TY - JOUR A1 - Cassan, A. A1 - Beaulieu, Jean-Philippe A1 - Brillant, Stephane A1 - Coutures, C. A1 - Dominik, M. A1 - Donatowicz, J. A1 - Jorgensen, Uffe Grae A1 - Kubas, Daniel A1 - Albrow, Michael D. A1 - Caldwell, John A. R. A1 - Fouque, P. A1 - Greenhill, John A1 - Hill, K. A1 - Horne, Keith A1 - Kane, Stephen R. A1 - Martin, Ralph A1 - Menzies, J. W. A1 - Pollard, K. R. A1 - Sahu, K. C. A1 - Vinter, C. A1 - Wambsganss, Joachim A1 - Watson, R. A1 - Williams, A. A1 - Fendt, Christian A1 - Hauschildt, P. A1 - Heinmueller, Janine A1 - Marquette, Jean-Baptiste A1 - Thurl, C. T1 - Probing the atmosphere of the bulge G5III star OGLE-2002-BUL-069 by analysis of microlensed H alpha line N2 - We discuss high-resolution, time-resolved spectra of the caustic exit of the binary microlensing event OGLE 2002-BLG-069 obtained with UVES on the VLT. The source star is a G5III giant in the Galactic Bulge. During such events, the source star is highly magnified, and a strong differential magnification around the caustic resolves its surface. Using an appropriate model stellar atmosphere generated by the PHOENIX v2.6 code we obtain a model light curve for the caustic exit and compare it with a dense set of photometric observations obtained by the PLANET microlensing follow up network. We further compare predicted variations in the Halpha equivalent width with those measured from our spectra. While the model and observations agree in the gross features, there are discrepancies suggesting shortcomings in the model, particularly for the Halpha line core, where we have detected amplified emission from the stellar chromosphere after the source star's trailing limb exited the caustic. This achievement became possible by the provision of the very efficient OGLE-III Early Warning System, a network of small telescopes capable of nearly-continuous round-the-clock photometric monitoring, on-line data reduction, daily near-real-time modelling in order to predict caustic crossing parameters, and a fast and efficient response of a 8 m class telescope to a "Target-of-Opportunity" observation request Y1 - 2004 ER - TY - JOUR A1 - Cemeljic, Miljenko A1 - Fendt, Christian T1 - Protostellar jets and magnetic diffusion N2 - We investigate the evolution of a disk wind into a collimated jet under the influence of magnetic diffusivity. Using the ZEUS-3D code in the axisymmetry option we solve the time-dependent resistive MHD equations for a model setup of a central star surrounded by an accretion disk. The disk is taken as a time-independent boundary condition for the mass flow rate and the magnetic flux distribution. We find that the diffusive jets propagate slower into the ambient medium, most probably due to the lower mass flow rate in axial direction. Close to the star we find that a quasi stationary state evolves after several hundreds (weak diffusion) or thousands (strong diffusion) of disk rotations. Magnetic diffusivity affects the protostellar jet structure de-collimating it. We explain these effects in the framework of the Lorentz force. Y1 - 2003 UR - http://arxiv.org/pdf/astro-ph/0211513 SN - 1-4020-1617-4 ER - TY - THES A1 - Fendt, Christian T1 - Formation of astrophysical jets N2 - populärwissenschaftlicher Abstract: Astrophysikalische Jets sind hochkollimierte Plasmaströmungen hoher Geschwindigkeit. Sie werden als allgemeines Phänomen bei unterschiedlichsten astronomischen Quellen gefunden - bei Objekten die sich sowohl in der Grössenskala als auch im Energieumsatz um viele Grössenordnungen unterscheiden. Jets werden beobachtet bei jungen stellaren Objekten (etwa TTauri-Sternen oder eingebettete IR-Quellen), bei sogenannten Mikroquasaren und bei aktiven galaktischen Kernen (etwa Radiogalaxien oder Quasare). So unterschiedlich die Jetquellen von ihrer Erscheinung sein mögen, zwei Tatsachen scheinen sie zu vereinen: Alle Jetquellen zeigen ebenfalls Hinweise auf die Existenz einer Akkretionsscheibe und von Magnetfeldern. Damit sind die wichtigsten Punkte einer Theorie der Jetentstehung schon umrissen. Sie muss sowohl die komplexe Struktur der Jetquelle berücksichtigen - ein System bestehend aus einem Zentralobjekt, der es umgebenden Scheibe, und dem Jet - als auch die magnetohydrodynamische Wechselwirkung zwischen diesen Komponenten. Die magnetohydrodynamischen Gleichungen für solch ein Problem sind derart kompliziert, dass sie meist nur numerisch, also nur mit dem Computer zu lösen sind. Zusätzlich sind viele vereinfachende Annahmen notwendig, da sonst auch der Computer überfordert waere. Im allgemeinen sind folgende Fragestellungen zu lösen: - Die Frage, wie ein Scheibenwind langsamer Geschwindigkeit beschleunigt wird und in einen Jet kollimiert wird ("jet formation"). - Die Frage, wie ein Ausfluss aus der Akkretionsscheibe überhaupt entsteht, d.h. die Frage wie die akkretierende Materie der Scheibe in den Scheibenwind umgelenkt wird ("jet launching"). - Die Frage, wie und wo das Magnetfeld, das zur Jetentstehung notwendig scheint, erzeugt wird. - Die Frage der Stabilität des asymptotischen Jets über weite Laengenskalen