@article{MantzoukiCampbellvanLoonetal.2018,
  author    = {Mantzouki, Evanthia and Campbell, James and van Loon, Emiel and Visser, Petra and Konstantinou, Iosif and Antoniou, Maria and Giuliani, Gregory and Machado-Vieira, Danielle and de Oliveira, Alinne Gurjao and Maronic, Dubravka Spoljaric and Stevic, Filip and Pfeiffer, Tanja Zuna and Vucelic, Itana Bokan and Zutinic, Petar and Udovic, Marija Gligora and Plenkovic-Moraj, Andelka and Tsiarta, Nikoletta and Blaha, Ludek and Geris, Rodan and Frankova, Marketa and Christoffersen, Kirsten Seestern and Warming, Trine Perlt and Feldmann, Tonu and Laas, Alo and Panksep, Kristel and Tuvikene, Lea and Kangro, Kersti and Haggqvist, Kerstin and Salmi, Pauliina and Arvola, Lauri and Fastner, Jutta and Straile, Dietmar and Rothhaupt, Karl-Otto and Fonvielle, Jeremy Andre and Grossart, Hans-Peter and Avagianos, Christos and Kaloudis, Triantafyllos and Triantis, Theodoros and Zervou, Sevasti-Kiriaki and Hiskia, Anastasia and Gkelis, Spyros and Panou, Manthos and McCarthy, Valerie and Perello, Victor C. and Obertegger, Ulrike and Boscaini, Adriano and Flaim, Giovanna and Salmaso, Nico and Cerasino, Leonardo and Koreiviene, Judita and Karosiene, Jurate and Kasperoviciene, Jurate and Savadova, Ksenija and Vitonyte, Irma and Haande, Sigrid and Skjelbred, Birger and Grabowska, Magdalena and Karpowicz, Maciej and Chmura, Damian and Nawrocka, Lidia and Kobos, Justyna and Mazur-Marzec, Hanna and Alcaraz-Parraga, Pablo and Wilk-Wozniak, Elzbieta and Krzton, Wojciech and Walusiak, Edward and Gagala, Ilona and Mankiewicz-Boczek, Joana and Toporowska, Magdalena and Pawlik-Skowronska, Barbara and Niedzwiecki, Michal and Peczula, Wojciech and Napiorkowska-Krzebietke, Agnieszka and Dunalska, Julita and Sienska, Justyna and Szymanski, Daniel and Kruk, Marek and Budzynska, Agnieszka and Goldyn, Ryszard and Kozak, Anna and Rosinska, Joanna and Szelag-Wasielewska, Elzbieta and Domek, Piotr and Jakubowska-Krepska, Natalia and Kwasizur, Kinga and Messyasz, Beata and Pelechata, Aleksandra and Pelechaty, Mariusz and Kokocinski, Mikolaj and Madrecka, Beata and Kostrzewska-Szlakowska, Iwona and Frak, Magdalena and Bankowska-Sobczak, Agnieszka and Wasilewicz, Michal and Ochocka, Agnieszka and Pasztaleniec, Agnieszka and Jasser, Iwona and Antao-Geraldes, Ana M. and Leira, Manel and Hernandez, Armand and Vasconcelos, Vitor and Morais, Joao and Vale, Micaela and Raposeiro, Pedro M. and Goncalves, Vitor and Aleksovski, Boris and Krstic, Svetislav and Nemova, Hana and Drastichova, Iveta and Chomova, Lucia and Remec-Rekar, Spela and Elersek, Tina and Delgado-Martin, Jordi and Garcia, David and Luis Cereijo, Jose and Goma, Joan and Carmen Trapote, Mari and Vegas-Vilarrubia, Teresa and Obrador, Biel and Garcia-Murcia, Ana and Real, Monserrat and Romans, Elvira and Noguero-Ribes, Jordi and Parreno Duque, David and Fernandez-Moran, Elisabeth and Ubeda, Barbara and Angel Galvez, Jose and Marce, Rafael and Catalan, Nuria and Perez-Martinez, Carmen and Ramos-Rodriguez, Eloisa and Cillero-Castro, Carmen and Moreno-Ostos, Enrique and Maria Blanco, Jose and Rodriguez, Valeriano and Juan Montes-Perez, Jorge and Palomino, Roberto L. and Rodriguez-Perez, Estela and Carballeira, Rafael and Camacho, Antonio and Picazo, Antonio and Rochera, Carlos and Santamans, Anna C. and Ferriol, Carmen and Romo, Susana and Soria, Juan Miguel and Hansson, Lars-Anders and Urrutia-Cordero, Pablo and Ozen, Arda and Bravo, Andrea G. and Buck, Moritz and Colom-Montero, William and Mustonen, Kristiina and Pierson, Don and Yang, Yang and Verspagen, Jolanda M. H. and Domis, Lisette N. de Senerpont and Seelen, Laura and Teurlincx, Sven and Verstijnen, Yvon and Lurling, Miquel and Maliaka, Valentini and Faassen, Elisabeth J. and Latour, Delphine and Carey, Cayelan C. and Paerl, Hans W. and Torokne, Andrea and Karan, Tunay and Demir, Nilsun and Beklioglu, Meryem and Filiz, Nur and Levi, Eti E. and Iskin, Ugur and Bezirci, Gizem and Tavsanoglu, Ulku Nihan and Celik, Kemal and Ozhan, Koray and Karakaya, Nusret and Kocer, Mehmet Ali Turan and Yilmaz, Mete and Maraslioglu, Faruk and Fakioglu, Ozden and Soylu, Elif Neyran and Yagci, Meral Apaydin and Cinar, Sakir and Capkin, Kadir and Yagci, Abdulkadir and Cesur, Mehmet and Bilgin, Fuat and Bulut, Cafer and Uysal, Rahmi and Koker, Latife and Akcaalan, Reyhan and Albay, Meric and Alp, Mehmet Tahir and Ozkan, Korhan and Sevindik, Tugba Ongun and Tunca, Hatice and Onem, Burcin and Richardson, Jessica and Edwards, Christine and Bergkemper, Victoria and Beirne, Eilish and Cromie, Hannah and Ibelings, Bastiaan W.},
  title     = {Data Descriptor: A European Multi Lake Survey dataset of environmental variables, phytoplankton pigments and cyanotoxins},
  series = {Scientific Data},
  volume    = {5},
  journal   = {Scientific Data},
  publisher = {Nature Publ. Group},
  address   = {London},
  issn      = {2052-4463},
  doi       = {10.1038/sdata.2018.226},
  pages     = {13},
  year      = {2018},
