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 - 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 - 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 - 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 - 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 - Oskinova, Lida A1 - Feldmeier, Achim A1 - Hamann, Wolf-Rainer T1 - X-ray line profiles from structured stellar winds N2 - Absorbing material compressed in a number of thin shells is effectively less opaque for X-rays than smoothly distributed gas. The calculated X-ray emission line profiles are red-shifted if the emission arises from the starward side of the shells. Y1 - 2003 SN - 1-58381-133-8 ER - TY - JOUR A1 - Oskinova, Lida A1 - Feldmeier, Achim A1 - Hamann, Wolf-Rainer T1 - X-ray emission lines from inhomogeneous stellar winds N2 - It is commonly adopted that X-rays from O stars are produced deep inside the stellar wind, and transported outwards through the bulk of the expanding matter which attenuates the radiation and affects the shape of emission line profiles. The ability of the X-ray observatories Chandra and XMM-Newton to resolve these lines spectroscopically provided a stringent test for the theory of the X-ray production. It turned out that none of the existing models was able to fit the observations consistently. The possible caveat of these models was the underlying assumption of a smooth stellar wind. Motivated by the evidence that the stellar winds are in fact structured, we present a 2-D numerical model of a stochastic, inhomogeneous wind. Small parcels of hot, X-ray emitting gas are permeated by cool, absorbing wind material which is compressed into thin shell fragments. Wind fragmentation alters the radiative transfer drastically, compared to homogeneous models of the same mass-loss rate. X-rays produced deep inside the wind, which would be totally absorbed in a homogeneous flow, can effectively escape from a fragmented wind. The wind absorption becomes wavelength independent if the individual fragments are optically thick. The X-ray line profiles are flat-topped in the blue part and decline steeply in the red part for the winds with a short acceleration zone. For the winds where the acceleration extends over significant distances, the lines can appear nearly symmetric and only slightly blueshifted, in contrast to the skewed, triangular line profiles typically obtained from homogeneous wind models of high optical depth. We show that profiles from a fragmented wind model can reproduce the observed line profiles from zeta Orionis. The present numerical modeling confirms the results from a previous study, where we derived analytical formulae from a statistical treatment Y1 - 2004 SN - 0004-6361 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 - Oskinova, Lida A1 - Feldmeier, Achim A1 - Hamann, Wolf-Rainer T1 - High-resolution X-ray spectroscopy of bright O-type stars JF - Monthly notices of the Royal Astronomical Society N2 - 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. KW - stars : individual : zeta Pup KW - stars : individual : zeta Ori KW - stars : individual : xi Per KW - stars : individual : zeta Oph KW - X-rays : stars Y1 - 2006 U6 - https://doi.org/10.1111/j.1365-2966.2006.10858.x SN - 0035-8711 VL - 372 SP - 313 EP - 326 PB - Oxford University Press CY - Oxford ER -