TY  - JOUR
A1  - Hajduk, Marcin
A1  - Todt, Helge Tobias
A1  - Hamann, Wolf-Rainer
A1  - Borek, Karolina
A1  - van Hoof, Peter A. M.
A1  - Zijlstra, Albert A.
T1  - The cooling-down central star of the planetary nebula SwSt 1
BT  - a late thermal pulse in a massive post-AGB star?
JF  - Monthly notices of the Royal Astronomical Society
N2  - SwSt 1 (PN G001.5-06.7) is a bright and compact planetary nebula containing a late [WC]-type central star. Previous studies suggested that the nebular and stellar lines are slowly changing with time. We studied new and archival optical and ultraviolet spectra of the object. The [O III] 4959 and 5007 angstrom to H beta line flux ratios decreased between about 1976 and 1997/2015. The stellar spectrum also shows changes between these epochs. We modelled the stellar and nebular spectra observed at different epochs. The analyses indicate a drop of the stellar temperature from about 42 kK to 40.5 kK between 1976 and 1993. We do not detect significant changes between 1993 and 2015. The observations show that the star performed a loop in the H-R diagram. This is possible when a shell source is activated during its post-AGB evolution. We infer that a late thermal pulse (LTP) experienced by a massive post-AGB star can explain the evolution of the central star. Such a star does not expand significantly as the result of the LTP and does not became a born-again red giant. However, the released energy can remove the tiny H envelope of the star.
KW  - stars: AGB and post-AGB
KW  - stars: atmospheres
KW  - stars: evolution
KW  - planetary
KW  - nebulae: general
KW  - planetary nebulae: individual: SwSt1
Y1  - 2020
U6  - https://doi.org/10.1093/mnras/staa2274
SN  - 0035-8711
SN  - 1365-2966
VL  - 498
IS  - 1
SP  - 1205
EP  - 1220
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Hainich, Rainer
A1  - Pasemann, Diana
A1  - Todt, Helge Tobias
A1  - Shenar, Tomer
A1  - Sander, Andreas Alexander Christoph
A1  - Hamann, Wolf-Rainer
T1  - Wolf-Rayet stars in the Small Magellanic Cloud I. Analysis of the single WN stars
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Wolf-Rayet (WR) stars have a severe impact on their environments owing to their strong ionizing radiation fields and powerful stellar winds. Since these winds are considered to be driven by radiation pressure, it is theoretically expected that the degree of the wind mass-loss depends on the initial metallicity of WR stars.
 Aims. Following our comprehensive studies of WR stars in the Milky Way, M31, and the LMC, we derive stellar parameters and mass-loss rates for all seven putatively single WN stars known in the SMC. Based on these data, we discuss the impact of a low-metallicity environment on the mass loss and evolution of WR stars.
 Methods. The quantitative analysis of the WN stars is performed with the Potsdam Wolf-Rayet (PoWR) model atmosphere code. The physical properties of our program stars are obtained from fitting synthetic spectra to multi-band observations.
 Results. In all SMC WN stars, a considerable surface hydrogen abundance is detectable. The majority of these objects have stellar temperatures exceeding 75 kK, while their luminosities range from 10(5.5) to 10(6.1) L-circle dot. The WN stars in the SMC exhibit on average lower mass-loss rates and weaker winds than their counterparts in the Milky Way, M31, and the LMC.
 Conclusions. By comparing the mass-loss rates derived for WN stars in different Local Group galaxies, we conclude that a clear dependence of the wind mass-loss on the initial metallicity is evident, supporting the current paradigm that WR winds are driven by radiation. A metallicity effect on the evolution of massive stars is obvious from the HRD positions of the SMC WN stars at high temperatures and high luminosities. Standard evolution tracks are not able to reproduce these parameters and the observed surface hydrogen abundances. Homogeneous evolution might provide a better explanation for their evolutionary past.
