TY - JOUR A1 - Bozzo, Enrico A1 - Oskinova, Lida A1 - Feldmeier, Achim A1 - Falanga, M. T1 - Clumpy wind accretion in supergiant neutron star high mass X-ray binaries JF - BMC neuroscience N2 - 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. KW - stars: neutron KW - X-rays: binaries KW - supergiants Y1 - 2016 U6 - https://doi.org/10.1051/0004-6361/201628341 SN - 1432-0746 VL - 589 SP - 369 EP - 389 PB - EDP Sciences CY - Les Ulis ER - TY - BOOK A1 - Feldmeier, Achim T1 - Theoretical Fluid Dynamics T3 - Theoretical and Mathematical Physics Y1 - 2019 SN - 978-3-030-31021-9 (online) SN - 978-3-030-31023-3 (print) U6 - https://doi.org/10.1007/978-3-030-31022-6 PB - Springer CY - Cham ER - TY - THES A1 - Feldmeier, Achim T1 - Hydrodynamics of astrophysical winds driven by scattering in spectral lines N2 - Liniengetriebene Winde werden durch Impulsübertrag von Photonen auf ein Plasma bei Absorption oder Streuung in zahlreichen Spektrallinien beschleunigt. Dieser Prozess ist besonders effizient für ultraviolette Strahlung und Plasmatemperaturen zwischen 10^4 K und 10^5 K. Zu den astronomischen Objekten mit liniengetriebenen Winden gehören Sterne der Spektraltypen O, B und A, Wolf-Rayet-Sterne sowie Akkretionsscheiben verschiedenster Größenordnung, von Scheiben um junge Sterne und in kataklysmischen Veränderlichen bis zu Quasarscheiben. Es ist bislang nicht möglich, das vollständige Windproblem numerisch zu lösen, also die Hydrodynamik, den Strahlungstransport und das statistische Gleichgewicht dieser Strömungen gleichzeitig zu behandeln. Die Betonung liegt in dieser Arbeit auf der Windhydrodynamik, mit starken Vereinfachungen in den beiden anderen Gebieten. Wegen persönlicher Beteiligung betrachte ich drei Themen im Detail. 1. Windinstabilität durch Dopplerde-shadowing des Gases. Die Instabilität 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 Röntgenflashes, die die beobachtete Röntgenstrahlung heißer Sterne erklären können. 2. Wind runway durch radiative Wellen. Der runaway zeigt, warum beobachtete liniengetriebene Winde schnelle, kritische Lösungen anstelle von Brisenlösungen (oder shallow solutions) annehmen. Unter bestimmten Bedingungen stabilisiert der Wind sich auf masseüberladenen Lösungen, 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 unterstützt. Ein Wirbelblatt, das am inneren Scheibenrand beginnt, kann zu stark erhöhten Massenverlustraten führen. N2 - Line driven winds are accelerated by the momentum transfer from photons to a plasma, by absorption and scattering in numerous spectral lines. Line driving is most efficient for ultraviolet radiation, and at plasma temperatures from 10^4 K to 10^5 K. Astronomical objects which show line driven winds include stars of spectral type O, B, and A, Wolf-Rayet stars, and accretion disks over a wide range of scales, from disks in young stellar objects and cataclysmic variables to quasar disks. It is not yet possible to solve the full wind problem numerically, and treat the combined hydrodynamics, radiative transfer, and statistical equilibrium of these flows. The emphasis in the present writing is on wind hydrodynamics, with severe simplifications in the other two areas. I consider three topics in some detail, for reasons of personal involvement. 1. Wind instability, as caused by Doppler de-shadowing of gas parcels. The instability causes the wind gas to be compressed into dense shells enclosed by strong shocks. Fast clouds occur in the space between shells, and collide with the latter. This leads to X-ray flashes which may explain the observed X-ray emission from hot stars. 2. Wind runaway, as caused by a new type of radiative waves. The runaway may explain why observed line driven winds adopt fast, critical solutions instead of shallow (or breeze) solutions. Under certain conditions the wind settles on overloaded solutions, which show a broad deceleration region and kinks in their velocity law. 3. Magnetized winds, as launched from accretion disks around stars or in active galactic nuclei. Line driving is assisted by centrifugal forces along co-rotating poloidal magnetic field lines, and by Lorentz forces due to toroidal field gradients. A vortex sheet starting at the inner disk rim can lead to highly enhanced mass loss rates. KW - Hydrodynamik KW - Strahlungstransport KW - Sternwinde KW - Akkretionsscheiben KW - hydrodynamics KW - radiative transfer KW - stellar winds KW - accretion disks Y1 - 2001 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0000388 ER - TY - CHAP A1 - Feldmeier, Achim A1 - Hamann, Wolf-Rainer A1 - Rätzel, D. A1 - Oskinova, Lida T1 - Hydrodynamic simulations of clumps N2 - 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. Y1 - 2007 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-17975 ER - TY - JOUR A1 - Feldmeier, Achim A1 - Nikutta, Robert T1 - Nonlocal radiative coupling in non monotonic stellar winds (Research note) N2 - There is strong observational evidence of shocks and clumping in radiation-driven stellar winds from hot, luminous stars. The resulting non nous monotonic velocity law allows for radiative coupling between distant locations, which is so far not accounted for in hydrodynamic wind simulations. In the present paper, we determine the Sobolev source function and radiative line force in the presence of radiative coupling in spherically