@article{FeldmeierShlosman2002,
  author    = {Feldmeier, Achim and Shlosman, Isak},
  title     = {Abbott wave-triggered runaway in line driven winds from stars and accretion disks},
  year      = {2002},
  abstract  = {Line driven winds from stars and accretion disks are accelerated by scattering in numerous line transitions. The wind is believed to adopt a unique critical solution, out of the infinite variety of shallow and steep solutions. We study the inherent dynamics of the transition towards the critical wind. A new runaway wind mechanism is analyzed in terms of radiative-acoustic (Abbott) waves which are responsible for shaping the wind velocity law and fixing the mass loss. Three different flow types result, depending on the location of perturbations. First, if the shallow solution is perturbed sufficiently far downstream, a single critical point forms in the flow, which is a barrier for Abbott waves, and the solution tends to the critical one. Second, if the shallow solution is perturbed upstream from this critical point, mass overloading results, and the critical point is shifted inwards. This wind exhibits a broad, stationary region of decelerating flow and its velocity law has kinks. Third, for perturbations even further upstream, the overloaded wind becomes time-dependent, and develops shocks and dense shells.},
  language  = {en}
}
@article{OskinovaFeldmeierKretschmar2012,
  author    = {Oskinova, Lida and Feldmeier, Achim and Kretschmar, Peter},
  title     = {Clumped stellar winds in supergiant high-mass X-ray binaries: X-ray variability and photoionization},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {421},
  journal   = {Monthly notices of the Royal Astronomical Society},
  number    = {4},
  publisher = {Wiley-Blackwell},
  address   = {Malden},
  issn      = {0035-8711},
  doi       = {10.1111/j.1365-2966.2012.20507.x},
  pages     = {2820 -- 2831},
  year      = {2012},
  abstract  = {The clumping of massive star winds is an established paradigm, which is confirmed by multiple lines of evidence and is supported by stellar wind theory. The purpose of this paper is to bridge the gap between detailed models of inhomogeneous stellar winds in single stars and the phenomenological description of donor winds in supergiant high-mass X-ray binaries (HMXBs). We use the results from time-dependent hydrodynamical models of the instability in the line-driven wind of a massive supergiant star to derive the time-dependent accretion rate on to a compact object in the BondiHoyleLyttleton approximation. The strong density and velocity fluctuations in the wind result in strong variability of the synthetic X-ray light curves. The model predicts a large-scale X-ray variability, up to eight orders of magnitude, on relatively short time-scales. The apparent lack of evidence for such strong variability in the observed HMXBs indicates that the details of the accretion process act to reduce the variability resulting from the stellar wind velocity and density jumps.},
  language  = {en}
}
@article{BozzoOskinovaFeldmeieretal.2016,
  author    = {Bozzo, Enrico and Oskinova, Lida and Feldmeier, Achim and Falanga, M.},
  title     = {Clumpy wind accretion in supergiant neutron star high mass X-ray binaries},
  series = {BMC neuroscience},
  volume    = {589},
  journal   = {BMC neuroscience},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201628341},
  pages     = {369 -- 389},
  year      = {2016},
  abstract  = {The accretion of the stellar wind material by a compact object represents the main mechanism powering the X-ray emission in classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. In this work we present the first attempt to simulate the accretion process of a fast and dense massive star wind onto a neutron star, taking into account the effects of the centrifugal and magnetic inhibition of accretion ("gating") due to the spin and magnetic field of the compact object. We made use of a radiative hydrodynamical code to model the nonstationary radiatively driven wind of an O-B supergiant star and then place a neutron star characterized by a fixed magnetic field and spin period at a certain distance from the massive companion. Our calculations follow, as a function of time (on a total timescale of several hours), the transitions of the system through all different accretion regimes that are triggered by the intrinsic variations in the density and velocity of the nonstationary wind. The X-ray luminosity released by the system is computed at each time step by taking