und die Rolle der Strahlungsprozesse. Die vorliegende Arbeit konzentriert sich auf die erste Frage. Der Versuch ihrer Beantwortung wird auf verschiedene Weise angestrebt. Zum einen durch Lösung der zeitunabhängigen Gleichungen, mit deren Hilfe das gesamte Jetentstehungsgebiet numerisch erfasst werden kann, zum anderen durch zeitabhängige Simulationen, die zwar nur einen Ausschnitt auflösen, dafür aber die zeitliche Entwicklung des Jets liefern koennen. Es werden relativistische und nicht-relativistische Lösungen diskutiert, Jets, die einen magnetisierten Stern im Ursprung haben und solche, wo dort ein schwarzes Loch existiert. Insgeamt sind grundlegenden Resultate aber allgemein gültig. Sie bestätigen die Vorstellung der magnetohydrodynamischen Entstehung astrophysikalischer Jets aus Akkretionsscheiben. N2 - Highly collimated, high velocity streams of hot plasma – the jets – are observed as a general phenomenon being found in a variety of astrophysical objects regarding their size and energy output. Known as jet sources are protostellar objects (T Tauri stars, embedded IR sources), galactic high energy sources ("microquasars"), and active galactic nuclei (extragalactic radio sources and quasars). Within the last two decades our knowledge regarding the processes involved in astro-physical jet formation has condensed in a kind of standard model. This is the scenario of a magnetohydrodynamically accelerated and collimated jet stream launched from the innermost part of an accretion disk close to the central object. Traditionally, the problem of jet formation is divided in two categories. One is the question how to collimate and accelerate an uncollimated low velocity disk wind into a jet. The second is the question how to initiate that outflow from a disk, i.e. how to turn accretion of matter into an ejection as a disk wind. My own work is mainly related to the first question, the collimation and acceleration process. Due to the complexity of both, the physical processes believed to be responsible for the jet launching and also the spatial configuration of the physical components of the jet source, the enigma of jet formation is not yet completely understood. On the theoretical side, there has been a substantial advancement during the last decade from purely station-ary models to time-dependent simulations lead by the vast increase of computer power. Observers, on the other hand, do not yet have the instruments at hand in order to spatially resolve observe the very jet origin. It can be expected that also the next years will yield a substantial improvement on both tracks of astrophysical research. Three-dimensional magnetohydrodynamic simu-lations will improve our understanding regarding the jet-disk interrelation and the time-dependent character of jet formation, the generation of the magnetic field in the jet source, and the interaction of the jet with the ambient medium. Another step will be the combina-tion of radiation transfer computations and magnetohydrodynamic simulations providing a direct link to the observations. At the same time, a new generation of telescopes (VLT, NGST) in combination with new instrumental techniques (IR-interferometry) will lead to a "quantum leap" in jet observation, as the resolution will then be sufficient in order to zoom into the innermost region of jet formation. T2 - Formation of astrophysical jets Y1 - 2002 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0000733 ER - TY - JOUR A1 - Fendt, Christian T1 - A stellar magnetic dipole connected to an accretion disk : MHD simulations of the long-term evolution Y1 - 2002 SN - 1-58381-101-X ER - TY - JOUR A1 - Fendt, Christian A1 - Cemeljic, Miljenko T1 - Formation of protostellar jets : effects of magnetic diffusion N2 - Protostellar jets most probably originate in turbulent accretion disks surrounding young stellar objects. We investigate the evolution of a disk wind into a collimated jet under the influence of magnetic diffusivity, assuming that the turbulent pattern in the disk will also enter the disk corona and the jet. Using the ZEUS-3D code in the axisymmetry option we solve the time-dependent resistive MHD equations for a model setup of a central star surrounded by an accretion disk. The disk is taken as a time-independent boundary condition for the mass flow rate and the magnetic flux distribution. We derive analytical estimates for the magnitude of magnetic diffusion in a protostellar jet connecting our results to earlier work in the limit of ideal MHD. We find that the diffusive jets propagate slower into the ambient medium, most probably due to the lower mass flow rate in the axial direction. Close to the star we find that a quasi stationary state evolves after several hundred (weak diffusion) or thousand (strong diffusion) disk rotations. Magnetic diffusivity affects the protostellar jet structure as follows. The jet poloidal magnetic field becomes de- collimated. The jet velocity increases with increasing diffusivity, while the degree of collimation for the hydrodynamic flow remains more or