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Tomographic Simulations of accretion disks in cataclysmic variables - Flickering and wind

RIBEIRO, Fabiola M. A.; DIAZ, Marcos P.
Fonte: ASTRONOMICAL SOC JAPAN Publicador: ASTRONOMICAL SOC JAPAN
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
66.67%
Both continuum and emission line flickering are phenomena directly associated with the mass-accretion process. In this work we simulated accretion-disk Doppler maps, including the effects of winds and flickering flares. Synthetic flickering Doppler maps were calculated and the effect of the flickering parameters on the maps was explored. Jets and winds occur in many astrophysical objects where accretion disks are present. Jets are generally absent among the cataclysmic variables (CVs), but there is evidence of mass loss by wind in many objects. CVs are ideal objects to study accretion disks, and consequently to study the wind associated with these disks. We also present simulations of accretion disks, including the presence of a wind with orbital phase resolution. Synthetic Ha line profiles in the optical region were obtained and their corresponding Doppler maps were calculated. The effect of the wind simulation parameters on the wind line profiles was also explored. From this study we verified that optically thick lines and/or emission by diffuse material into the primary Roche lobe are necessary to generate single peaked line profiles, often seen in CVs. The future accounting of these effects is suggested for interpreting Doppler tomography reconstructions.

ON THE DOUBLE NATURED SOLUTIONS OF THE TWO-TEMPERATURE EXTERNAL SOFT PHOTON COMPTONIZED ACCRETION DISKS

MEIRELLES FILHO, Cesar
Fonte: IOP PUBLISHING LTD Publicador: IOP PUBLISHING LTD
Tipo: Artigo de Revista Científica
ENG
Relevância na Pesquisa
56.4%
We have analyzed pair production in the innermost region of a two-temperature external soft photon Comptonized accretion disk. We have shown that, if the viscosity parameter is greater than a critical value alpha(c), the solution to the disk equation is double valued: one, advection dominated, and the other, radiation dominated. When alpha <= alpha(c), the accretion rate has to satisfy (m) over dot(1) <= (m) over dot <= (m) over dot(c) in order to have two steady-state solutions. It is shown that these critical parameters (m) over dot(1), (m) over dot(c) are functions of r, alpha, and theta(e), and alpha(c) is a function of r and theta(e). Depending on the combination of the parameters, the advection-dominated solution may not be physically consistent. It is also shown that the electronic temperature is maximum at the onset of the thermal instability, from which results this inner region. These solutions are stable against perturbations in the electron temperature and in the density of pairs.; FAPESP

Uma análise espectroscópica de discos de acresção em variáveis cataclísmicas; A Spectroscopic Analysis of Accretion Disks in Cataclysmic Variables

Ribeiro, Fabíola Mariana Aguiar
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Tese de Doutorado Formato: application/pdf
Publicado em 27/10/2006 PT
Relevância na Pesquisa
66.59%
Neste trabalho é apresentado um estudo observacional de discos de acresção em Variáveis Cataclísmicas (VCs). São analisadas medidas espectrofotométricas com resolução temporal dos perfis de linhas de emissão. A emissividade em linhas dos sistemas é mapeada utilizando a técnica de tomografia Doppler. Os parâmetros básicos das binárias, tais como período orbital, massas, inclinação orbital, são determinados quando necessário. Um código foi desenvolvido para simular a variabilidade das linhas de emissão em sistemas binários, além da presença de vento. O código foi utilizado para quantificar os parâmetros necessários para um estudo adequado de tomografia do flickering, tais como número de espectros, relação sinal-ruído destes, e frequência e amplitude do flickering em questão. Três sistemas são abordados: V3885 Sgr, RR Pic e V841 Oph. A variabilidade intrínseca de V3885 Sgr é mapeada através da técnica de tomografia do flickering. O flickering foi simulado e verificou-se que a fonte principal de flickering observada em V3885 Sgr não poderia se originar em um disco de acresção Kepleriano, mas sim na face iluminada da estrela secundária. Uma interpretação proposta para este fenômeno seria de um cenário onde o flickering no contínuo UV originado nas regiões centrais do disco ou na mancha quente é reprocessado na face iluminada da secundária. Obtivemos a primeira confirmação...

