M. and Hamann, Wolf-Rainer and Sandin, Christer and Sch{\"o}nberner, Detlef},
  title     = {Weak magnetic fields in central stars of planetary nebulae?},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {570},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201423842},
  pages     = {15},
  year      = {2014},
  abstract  = {Context. It is not yet clear whether magnetic fields play an essential role in shaping planetary nebulae (PNe), or whether stellar rotation alone and/or a close binary companion, stellar or substellar, can account for the variety of the observed nebular morphologies. Aims. In a quest for empirical evidence verifying or disproving the role of magnetic fields in shaping planetary nebulae, we follow up on previous attempts to measure the magnetic field in a representative sample of PN central stars. Methods. We obtained low-resolution polarimetric spectra with FORS2 installed on the Antu telescope of the VLT for a sample of 12 bright central stars of PNe with different morphologies, including two round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets are Wolf-Rayet type central stars. Results. For the majority of the observed central stars, we do not find any significant evidence for the existence of surface magnetic fields. However, our measurements may indicate the presence of weak mean longitudinal magnetic fields of the order of 100 Gauss in the central star of the young elliptical planetary nebula IC 418 as well as in the Wolf-Rayet type central star of the bipolar nebula Hen 2-113 and the weak emission line central star of the elliptical nebula Hen 2-131. A clear detection of a 250 G mean longitudinal field is achieved for the A-type companion of the central star of NGC 1514. Some of the central stars show a moderate night-to-night spectrum variability, which may be the signature of a variable stellar wind and/or rotational modulation due to magnetic features. Conclusions. Since our analysis indicates only weak fields, if any, in a few targets of our sample, we conclude that strong magnetic fields of the order of kG are not widespread among PNe central stars. Nevertheless, simple estimates based on a theoretical model of magnetized wind bubbles suggest that even weak magnetic fields below the current detection limit of the order of 100 G may well be sufficient to contribute to the shaping of the surrounding nebulae throughout their evolution. Our current sample is too small to draw conclusions about a correlation between nebular morphology and the presence of stellar magnetic fields.},
  language  = {en}
}
@article{HerwigSchoenbernerBloecker1998,
  author    = {Herwig, Falk and Sch{\"o}nberner, Detlef and Bl{\"o}cker, Thomas},
  title     = {On the validity of the core-mass luminosity relation for TP-AGB stars with efficient dredge-up},
  year      = {1998},
  language  = {en}
}
@article{HerwigSchoenbernerBloecker1999,
  author    = {Herwig, Falk and Sch{\"o}nberner, Detlef and Bl{\"o}cker, Thomas},
  title     = {Violation of the Core Mass - Luminosity relation for AGB models wich experience the thord dredge-up},
  year      = {1999},
  language  = {en}
}
@article{HerwigBloeckerSchoenberner1999,
  author    = {Herwig, Falk and Bl{\"o}cker, Thomas and Sch{\"o}nberner, Detlef},
  title     = {The role of convective boundaries},
  year      = {1999},
  language  = {en}
}
@article{GuerreroRuizHamannetal.2012,
  author    = {Guerrero, Mart{\´i}n A. and Ruiz, N. and Hamann, Wolf-Rainer and Chu, Y.-H. and Todt, Helge Tobias and Sch{\"o}nberner, Detlef and Oskinova, Lida and Gr{\"u}ndl, R. A. and Steffen, M. and Blair, William P. and Toala, Jes{\´u}s Alberto},
  title     = {Rebirth of X-Ray emission from the born-again planetary Nebula A30},
  series = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  volume    = {755},
  journal   = {The astrophysical journal : an international review of spectroscopy and astronomical physics},
  number    = {2},
  publisher = {IOP Publ. Ltd.},
  address   = {Bristol},
  issn      = {0004-637X},
  doi       = {10.1088/0004-637X/755/2/129},
  pages     = {15},
  year      = {2012},
  abstract  = {The planetary nebula A30 is believed to have undergone a very late thermal pulse resulting in the ejection of knots of hydrogen-poor material. Using multi-epoch Hubble Space Telescope images, we have detected the angular expansion of these knots and derived an age of 850(-150)(+280) 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 A30. The X-ray emission from A30 can be separated into two components: a point source at the central star and diffuse 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 A30 using a non-LTE model fit to its optical and UV spectra. The spatial distribution and spectral properties of the diffuse X-ray emission are 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. This emission can be attributed to shock-heated plasma, as the hydrogen-poor knots are ablated by the stellar winds, under which circumstances the efficient mass loading of the present fast stellar wind raises its density and damps its velocity to produce the observed diffuse soft X-rays. Charge transfer reactions between the ions of the stellar winds and material of the born-again ejecta have also been considered as a possible mechanism for the production of diffuse X-ray emission, and upper limits on the expected X-ray production by this mechanism have been derived. The origin of the X-ray emission from the central star of A30 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.},
  language  = {en}
}
@article{DriebeSchoenbernerBloeckeretal.1998,
  author    = {Driebe, Thomas and Sch{\"o}nberner, Detlef and Bl{\"o}cker, Thomas and Herwig, Falk},
  title     = {The evolution of helium white dwarfs : I. the companion of the millisecond pular PSR J1012+5307},
  year      = {1998},
  language  = {en}
}
@article{DriebeBloeckerSchoenberneretal.1999,
  author    = {Driebe, Thomas and Bl{\"o}cker, Thomas and Sch{\"o}nberner, Detlef and Herwig, Falk},
  title     = {The evolution of helium white dwarfs : II. Thermal instabilities},
  year      = {1999},
  abstract  = {We calculated a grid of evolutionary models for white dwarfs with helium cores (He-WDs) and investigated the occurrence of hydrogen-shell flashes due to unstable hydrogen burning via CNO cycling. Our calculations show that such thermal instabilities are restricted to a certain mass range (M approx 0.21 ... 0.30 Msun), consistent with earlier studies. Models within this mass range undergo the more hydrogen shell flashes the less massive they are. This is caused by the strong dependence of the envelope mass on the white dwarf core mass. The maximum luminosities from hydrogen burning during the flashes are of the order of 105 Lsun. Because of the development of a pulse-driven convection zone whose upper boundary temporarily reaches the surface layers, the envelope's hydrogen content decreases by Delta X approx 0.06 per flash. Our study further shows that an additional high mass-loss episode during a flash-driven Roche lobe overflow to the white dwarf's companion does not affect the final cooling behaviour of the models. Independent of hydrogen shell flashes the evolution along the final white dwarf cooling branch is determined by hydrogen burning via pp-reactions down to effective temperatures as low as approx 8000 K.},
  language  = {en}
}
@article{DriebeBloeckerHerwigetal.1998,
  author    = {Driebe, Thomas and Bl{\"o}cker, Thomas and Herwig, Falk and Sch{\"o}nberner, Detlef},
  title     = {Diffusive overshooting in hot bottom burning AGB models},
  year      = {1998},
  language  = {en}
}