less the same. We suggest that the mass flux is a proper tracer for the degree of jet collimation and find indications of a critical value for the magnetic diffusivity above which the jet collimation is only weak. We finally develop a self-consistent picture in which all these effects can be explained in the framework of the Lorentz force. Y1 - 2003 UR - http://www.edpsciences.org/articles/aa/pdf/2002/45/aa2729.pdf ER - TY - JOUR A1 - Fendt, Christian A1 - Ouyed, R. T1 - Ultrarelativistic magnetohydrodynamic jets in the context of gamma-ray bursts N2 - We present a detailed numerical study of the dynamics and evolution of ultrarelativistic magnetohydrodynamic jets in the black hole-disk system under extreme magnetization conditions. We find that Lorentz factors of up to 3000 are achieved and derived a modified Michel scaling (Gamma similar to sigma) that allows for a wide variation in the flow Lorentz factor. Pending contamination induced by mass entrainment, the linear Michel scaling links modulations in the ultrarelativistic wind to variations in mass accretion in the disk for a given magnetization. The jet is asymptotically dominated by the toroidal magnetic field allowing for efficient collimation. We discuss our solutions ( jets) in the context of gamma-ray bursts and describe the relevant features such as the high variability in the Lorentz factor and how high collimation angles (similar to0degrees-5degrees), or cylindrical jets, can be achieved. We isolate a jet instability mechanism we refer to as the "bottleneck'' instability, which essentially relies on a high magnetization and a recollimation of the magnetic flux surfaces. The instability occurs at large radii where any dissipation of the magnetic energy into radiation would in principle result in an optically thin emission Y1 - 2004 SN - 0004-637X ER - TY - JOUR A1 - Kubas, Daniel A1 - Cassan, A. A1 - Beaulieu, Jean-Philippe A1 - Coutures, C. A1 - Dominik, M. A1 - Albrow, Michael D. A1 - Brillant, Stephane A1 - Caldwell, John A. R. A1 - Dominis, Dijana A1 - Donatowicz, J. A1 - Fendt, Christian A1 - Fouque, P. A1 - Jorgensen, Uffe Grae A1 - Greenhill, John A1 - Hill, K. A1 - Heinmüller, Janine A1 - Horne, Keith A1 - Kane, Stephen R. A1 - Marquette, Jean-Baptiste A1 - Martin, Ralph A1 - Menzies, J. W. A1 - Pollard, K. R. A1 - Sahu, K. C. A1 - Vinter, C. A1 - Wambsganss, Joachim A1 - Watson, R. A1 - Williams, A. A1 - Thurl, C. T1 - Full characterization of binary-lens event OGLE-2002-BLG-069 from PLANET observations N2 - We analyze the photometric data obtained by PLANET and OGLE on the caustic-crossing binary-lens microlensing event OGLE-2002-BLG-069. Thanks to the excellent photometric and spectroscopic coverage of the event, we are able to constrain the lens model up to the known ambiguity between close and wide binary lenses. The detection of annual parallax in combination with measurements of extended-source effects allows us to determine the mass, distance and velocity of the lens components for the competing models. While the model involving a close binary lens leads to a Bulge- Disc lens scenario with a lens mass of M = (0.51 ± 0.15) M-⊙ and distance of D-L = (2.9 ± 0.4) kpc, the wide binary lens solution requires a rather implausible binary black-hole lens ( M ≳ 126 M-⊙). Furthermore we compare current state-of-the-art numerical and empirical models for the surface brightness profile of the source, a G5III Bulge giant. We find that a linear limb-darkening model for the atmosphere of the source star is consistent with the data whereas a PHOENIX atmosphere model assuming LTE and with no free parameter does not match our observations Y1 - 2005 SN - 0004-6361 ER - TY - JOUR A1 - Memola, Elisabetta A1 - Fendt, Christian A1 - Brinkmann, W. T1 - Theoretical thermal X-ray spectra of relativistic MHD jets N2 - Highly relativistic jets are most probably driven by strong magnetic fields and launched from the accretion disk surrounding a central black hole. Applying the jet flow parameters (velocity, density, temperature) calculated from the magnetohydrodynamic (MHD) equations, we derive the thermal X-ray luminosity along the inner jet flow in the energy range 0.2-10.1 keV. Here, we concentrate on the case of Galactic microquasars emitting highly relativistic jets. For a 5 Msun central object and a jet mass flow rate of dot {M}j = 10-8 Msun yr-1 we obtain a jet X-ray luminosity LX ~ 1033 erg s-1. Emission lines of Fe XXV and Fe XXVI are clearly visible. Relativistic effects such as Doppler shift and boosting were considered for different inclinations of the jet axis. Due to the chosen geometry of the MHD jet the inner X-ray emitting part is not yet collimated. Therefore, depending on the viewing angle, the Doppler boosting does not play a major role in the total spectra. Y1 - 2002 UR - http://adsabs.harvard.edu/cgi-bin/nph- data_query?bibcode=2002A%26A...385.1089M&link_type=ARTICLE&db_key=AST ER - TY - JOUR A1 - Memola, Elisabetta A1 - Fendt, Christian A1 - Brinkmann, W. T1 - Relativistic Jets from Blazars and Microquasars Y1 - 2001 ER -