into account the relevant physical processes occurring in the different accretion regimes. Synthetic lightcurves are derived and qualitatively compared with those observed from classical supergiant high mass X-ray binaries and supergiant fast X-ray transients. Although a number of simplifications are assumed in these calculations, we show that taking into account the effects of the centrifugal and magnetic inhibition of accretion significantly reduces the average X-ray luminosity expected for any neutron star wind-fed binary. The present model calculations suggest that long spin periods and stronger magnetic fields are favored in order to reproduce the peculiar behavior of supergiant fast X-ray transients in the X-ray domain.},
  language  = {en}
}
@article{OskinovaHamannFeldmeieretal.2009,
  author    = {Oskinova, Lida and Hamann, Wolf-Rainer and Feldmeier, Achim and Ignace, Richard and Chu, You-Hua},
  title     = {Discovery of X-ray emission from the Wolf-Rayet star WR 142 of oxygen subtype},
  issn      = {0004-637X},
  doi       = {10.1088/0004-637x/693/1/L44},
  year      = {2009},
  abstract  = {We report the discovery of weak yet hard X-ray emission from the Wolf-Rayet (WR) star WR 142 with the XMM- Newton X-ray telescope. Being of spectral subtype WO2, WR 142 is a massive star in a very advanced evolutionary stage shortly before its explosion as a supernova or. gamma-ray burst. This is the first detection of X-ray emission from a WO- type star. We rule out any serendipitous X-ray sources within approximate to 1 '' of WR 142. WR 142 has an X- ray luminosity of L-X approximate to 7 x 10(30) erg s(-1), which constitutes only less than or similar to 10(-8) of its bolometric luminosity. The hard X-ray spectrum suggests a plasma temperature of about 100 MK. Commonly, X-ray emission from stellar winds is attributed to embedded shocks due to the intrinsic instability of the radiation driving. From qualitative considerations we conclude that this mechanism cannot account for the hardness of the observed radiation. There are no hints for a binary companion. Therefore the only remaining, albeit speculative explanation must refer to magnetic activity. Possibly related, WR 142 seems to rotate extremely fast, as indicated by the unusually round profiles of its optical emission lines. Our detection implies that the wind of WR 142 must be relatively transparent to X-rays, which can be due to strong wind ionization, wind clumping, or nonspherical geometry from rapid rotation.},
  language  = {en}
}
@article{OskinovaFeldmeierHamann2006,
  author    = {Oskinova, Lida and Feldmeier, Achim and Hamann, Wolf-Rainer},
  title     = {High-resolution X-ray spectroscopy of bright O-type stars},
  series = {Monthly notices of the Royal Astronomical Society},
  volume    = {372},
  journal   = {Monthly notices of the Royal Astronomical Society},
  publisher = {Oxford University Press},
  address   = {Oxford},
  issn      = {0035-8711},
  doi       = {10.1111/j.1365-2966.2006.10858.x},
  pages     = {313 -- 326},
  year      = {2006},
  abstract  = {Archival X-ray spectra of the four prominent single, non-magnetic O stars zeta Pup, zeta Ori, xi Per and zeta Oph, obtained in high resolution with Chandra HETGS/MEG have been studied. The resolved X-ray emission line profiles provide information about the shocked, hot gas which emits the X-radiation, and about the bulk of comparably cool stellar wind material which partly absorbs this radiation. In this paper, we synthesize X-ray line profiles with a model of a clumpy stellar wind. We find that the geometrical shape of the wind inhomogeneities is important: better agreement with the observations can be achieved with radially compressed clumps than with spherical clumps. The parameters of the model, i.e. chemical abundances, stellar radius, mass-loss rate and terminal wind velocity, are taken from existing analyses of UV and optical spectra of the programme stars. On this basis, we also calculate the continuum-absorption coefficient of the cool-wind material, using the Potsdam Wolf-Rayet (POWR) model atmosphere code. The radial location of X-ray emitting gas is restricted from analysing the FIR line ratios of helium-like ions. The only remaining free parameter of our model is the typical distance between the clumps; here, we assume that at any point in the wind there is one clump passing by per one dynamical time-scale of the wind. The total emission in a model line is scaled to the observation. There is a good agreement between synthetic and observed line profiles. We conclude that the X-ray emission line profiles in O stars can be explained by hot plasma embedded in a cool wind which is highly clumped in the form of radially compressed shell fragments.},