Reconexão magnética em discos de acreção e seus efeitos sobre a formação e aceleração de jatos: um estudo teórico-numérico; Magnetic reconnection in accretion disks and their effects on the formation and acceleration of jets: a theoretical and numerical study

Kadowaki, Luis Henrique Sinki
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 09/12/2011 PT
Relevância na Pesquisa
56.59%
Jatos e discos de acreção associados a objetos galácticos e extragalácticos tais como, microquasares (i.e., buracos negros de massa estelar presentes em alguns sistemas binários estelares), núcleos ativos de galáxias (NAGs) e objetos estelares jovens (OEJs), frequentemente exibem eventos de ejeção de matéria quase periódicos que podem fornecer importantes informações sobre os processos físicos que ocorrem nas suas regiões mais internas. Entre essas classes de objetos, os microquasares com emissão transiente em raios-X vêm sendo identificados em nossa Galáxia desde a última década, e tal como os NAGs e quasares distantes, alguns desses sistemas também produzem jatos colimados com velocidades aparentemente superluminais, não deixando dúvidas de que se tratam de um gás ejetado com velocidades relativísticas. Um exemplo amplamente observado em comprimentos de onda do rádio aos raios-X é o microquasar GRS 1915+105 (e.g., Dhawan et al.,2000), que foi o primeiro objeto galáctico a exibir evidências de um jato com movimento aparentemente superluminal (Mirabel e Rodríguez, 1998, 1994). Um modelo para explicar a origem dessas ejeções superluminais, bem como a emissão rádio sincrotrônica em flares não muito diferentes dos que ocorrem na coroa solar...

Acreção esfericamente simétrica de matéria: Conceitos básicos e aplicações em cosmologia; Spherically symmetrical accretion of matter: Basic concepts and cosmological applications

Silva, Michel Aguena da
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 25/06/2012 PT
Relevância na Pesquisa
56.5%
Nesta dissertação discutimos o processo da acreção de materia sobre objetos compactos em suas diferentes abordagens. Iniciando com o caso clássico, estudamos sua contraparte relativística e, por fim, investigamos a acreção de fluidos cosmológicos (energia escura e matéria escura) em buracos negros. Devido a simetria esférica adotada, a formação dos chamados discos de acréscimo é proibida (tanto no caso clássico quanto no relativístico) e, portanto, os problemas relacionados com a física dos discos (sua formação e evolução) não foram investigados. No contexto clássico, analisamos inicialmente a chamada acreçao de Bondi, onde o fluido acretado obedece a uma equação de estado politropica e o processo de acreção é descrito pela hidrodinâmica euleriana. A existância de 6 tipos possíveis de solucões para o campo de velocidades é identicada e suas consequências fsicas são discutidas em detalhe. Apenas uma dessas soluções descreve de forma fisicamente consistente o processo de acreção. A taxa de materia acretada é constante, um resultado esperado devido a hipotese de regime estacionário. O estudo do caso relativstico é completamente baseado na Teoria da Relatividade Geral, com o campo gravitacional do corpo central sendo descrito pela metrica de Schwarzschild. O processo relativstico também ocorre sob condições estacionárias e...