KW  - stars: Wolf-Rayet
KW  - Magellanic Clouds
KW  - stars: early-type
KW  - stars: atmospheres
KW  - stars: winds, outflows
KW  - stars: mass-loss
Y1  - 2015
U6  - https://doi.org/10.1051/0004-6361/201526241
SN  - 1432-0746
VL  - 581
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Sander, Andreas Alexander Christoph
A1  - Todt, Helge Tobias
A1  - Hainich, Rainer
A1  - Hamann, Wolf-Rainer
T1  - The Wolf-Rayet stars in M31 I. Analysis of the late-type WN stars
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Comprehensive studies of Wolf-Rayet stars were performed in the past for the Galactic and the LMC population. The results revealed significant differences, but also unexpected similarities between the WR populations of these different galaxies. Analyzing the WR stars in M 31 will extend our understanding of these objects in different galactic environments.
 Aims. The present study aims at the late-type WN stars in M 31. The stellar and wind parameters will tell about the formation of WR stars in other galaxies with different metallicity and star formation histories. The obtained parameters will provide constraints to the evolution of massive stars in the environment of M 31.
 Methods. We used the latest version of the Potsdam Wolf-Rayet model atmosphere code to analyze the stars via fitting optical spectra and photometric data. To account for the relatively low temperatures of the late WN I 0 and WN I I subtypes, our WN models have been extended into this temperature regime.
 Results. Stellar and atmospheric parameters are derived for all known late-type WN stars in M 31 with available spectra. All of these stars still have hydrogen in their outer envelopes, some of them up to 50% by mass. The stars are located on the cool side of the zero age main sequence in the Hertzsprung-Russell diagram, while their luminosities range from 105 to 1064). It is remarkable that no star exceeds 106 L.
 Conclusions. If formed via single-star evolution, the late-type WN stars in M 31 stem from an initial mass range between 20 and 60 M-circle dot. From the very late-type WN9-11 stars, only one star is located in the S Doradus instability strip. We do not find any late-type WN stars with the high luminosities known in the Milky Way.
KW  - stars: massive
KW  - stars: evolution
KW  - stars: mass-loss
KW  - stars: Wolf-Rayet
KW  - stars: atmospheres
KW  - stars: winds
KW  - outflows
Y1  - 2014
U6  - https://doi.org/10.1051/0004-6361/201323240
SN  - 0004-6361
SN  - 1432-0746
VL  - 563
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Gruner, David
A1  - Hainich, Rainer
A1  - Sander, Andreas Alexander Christoph
A1  - Shenar, Tomer
A1  - Todt, Helge Tobias
A1  - Oskinova, Lida
A1  - Ramachandran, Varsha
A1  - Ayres, T.
A1  - Hamann, Wolf-Rainer
T1  - The extreme O-type spectroscopic binary HD 93129A A quantitative, multiwavelength analysis
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. HD 93129A was classified as the earliest O-type star in the Galaxy (O2 If*) and is considered as the prototype of its spectral class. However, interferometry shows that this object is a binary system, while recent observations even suggest a triple configuration. None of the previous spectral analyses of this object accounted for its multiplicity. With new high-resolution UV and optical spectra, we have the possibility to reanalyze this key object, taking its binary nature into account for the first time. Aims. We aim to derive the fundamental parameters and the evolutionary status of HD 93129A, identifying the contributions of both components to the composite spectrum Results. Despite the similar spectral types of the two components, we are able to find signatures from each of the components in the combined spectrum, which allows us to estimate the parameters of both stars. We derive log(L/L-circle dot) = 6.15, T-eff = 52 kK, and log (M)over dot = -4.7[M-circle dot yr(-1)] for the primary Aa, and log(L/L-circle dot) = 5.58, T-eff = 45 kK, and log (M)over dot = -5.8 [M(circle dot)yr(-1)] for the secondary Ab. Conclusions. Even when accounting for the binary nature, the primary of HD 93129A is found to be one of the hottest and most luminous O stars in our Galaxy. Based on the theoretical decomposition of the spectra, we assign spectral types O2 If* and O3 III(f*) to components Aa and Ab, respectively. While we achieve a good fit for a wide spectral range, specific spectral features are not fully reproduced. The data are not sufficient to identify contributions from a hypothetical third component in the system.