  language  = {en}
}
@inproceedings{FeldmeierHamannRaetzeletal.2007,
  author    = {Feldmeier, Achim and Hamann, Wolf-Rainer and R{\"a}tzel, D. and Oskinova, Lida},
  title     = {Hydrodynamic simulations of clumps},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-17975},
  year      = {2007},
  abstract  = {Clumps in hot star winds can originate from shock compression due to the line driven instability. One-dimensional hydrodynamic simulations reveal a radial wind structure consisting of highly compressed shells separated by voids, and colliding with fast clouds. Two-dimensional simulations are still largely missing, despite first attempts. Clumpiness dramatically affects the radiative transfer and thus all wind diagnostics in the UV, optical, and in X-rays. The microturbulence approximation applied hitherto is currently superseded by a more sophisticated radiative transfer in stochastic media. Besides clumps, i.e. jumps in the density stratification, so-called kinks in the velocity law, i.e. jumps in dv/dr, play an eminent role in hot star winds. Kinks are a new type of radiative-acoustic shock, and propagate at super-Abbottic speed.},
  language  = {en}
}
@phdthesis{Feldmeier2001,
  author    = {Feldmeier, Achim},
  title     = {Hydrodynamics of astrophysical winds driven by scattering in spectral lines},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0000388},
  school      = {Universit{\"a}t Potsdam},
  year      = {2001},
  abstract  = {Liniengetriebene Winde werden durch Impuls{\"u}bertrag von Photonen auf ein Plasma bei Absorption oder Streuung in zahlreichen Spektrallinien beschleunigt. Dieser Prozess ist besonders effizient f{\"u}r ultraviolette Strahlung und Plasmatemperaturen zwischen 10^4 K und 10^5 K. Zu den astronomischen Objekten mit liniengetriebenen Winden geh{\"o}ren Sterne der Spektraltypen O, B und A, Wolf-Rayet-Sterne sowie Akkretionsscheiben verschiedenster Gr{\"o}ßenordnung, von Scheiben um junge Sterne und in kataklysmischen Ver{\"a}nderlichen bis zu Quasarscheiben. Es ist bislang nicht m{\"o}glich, das vollst{\"a}ndige Windproblem numerisch zu l{\"o}sen, also die Hydrodynamik, den Strahlungstransport und das statistische Gleichgewicht dieser Str{\"o}mungen gleichzeitig zu behandeln. Die Betonung liegt in dieser Arbeit auf der Windhydrodynamik, mit starken Vereinfachungen in den beiden anderen Gebieten. Wegen pers{\"o}nlicher Beteiligung betrachte ich drei Themen im Detail. 1. Windinstabilit{\"a}t durch Dopplerde-shadowing des Gases. Die Instabilit{\"a}t bewirkt, dass Windgas in dichte Schalen komprimiert wird, die von starken Stoßfronten begrenzt sind. Schnelle Wolken entstehen im Raum zwischen den Schalen und stoßen mit diesen zusammen. Dies erzeugt R{\"o}ntgenflashes, die die beobachtete R{\"o}ntgenstrahlung heißer Sterne erkl{\"a}ren k{\"o}nnen. 2. Wind runway durch radiative Wellen. Der runaway zeigt, warum beobachtete liniengetriebene Winde schnelle, kritische L{\"o}sungen anstelle von Brisenl{\"o}sungen (oder shallow solutions) annehmen. Unter bestimmten Bedingungen stabilisiert der Wind sich auf masse{\"u}berladenen L{\"o}sungen, mit einem breiten, abbremsenden Bereich und Knicken im Geschwindigkeitsfeld. 3. Magnetische Winde von Akkretionsscheiben um Sterne oder in aktiven Galaxienzentren. Die Linienbeschleunigung wird hier durch die Zentrifugalkraft entlang korotierender poloidaler Magnetfelder und die Lorentzkraft aufgrund von Gradienten im toroidalen Feld unterst{\"u}tzt. Ein Wirbelblatt, das am inneren Scheibenrand beginnt, kann zu stark erh{\"o}hten Massenverlustraten f{\"u}hren.},
  language  = {en}
}
@article{KrtickaFeldmeier2018,
  author    = {Krticka, Jiri and Feldmeier, Achim},
  title     = {Light variations due to the line-driven wind instability and wind blanketing in O stars},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {617},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {1432-0746},
  doi       = {10.1051/0004-6361/201731614},
  pages     = {7},
  year      = {2018},