Numerische Simulationen von Akkretionsscheiben in Kataklysmischen Variablen mit Smoothed Particle Hydrodynamics; Numerical Simulations of Accretion Disks in Cataclysmic Variables with Smoothes Particle Hydrodynamics

Kunze, Stefan
Fonte: Universidade de Tubinga Publicador: Universidade de Tubinga
Tipo: Dissertação
DE_DE
Relevância na Pesquisa
56.62%
Diese Arbeit befaßt sich mit der Untersuchung von Akkretionsscheiben in Kataklysmischen Variablen (CVs) mit der numerischen Methode Smoothed Particle Hydrodynamics. CVs sind enge Doppelsterne, die aus einem leichten Hauptreihenstern und einem Weißen Zwerg bestehen. Ein Gasstrom vom Begleitstern speist eine dünne Akkretionsscheibe um den Weißen Zwerg. Die Akkretion wird durch die viskose Entwicklung der Scheibe bestimmt. Ein Aspekt der Vorgänge in den Akkretionsscheiben von CVs ist die Wechselwirkung zwischen dem Akkretionsstrom und dem Rand der Akkretionsscheibe. In dem Aufprallgebiet (Bright Spot) wird viel Strahlung frei. Ich untersuche, ob der Akkretionsstrom am Scheibenrand gestoppt wird oder ob ein Teil über und unter der Scheibe zu kleineren Radien strömen kann. Es stellt sich heraus, daß der größte Teil des Akkretionsstroms tatsächlich direkt nach innen fließt. Bisher wurde davon ausgegangen, daß nur vertikal außenliegende Teile des Akkretionsstroms, die die Scheibenhöhe überragen, überströmen können, und der Rest am Rand der Scheibe verbleibt. Hier stellt sich jedoch heraus, daß auch innenliegende Teile des Stroms in der Bright Spot-Region vertikal abgelenkt werden. Das sollte beispielsweise für das Ausbruchsverhalten von Zwergnovae eine Rolle spielen. AM CVn-Sterne sind nahe Verwandte der Kataklysmischen Variablen. In zweien dieser Systeme wurden Superhumps entdeckt. Durch Simulationen der Akkretionsscheiben in diesen Systemen unterstütze ich die Interpretation dieser Systeme als Helium-transferierende Doppel-Weiße Zwerg-Sterne. Bei schwach magnetischen Weißen Zwergen kann die Ausbildung einer Akkretionsscheibe nahe dem Weißen Zwerg verhindert werden. Unter der Annahme...

Synthetische Spektren und Vertikalschichtungen von Akkretionscheiben; Synthetic Spectra and Vertical Structures of Accretion Disks

Nagel, Thorsten
Fonte: Universidade de Tubinga Publicador: Universidade de Tubinga
Tipo: Dissertação
DE_DE
Relevância na Pesquisa
46.69%
Akkretionsscheiben beobachtet man in so unterschiedlichen Objekten wie protoplanetaren Systemen, Kernen aktiver Galaxien, kataklysmischen Variablen oder Röntgendoppelsternen. Sie generieren hierbei mitunter einen Großteil der Leuchtkraft des Gesamtsystems, weshalb eine realistische physikalische Beschreibung der Akkretionsscheibe zunehmend an Bedeutung gewinnt, um die immer besser werdenden Beobachtungsdaten analysieren zu können. Ziel dieser Arbeit ist es, ein Modell zu entwickeln, das die Berechnung synthetischer Spektren und Vertikalstrukturen von Akkretionsscheiben unter möglichst exakter Berücksichtigung der in der Akkretionsscheibe ablaufenden physikalischen Prozesse erlaubt. Eine detaillierte strahlungshydrodynamische Behandlung der Akkretionsscheibe in allen drei Raumrichtungen gelingt auf Grund des numerischen Aufwandes derzeit allerdings noch nicht. Im Falle einer geometrisch dünnen alpha-Scheibe (Shakura & Sunyaev 1973), bei der die Scheibendicke sehr viel kleiner ist als der -durchmesser, lassen sich jedoch Radial- und Vertikalstruktur entkoppeln. Unter der Annahme von Axialsymmetrie und durch die Unterteilung der Scheibe in konzentrische Ringe vereinfacht sich so die Bestimmung der Vertikalstruktur deutlich. Die in jedem Scheibenring dissipativ erzeugte Energie wird abgestrahlt...