KW  - stars: individual: HD 93129A
KW  - stars: atmospheres
KW  - stars: fundamental parameters
KW  - stars: early-typeP
Y1  - 2018
U6  - https://doi.org/10.1051/0004-6361/201833178
SN  - 1432-0746
VL  - 621
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Sander, A.
A1  - Hamann, Wolf-Rainer
A1  - Todt, Helge Tobias
T1  - The Galactic WC stars Stellar parameters from spectral analyses indicate a new evolutionary sequence
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. The life cycles of massive stars from the main sequence to their explosion as supernovae or gamma ray bursts are not yet fully clear, and the empirical results from spectral analyses are partly in conflict with current evolutionary models. The spectral analysis of Wolf-Rayet stars requires the detailed modeling of expanding stellar atmospheres in non-LTE. The Galactic WN stars have been comprehensively analyzed with such models of the latest stage of sophistication, while a similarly comprehensive study of the Galactic WC sample remains undone.
 Aims. We aim to establish the stellar parameters and mass-loss rates of the Galactic WC stars. These data provide the empirical basis of studies of (i) the role of WC stars in the evolution of massive stars, (ii) the wind-driving mechanisms, and (iii) the feedback of WC stars as input to models of the chemical and dynamical evolution of galaxies.
 Methods. We analyze the nearly complete sample of un-obscured Galactic WC stars, using optical spectra as well as ultraviolet spectra when available. The observations are fitted with theoretical spectra, using the Potsdam Wolf-Rayet (PoWR) model atmosphere code. A large grid of line-blanked models has been established for the range of WC subtypes WC4 - WC8, and smaller grids for the WC9 parameter domain. Both WO stars and WN/WC transit types are also analyzed using special models.
 Results. Stellar and atmospheric parameters are derived for more than 50 Galactic WC and two WO stars, covering almost the whole Galactic WC population as far as the stars are single, and un-obscured in the visual. In the Hertzsprung-Russell diagram, the WC stars reside between the hydrogen and the helium zero-age main sequences, having luminosities L from 10(4.9) to 10(5.6) L-circle dot. The mass-loss rates scale very tightly with L-0.8. The two WO stars in our sample turn out to be outstandingly hot (approximate to 200 kK) and do not fit into the WC scheme.
 Conclusions. By comparing the empirical WC positions in the Hertzsprung-Russell diagram with evolutionary models, and from recent supernova statistics, we conclude that WC stars have evolved from initial masses between 20 solar masses and 45 M-circle dot. In contrast to previous assumptions, it seems that WC stars in general do not descend from the most massive stars. Only the WO stars might stem from progenitors that have been initially more massive than 45 M-circle dot.
KW  - stars: massive
KW  - stars: mass-loss
KW  - stars: Wolf-Rayet
KW  - stars: evolution
KW  - stars: atmospheres
KW  - stars: winds, outflows
Y1  - 2012
U6  - https://doi.org/10.1051/0004-6361/201117830
SN  - 0004-6361
VL  - 540
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Löbling, Lisa
A1  - Rauch, Thomas
A1  - Bertolami Miller, Marcelo Miguel
A1  - Todt, Helge Tobias
A1  - Friederich, F.
A1  - Ziegler, M.
A1  - Werner, Klaus
A1  - Kruk, J. W.
T1  - Spectral analysis of the hybrid PG 1159-type central stars of the planetary nebulae Abell 43 and NGC7094
JF  - Monthly notices of the Royal Astronomical Society
N2  - 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.