  abstract  = {Under ongoing climate change and increasing anthropogenic activity, which continuously challenge ecosystem resilience, an in-depth understanding of ecological processes is urgently needed. Lakes, as providers of numerous ecosystem services, face multiple stressors that threaten their functioning. Harmful cyanobacterial blooms are a persistent problem resulting from nutrient pollution and climate-change induced stressors, like poor transparency, increased water temperature and enhanced stratification. Consistency in data collection and analysis methods is necessary to achieve fully comparable datasets and for statistical validity, avoiding issues linked to disparate data sources. The European Multi Lake Survey (EMLS) in summer 2015 was an initiative among scientists from 27 countries to collect and analyse lake physical, chemical and biological variables in a fully standardized manner. This database includes in-situ lake variables along with nutrient, pigment and cyanotoxin data of 369 lakes in Europe, which were centrally analysed in dedicated laboratories. Publishing the EMLS methods and dataset might inspire similar initiatives to study across large geographic areas that will contribute to better understanding lake responses in a changing environment.},
  language  = {en}
}
@article{RauchQuinetHoyeretal.2016,
  author    = {Rauch, Thomas and Quinet, P. and Hoyer, D. and Werner, K. and Richter, Philipp and Kruk, J. W. and Demleitner, M.},
  title     = {VII. New Kr IV - VII oscillator strengths and an improved spectral analysis of the hot, hydrogen-deficient DO-type white dwarf RE 0503-289},
  series = {Plant physiology : an international journal devoted to physiology, biochemistry, cellular and molecular biology, biophysics and environmental biology of plants},
  volume    = {590},
  journal   = {Plant physiology : an international journal devoted to physiology, biochemistry, cellular and molecular biology, biophysics and environmental biology of plants},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  organization    = {VERITAS Collaboration},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201628131},
  pages     = {26},
  year      = {2016},
  abstract  = {Context. For the spectral analysis of high-resolution and high signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. Aims. New Kr IV-VII oscillator strengths for a large number of lines enable us to construct more detailed model atoms for our NLTE model-atmosphere calculations. This enables us to search for additional Kr lines in observed spectra and to improve Kr abundance determinations. Methods. We calculated Kr IV-VII oscillator strengths to consider radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Kr lines that are exhibited in high-resolution and high S/N ultraviolet (UV) observations of the hot white dwarf RE 0503-289. Results. We reanalyzed the effective temperature and surface gravity and determined T-eff = 70 000 +/- 2000 K and log (g/cm s(-2)) = 7.5 +/- 0.1. We newly identified ten Kr V lines and one Kr vi line in the spectrum of RE 0503-289. We measured a Kr abundance of 3.3 +/- 0.3 (logarithmic mass fraction). We discovered that the interstellar absorption toward RE 0503-289 has a multi-velocity structure within a radial-velocity interval of -40 km s(-1) < upsilon(rad) < +18 km s(-1). Conclusions. Reliable measurements and calculations of atomic data are a prerequisite for state-of-the-art NLTE stellar-atmosphere modeling. Observed Kr V-VII line profiles in the UV spectrum of the white dwarf RE 0503-289 were simultaneously well reproduced with our newly calculated oscillator strengths.},
  language  = {en}
}
@article{ReindlRauchParthasarathyetal.2014,
  author    = {Reindl, Nicole and Rauch, Thomas and Parthasarathy, M. and Werner, K. and Kruk, J. W. and Hamann, Wolf-Rainer and Sander, Andreas Alexander Christoph and Todt, Helge Tobias},
  title     = {The rapid evolution of the exciting star of the Stingray nebula},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {565},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201323189},
  pages     = {14},
  year      = {2014},
  abstract  = {Context. SAO 244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21 kK in 1971 to over 50 kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Aims. A comprehensive model-atmosphere analysis of UV and optical spectra taken during 1988-2006 should reveal the detailed temporal evolution of its atmospheric parameters and provide explanations for the unusually fast evolution. Methods. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed spectra allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. Results. We find that the central star has steadily increased its effective temperature from 38 kK in 1988 to a peak value of 60 kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log(M/M-circle dot yr(-1)) = -9.0 to -11.6 and the terminal wind velocity increased from v(infinity) = 1800 km s(-1) to 2800 km s(-1). Since around 2002, the star stopped heating and has cooled down again to 55 kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. The results are discussed by considering different evolutionary scenarios. Conclusions. The position of SAO 244567 in the log T-eff-log g plane places the star in the region of sdO stars. By comparison with stellar-evolution calculations, we confirm that SAO 244567 must be a low-mass star (M < 0.55 M-circle dot). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAD 244567 would be a low-mass (0.354 M-circle dot) helium pre-white dwarf after the common-envelope phase, during which the planetary nebula was ejected.},