  abstract  = {A small fraction of the radiative flux emitted by hot stars is absorbed by their winds and redistributed towards longer wavelengths. This effect, which leads also to the heating of the stellar photosphere, is termed wind blanketing. For stars with variable winds, the effect of wind blanketing may lead to the photometric variability. We have studied the consequences of line driven wind instability and wind blanketing for the light variability of O stars. We combined the results of wind hydrodynamic simulations and of global wind models to predict the light variability of hot stars due to the wind blanketing and instability. The wind instability causes stochastic light variability with amplitude of the order of tens of millimagnitudes and a typical timescale of the order of hours for spatially coherent wind structure. The amplitude is of the order of millimagnitudes when assuming that the wind consists of large number of independent concentric cones. The variability with such amplitude is observable using present space borne photometers. We show that the simulated light curve is similar to the light curves of O stars obtained using BRITE and CoRoT satellites.},
  language  = {en}
}
@misc{KurfuerstFeldmeierKrtička2017,
  author    = {Kurf{\"u}rst, P. and Feldmeier, Achim and Krtička, Jiri},
  title     = {Modeling sgB[e] Circumstellar Disks},
  series = {The B(e) Phenomenon: Forty Years of Studies : proceedings of a conference held at Charles University, Prague, Czech Republic, 27 June-1 July 2016},
  volume    = {508},
  journal   = {The B(e) Phenomenon: Forty Years of Studies : proceedings of a conference held at Charles University, Prague, Czech Republic, 27 June-1 July 2016},
  publisher = {Astronomical Scoeity of the Pacific},
  address   = {San Fransisco},
  isbn      = {978-1-58381-900-5},
  pages     = {17 -- 22},
  year      = {2017},
  abstract  = {During their evolution, massive stars are characterized by a significant loss of mass either via spherically symmetric stellar winds or by aspherical mass-loss mechanisms, namely outflowing equatorial disks. However, the scenario that leads to the formation of a disk or rings of gas and dust around these objects is still under debate. Is it a viscous disk or an ouftlowing disk-forming wind or some other mechanism? It is also unclear how various physical mechanisms that act on the circumstellar environment of the stars affect its shape, density, kinematic, and thermal structure. We assume that the disk-forming mechanism is a viscous transport within an equatorial outflowing disk of a rapidly or even critically rotating star. We study the hydrodynamic and thermal structure of optically thick dense parts of outflowing circumstellar disks that may form around,e.g., Be stars, sgB[e] stars, or Pop m stars. We calculate self-consistent time dependent models of the inner dense region of the disk that is strongly affected either by irradiation from the central star and by contributions of viscous heating effects. We also simulate the dynamic effects of collision between expanding ejecta of supernovae and circumstellar disks that may be form in sgB[e] stars and, e.g., LBVs or Pop in stars.},
  language  = {en}
}
@article{KustererNagelHartmannetal.2014,
  author    = {Kusterer, D. -J. and Nagel, T. and Hartmann, S. and Werner, K. and Feldmeier, Achim},
  title     = {Monte Carlo radiation transfer in CV disk winds: application to the AM CVn prototype},
  series = {Astronomy and astrophysics : an international weekly journal},
  volume    = {561},
  journal   = {Astronomy and astrophysics : an international weekly journal},
  publisher = {EDP Sciences},
  address   = {Les Ulis},
  issn      = {0004-6361},
  doi       = {10.1051/0004-6361/201321438},
  pages     = {10},
  year      = {2014},
  abstract  = {Context. AMCVn systems are ultracompact binaries in which a (semi-) degenerate star transfers helium-dominated matter onto a white dwarf. They are effective gravitational-wave emitters and potential progenitors of Type Ia supernovae. Aims. To understand the evolution of AMCVn systems it is necessary to determine their mass-loss rate through their radiation-driven accretion-disk wind. We constructed models to perform quantitative spectroscopy of P Cygni line profiles that were detected in UV spectra. Methods. We performed 2.5D Monte Carlo radiative transfer calculations in hydrodynamic wind structures by making use of realistic NLTE spectra from the accretion disk and by accounting for the white dwarf as an additional photon source. Results. We present first results from calculations in which LTE opacities are used in the wind model. A comparison with UV spectroscopy of the AMCVn prototype shows that the modeling procedure is potentially a good tool for determining mass-loss rates and abundances of trace metals in the helium-rich wind.},
  language  = {en}
}