KW  - stars: abundances
KW  - stars: AGB and post-AGB
KW  - stars: atmospheres
KW  - stars: evolution
KW  - stars: individual: WD1751+106
KW  - stars: individual: WD2134+125
Y1  - 2019
U6  - https://doi.org/10.1093/mnras/stz1994
SN  - 0035-8711
SN  - 1365-2966
VL  - 489
IS  - 1
SP  - 1054
EP  - 1071
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - Sander, Andreas Alexander Christoph
A1  - Hamann, Wolf-Rainer
A1  - Todt, Helge Tobias
A1  - Hainich, Rainer
A1  - Shenar, Tomer
T1  - Coupling hydrodynamics with comoving frame radiative transfer I. A unified approach for OB and WR stars
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. For more than two decades, stellar atmosphere codes have been used to derive the stellar and wind parameters of massive stars. Although they have become a powerful tool and sufficiently reproduce the observed spectral appearance, they can hardly be used for more than measuring parameters. One major obstacle is their inconsistency between the calculated radiation field and the wind stratification due to the usage of prescribed mass-loss rates and wind-velocity fields. Aims. We present the concepts for a new generation of hydrodynamically consistent non-local thermodynamical equilibrium (nonLTE) stellar atmosphere models that allow for detailed studies of radiation-driven stellar winds. As a first demonstration, this new kind of model is applied to a massive O star. Methods. Based on earlier works, the PoWR code has been extended with the option to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer in order to obtain a hydrodynamically consistent atmosphere stratification. In these models, the whole velocity field is iteratively updated together with an adjustment of the mass-loss rate. Results. The concepts for obtaining hydrodynamically consistent models using a comoving-frame radiative transfer are outlined. To provide a useful benchmark, we present a demonstration model, which was motivated to describe the well-studied O4 supergiant zeta Pup. The obtained stellar and wind parameters are within the current range of literature values. Conclusions. For the first time, the PoWR code has been used to obtain a hydrodynamically consistent model for a massive O star. This has been achieved by a profound revision of earlier concepts used for Wolf-Rayet stars. The velocity field is shaped by various elements contributing to the radiative acceleration, especially in the outer wind. The results further indicate that for more dense winds deviations from a standard beta-law occur.
KW  - stars: mass-loss
KW  - stars: winds, outflows
KW  - stars: early-type
KW  - stars: atmospheres
KW  - stars: fundamental parameters
KW  - stars: massive
Y1  - 2017
U6  - https://doi.org/10.1051/0004-6361/201730642
SN  - 1432-0746
VL  - 603
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Sander, Andreas Alexander Christoph
A1  - Shenar, Tomer
A1  - Hainich, Rainer
A1  - Gimenez-Garcia, Ana
A1  - Todt, Helge Tobias
A1  - Hamann, Wolf-Rainer
T1  - On the consistent treatment of the quasi-hydrostatic layers in hot star atmospheres
JF  - Astronomy and astrophysics : an international weekly journal
N2  - Context. Spectroscopic analysis remains the most common method to derive masses of massive stars, the most fundamental stellar parameter. While binary orbits and stellar pulsations can provide much sharper constraints on the stellar mass, these methods are only rarely applicable to massive stars. Unfortunately, spectroscopic masses of massive stars heavily depend on the detailed physics of model atmospheres.
 Aims. We demonstrate the impact of a consistent treatment of the radiative pressure on inferred gravities and spectroscopic masses of massive stars. Specifically, we investigate the contribution of line and continuum transitions to the photospheric radiative pressure. We further explore the effect of model parameters, e.g., abundances, on the deduced spectroscopic mass. Lastly, we compare our results with the plane-parallel TLUSTY code, commonly used for the analysis of massive stars with photospheric spectra.
 Methods. We calculate a small set of O-star models with the Potsdam Wolf-Rayet (PoWR) code using different approaches for the quasi-hydrostatic part. These models allow us to quantify the effect of accounting for the radiative pressure consistently. We further use PoWR models to show how the Doppler widths of line profiles and abundances of elements such as iron affect the radiative pressure, and, as a consequence, the derived spectroscopic masses.
 Results. Our study implies that errors on the order of a factor of two in the inferred spectroscopic mass are to be expected when neglecting the contribution of line and continuum transitions to the radiative acceleration in the photosphere. Usage of implausible microturbulent velocities, or the neglect of important opacity sources such as Fe, may result in errors of approximately 50% in the spectroscopic mass. A comparison with TLUSTY model atmospheres reveals a very good agreement with PoWR at the limit of low mass-loss rates.