symmetric flows, extending the geometry-free formalism of Rybicki & Hummer (1978, ApJ, 219, 654) to the case of three-point coupling, which can result from, e. g., corotating interaction regions, wind shocks, or mass overloading. For a simple model of an overloaded wind, we find that, surprisingly, the flow decelerates at all radii above a certain height when nonlocal radiative coupling is accounted for. We discuss whether radiation-driven winds might in general not be able to re- accelerate after a non monotonicity has occurred in the velocity law Y1 - 2006 UR - http://www.aanda.org/ U6 - https://doi.org/10.1051/0004-6361:20053380 SN - 0004-6361 ER - TY - JOUR A1 - Feldmeier, Achim A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer T1 - X-ray line emission from a fragmented stellar wind N2 - We discuss X-ray line formation in dense O star winds. A random distribution of wind shocks is assumed to emit X-rays that are partially absorbed by cooler wind gas. The cool gas resides in highly compressed fragments oriented perpendicular to the radial flow direction. For fully opaque fragments, we find that the blueshifted part of X-ray line profiles remains flat-topped even after severe wind attenuation, whereas the red part shows a steep decline. These box- type, blueshifted profiles resemble recent Chandra observations of the O3 star zeta Pup. For partially transparent fragments, the emission lines become similar to those from a homogeneous wind. Y1 - 2003 ER - TY - JOUR A1 - Feldmeier, Achim A1 - Oskinova, Lida A1 - Hamann, Wolf-Rainer A1 - Owocki, S. P. T1 - Overloaded and fractured winds N2 - We discuss the connection between wind overloading and discrete absorption components in P Cygni line profiles from O stars. Overloading can create horizontal plateaus in the radial wind speed that cause the extra absorption in the line profile. The upstream propagation speed of these velocity plateaus is analyzed. The second part of the paper deals with X-ray emission from O stars. X-ray line profiles observed with Chandra and XMM are often symmetric, contrary to what is expected for lines from a homogeneous wind. We discuss the influence on line symmetry of photon escape channels in a strongly clumped wind. Y1 - 2003 SN - 1-58381-133-8 ER - TY - JOUR A1 - Feldmeier, Achim A1 - Shlosman, Isak T1 - Abbott wave-triggered runaway in line driven winds from stars and accretion disks N2 - 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. Y1 - 2002 ER - TY - JOUR A1 - Feldmeier, Achim A1 - Shlosman, Isak A1 - Hamann, Wolf-Rainer T1 - Runaway acceleration of line-driven winds : the role of the outer boundary N2 - Observations and theory suggest that line driven winds from hot stars and luminous accretion disks adopt a unique, critical solution which corresponds to maximum mass loss rate. We analyze the numerical stability of the infinite family of shallow wind solutions, which resemble solar wind breezes, and their transition to the critical wind. Shallow solutions are sub-critical with respect to radiative (or Abbott) waves. These waves can propagate upstream through shallow winds at high speeds. If the waves are not accounted for in the Courant time step, numerical runaway results. The outer boundary condition is equally important for wind stability. Assuming pure outflow conditions, as is done in the literature, triggers runaway of shallow winds to the critical solution or to accretion flow. Y1 - 2002 ER - TY - JOUR A1 - Hamann, Wolf-Rainer A1 - Brown, John C. A1 - Feldmeier, Achim A1 - Oskinova, Lida T1 - On the wavelength drift of spectral features from structured hot star winds N2 - Spectral lines formed in stellar winds from OB stars are observed to exhibit profile variations. Discrete Absorption Components (DACs) show a remarkably slow wavelength drift with time. In a straightforward interpretation, this is in sharp contradiction to the steep velocity law predicted by the radiation-driven wind theory, and by semi- empirical profile fitting. In the present paper we re-discuss the interpretation of the drift rate. We show that the Co- rotating Interaction Region (CIR) model for the formation of DACs does not explain their slow drift rate as a consequence of rotation. On the contrary, the apparent acceleration of a spectral CIR feature is even higher than for the corresponding kinematical model without rotation. However, the observations can be understood by distinguishing between the velocity field of the matter flow, and the velocity law for the motion of the patterns in which the DAC features are formed. If the latter propagate upstream against the matter flow, the resulting wavelength drift mimics a much slower acceleration although the matter is moving fast. Additional to the DACs, a second type of recurrent structures is present in observed OB star spectra, the so-called modulations. In contrast to the DACs, these structures show a steep acceleration compatible with the theoretically predicted velocity law. We see only two possible consistent scenarios. Either, the wind is accelerated fast, and the modulations are formed in advected structures, while the DACs come from structures which are propagating upstream. Or, alternatively, steep and shallow velocity laws may co-exist at the same time in different spatial regions or directions of the wind. Y1 - 2001 ER -