@misc{VinkHegerKrumholzetal.2012, author = {Vink, Jorick Sandor and Heger, Alexander and Krumholz, Mark R. and Puls, Joachim and Banerjee, Shiladitya and Castro, Norberto and Chen, K.-J. and Chen{\`e}, A.-N. and Crowther, P. A. and Daminelli, A. and Gr{\"a}fener, G. and Groh, J. H. and Hamann, Wolf-Rainer and Heap, S. and Herrero, A. and Kaper, L. and Najarro, F. and Oskinova, Lida and Roman-Lopes, A. and Rosen, A. and Sander, A. and Shirazi, M. and Sugawara, Y. and Tramper, F. and Vanbeveren, D. and Voss, R. and Wofford, A. and Zhang, Y.}, title = {Very massive stars in the local universe}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {601}, issn = {1866-8372}, doi = {10.25932/publishup-41522}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-415220}, pages = {29}, year = {2012}, abstract = {Recent studies have claimed the existence of very massive stars (VMS) up to 300 M⊙ in the local Universe. As this finding may represent a paradigm shift for the canonical stellar upper-mass limit of 150 M⊙, it is timely to discuss the status of the data, as well as the far-reaching implications of such objects. We held a Joint Discussion at the General Assembly in Beijing to discuss (i) the determination of the current masses of the most massive stars, (ii) the formation of VMS, (iii) their mass loss, and (iv) their evolution and final fate. The prime aim was to reach broad consensus between observers and theorists on how to identify and quantify the dominant physical processes.}, language = {en} } @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{MorelCastroFossatietal.2014, author = {Morel, T. and Castro, Norberto and Fossati, Luca and Hubrig, Swetlana and Langer, N. and Przybilla, Norbert and Sch{\"o}ller, Markus and Carroll, Thorsten Anthony and Ilyin, Ilya and Irrgang, Andreas and Oskinova, Lida and Schneider, Fabian R. N. and Simon D{\´i}az, Sergio and Briquet, Maryline and Gonz{\´a}lez, Jean-Francois and Kharchenko, Nina and Nieva, M.-F. and Scholz, Ralf-Dieter and de Koter, Alexander and Hamann, Wolf-Rainer and Herrero, Artemio and Ma{\´i}z Apell{\´a}niz, Jesus and Sana, Hugues and Arlt, Rainer and Barb{\´a}, Rodolfo H. and Dufton, Polly and Kholtygin, Alexander and Mathys, Gautier and Piskunov, Anatoly E. and Reisenegger, Andreas and Spruit, H. and Yoon, S.-C.}, title = {The B fields in OB stars (BOB) survey}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {821}, issn = {1866-8372}, doi = {10.25932/publishup-41523}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-415238}, pages = {8}, year = {2014}, abstract = {The B fields in OB stars (BOB) survey is an ESO large programme collecting spectropolarimetric observations for a large number of early-type stars in order to study the occurrence rate, properties, and ultimately the origin of magnetic fields in massive stars. As of July 2014, a total of 98 objects were observed over 20 nights with FORS2 and HARPSpol. Our preliminary results indicate that the fraction of magnetic OB stars with an organised, detectable field is low. This conclusion, now independently reached by two different surveys, has profound implications for any theoretical model attempting to explain the field formation in these objects. We discuss in this contribution some important issues addressed by our observations (e.g., the lower bound of the field strength) and the discovery of some remarkable objects.}, language = {en} } @misc{MartinsBergemannBestenlehneretal.2012, author = {Martins, Fabrice and Bergemann, Maria and Bestenlehner, Joachim M. and Crowther, Paul A. and Hamann, Wolf-Rainer and Najarro, Francisco and Nieva, Maria Fernanda and Przybilla, Norbert and Freimanis, Juris and Hou, Weizhen and Kaper, Lex}, title = {SpS5 - II. Stellar and wind parameters}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {688}, issn = {1866-8372}, doi = {10.25932/publishup-41495}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-414956}, pages = {9}, year = {2012}, abstract = {The development of infrared observational facilities has revealed a number of massive stars in obscured environments throughout the Milky Way and beyond. The determination of their stellar and wind properties from infrared diagnostics is thus required to take full advantage of the wealth of observations available in the near and mid infrared. However, the task is challenging. This session addressed some of the problems encountered and showed the limitations and successes of infrared studies of massive stars.}, 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} }