  language  = {en}
}
@article{ReindlRauchWerneretal.2014,
  author    = {Reindl, Nicole and Rauch, Thomas and Werner, Klaus and Kruk, J. W. and Todt, Helge Tobias},
  title     = {On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {566},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201423498},
  pages     = {23},
  year      = {2014},
  abstract  = {Context. About a quarter of all post-asymptotic giant branch (AGB) stars are hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated H-deficient stars by a (very) late thermal pulse scenario where the hydrogen-rich envelope is mixed with the helium-rich intershell layer. Depending on the particular time at which the final flash occurs, the entire hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars and their evolution are not yet understood. Aims. A small group of very hot, helium-dominated stars is formed by O(He)-type stars. A precise analysis of their photospheric abundances will establish constraints to their evolution. Methods. We performed a detailed spectral analysis of ultraviolet and optical spectra of four O(He) stars by means of state-of-the-art non-LTE model-atmosphere techniques. Results. We determined effective temperatures, surface gravities, and the abundances of H, He, C, N, O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss. The comparison with evolutionary models shows that the status of the O(He) stars remains uncertain. Their abundances match predictions of a double helium white dwarf (WD) merger scenario, suggesting that they might be the progeny of the compact and of the luminous helium-rich sdO-type stars. The existence of planetary nebulae that do not show helium enrichment around every other O(He) star precludes a merger origin for these stars. These stars must have formed in a different way, for instance via enhanced mass-loss during their post-AGB evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant or AGB star. Conclusions. A helium-dominated stellar evolutionary sequence exists that may be fed by different types of mergers or CE scenarios. It appears likely that all these pass through the O(He) phase just before they become WDs.},
  language  = {en}
}
@article{LoeblingRauchBertolamiMilleretal.2019,
  author    = {L{\"o}bling, Lisa and Rauch, Thomas and Bertolami Miller, Marcelo Miguel and Todt, Helge Tobias and Friederich, F. and Ziegler, M. and Werner, Klaus and Kruk, J. W.},
  title     = {Spectral analysis of the hybrid PG 1159-type central stars of the planetary nebulae Abell 43 and NGC7094},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {489},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {1},
  publisher = {Oxford Univ. Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1093/mnras/stz1994},
  pages     = {1054 -- 1071},
  year      = {2019},
  abstract  = {Stellar post asymptotic giant branch (post-AGB) evolution can be completely altered by a final thermal pulse (FTP) which may occur when the star is still leaving the AGB (AFTP), at the departure from the AGB at still constant luminosity (late TP, LTP) or after the entry to the white-dwarf cooling sequence (very late TP, VLTP). Then convection mixes the Herich material with the H-rich envelope. According to stellar evolution models the result is a star with a surface composition of H approximate to 20 per cent by mass (AFTP), approximate to 1 per cent (LTP), or (almost) no H (VLTP). Since FTP stars exhibit intershell material at their surface, spectral analyses establish constraints for AGB nucleosynthesis and stellar evolution. We performed a spectral analysis of the so-called hybrid PG 1159-type central stars (CS) of the planetary nebulae Abell 43 and NGC7094 by means of non-local thermodynamical equilibrium models. We confirm the previously determined effective temperatures of T-eff = 115 000 +/- 5 000K and determine surface gravities of log (g /(cm s(-2))) = 5.6 +/- 0.1 for both. From a comparison with AFTP evolutionary tracks, we derive stellar masses of 0.57(-0.04)(+0.07)M(circle dot) and determine the abundances of H, He, and metals up to Xe. Both CS are likely AFTP stars with a surface H mass fraction of 0.25 +/- 0.03 and 0.15 +/- 0.03, respectively, and an Fe deficiency indicating subsolar initial metallicities. The light metals show typical PG 1159-type abundances and the elemental composition is in good agreement with predictions from AFTP evolutionary models. However, the expansion ages do not agree with evolution time-scales expected from the AFTP scenario and alternatives should be explored.},
  language  = {en}
}