KW  - stars: early-type
KW  - stars: mass-loss
KW  - stars: winds, outflows
KW  - stars: atmospheres
KW  - stars: fundamental parameters
KW  - stars: massive
Y1  - 2015
U6  - https://doi.org/10.1051/0004-6361/201425356
SN  - 0004-6361
SN  - 1432-0746
VL  - 577
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Todt, Helge Tobias
A1  - Sander, Angelika
A1  - Hainich, Rainer
A1  - Hamann, Wolf-Rainer
A1  - Quade, Markus
A1  - Shenar, Tomer
T1  - Potsdam Wolf-Rayet model atmosphere grids for WN stars
JF  - Astronomy and astrophysics : an international weekly journal
N2  - We present new grids of Potsdam Wolf-Rayet (PoWR) model atmospheres for Wolf-Rayet stars of the nitrogen sequence (WN stars). The models have been calculated with the latest version of the PoWR stellar atmosphere code for spherical stellar winds. The WN model atmospheres include the non-LTE solutions of the statistical equations for complex model atoms, as well as the radiative transfer equation in the co-moving frame. Iron-line blanketing is treated with the help of the superlevel approach, while wind inhomogeneities are taken into account via optically thin clumps. Three of our model grids are appropriate for Galactic metallicity. The hydrogen mass fraction of these grids is 50%, 20%, and 0%, thus also covering the hydrogen-rich late-type WR stars that have been discovered in recent years. Three grids are adequate for LMC WN stars and have hydrogen fractions of 40%, 20%, and 0%. Recently, additional grids with SMC metallicity and with 60%, 40%, 20%, and 0% hydrogen have been added. We provide contour plots of the equivalent widths of spectral lines that are usually used for classification and diagnostics.
KW  - stars: evolution
KW  - stars: mass-loss
KW  - stars: winds, outflows
KW  - stars: Wolf-Rayet
KW  - stars: atmospheres
KW  - stars: massive
Y1  - 2015
U6  - https://doi.org/10.1051/0004-6361/201526253
SN  - 0004-6361
SN  - 1432-0746
VL  - 579
PB  - EDP Sciences
CY  - Les Ulis
ER  - 
TY  - JOUR
A1  - Reindl, Nicole
A1  - Rauch, Thomas
A1  - Miller Bertolami, Marcelo Miguel
A1  - Todt, Helge Tobias
A1  - Werner, K.
T1  - Breaking news from the HST
BT  - the central star of the Stingray Nebula is now returning towards the AGB
JF  - Monthly notices of the Royal Astronomical Society
N2  - SAO 244567 is a rare example of a star that allows us to witness stellar evolution in real time. Between 1971 and 1990, it changed from a B-type star into the hot central star of the Stingray Nebula. This observed rapid heating has been a mystery for decades, since it is in strong contradiction with the low mass of the star and canonical post-asymptotic giant branch (AGB) evolution. We speculated that SAO 244567 might have suffered from a late thermal pulse (LTP) and obtained new observations with Hubble Space Telescope (HST)/COS to follow the evolution of the surface properties of SAO 244567 and to verify the LTP hypothesis. Our non-LTE spectral analysis reveals that the star cooled significantly since 2002 and that its envelope is now expanding. Therefore, we conclude that SAO 244567 is currently on its way back towards the AGB, which strongly supports the LTP hypothesis. A comparison with state-of-the-art LTP evolutionary calculations shows that these models cannot fully reproduce the evolution of all surface parameters simultaneously, pointing out possible shortcomings of stellar evolution models. Thereby, SAO 244567 keeps on challenging stellar evolution theory and we highly encourage further investigations.
KW  - stars: AGB and post-AGB
KW  - stars: atmospheres
KW  - stars: evolution
Y1  - 2016
U6  - https://doi.org/10.1093/mnrasl/slw175
SN  - 0035-8711
SN  - 1365-2966
VL  - 464
SP  - L51
EP  - L55
PB  - Oxford Univ. Press
CY  - Oxford
ER  -