Linkage between accretion disks and blazars

Bicknell, Geoffrey; Li, Jianke
Fonte: Kluwer Academic Publishers Publicador: Kluwer Academic Publishers
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.36%
The magnetic field in an accretion disk is estimated assuming that all of the angular momentum within prescribed accretion disk radii is removed by a jet. The magnetic field estimated at the base of the jet is extrapolated to the blazar emission region us

Warped accretion disks and the unification of Active Galactic Nuclei

Nayakshin, Sergei
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/11/2004
Relevância na Pesquisa
46.66%
Orientation of parsec-scale accretion disks in AGN is likely to be nearly random for different black hole feeding episodes. Since AGN accretion disks are unstable to self-gravity on parsec scales, star formation in these disks will create young stellar disks, similar to those recently discovered in our Galactic Center. The disks blend into the quasi-spherical star cluster enveloping the AGN on time scales much longer than a likely AGN lifetime. Therefore, the gravitational potential within the radius of the black hole influence is at best axi-symmetric rather than spherically symmetric. Here we show that as a result, a newly formed accretion disk will be warped. For the simplest case of a potential resulting from a thin stellar ring, we calculate the disk precession rates, and the time dependent shape. We find that, for a realistic parameter range, the disk becomes strongly warped in few hundred orbital times. We suggest that this, and possibly other mechanisms of accretion disk warping, have a direct relevance to the problem of AGN obscuration, masing warped accretion disks, narrow Fe K-alpha lines, etc.; Comment: 6 pages and 2 figures; submitted to MNRAS

Turbulent Angular Momentum Transport in Weakly-Ionized Accretion Disks

Johnson, Bryan Mark
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 13/09/2005
Relevância na Pesquisa
46.67%
Understanding the mechanism that drives accretion has been the primary challenge in accretion disk theory. Turbulence provides a natural means of dissipation and the removal of angular momentum, but firmly establishing its presence in disks proved for many years to be difficult. The realization in the 1990s that a weak magnetic field will destabilize a disk and result in a vigorous turbulent transport of angular momentum has revolutionized the field. Much of accretion disk research now focuses on understanding the implications of this mechanism for astrophysical observations. At the same time, the success of this mechanism depends upon a sufficient ionization level in the disk for the flow to be well-coupled to the magnetic field. Many disks, such as disks around young stars and disks in binary systems that are in quiescence, are too cold to be sufficiently ionized, and so efforts to establish the presence of turbulence in these disks continues. This dissertation focuses on several possible mechanisms for the turbulent transport of angular momentum in weakly-ionized accretion disks: gravitational instability, radial convection and vortices driving compressive motions. It appears that none of these mechanisms are very robust in driving accretion. A discussion is given...

The Observational Appearance of Slim Accretion Disks

Szuszkiewicz, Ewa; Malkan, Matthew A.; Abramowicz, Marek A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 07/09/1995
Relevância na Pesquisa
46.69%
We reexamine the hypothesis that the optical/UV/soft X-ray continuum of Active Galactic Nuclei is thermal emission from an accretion disk. Previous studies have shown that fitting the spectra with the standard, optically thick and geometrically thin accretion disk models often led to luminosities which contradict the basic assumptions adopted in the standard model. There is no known reason why the accretion rates in AGN should not be larger than the thin disk limit. In fact, more general, slim accretion disk models are self-consistent even for moderately super-Eddington luminosities. We calculate here spectra from a set of thin and slim, optically thick accretion disks. We discuss the differences between the thin and slim disk models, stressing the implications of these differences for the interpretation of the observed properties of AGN. We found that the spectra can be fitted not only by models with a high mass and a low accretion rate (as in the case of thin disk fitting) but also by models with a low mass and a high accretion rate. In the first case fitting the observed spectra in various redshift categories gives black hole masses around 10^9 solar masses for a wide range of redshifts, and for accretion rates ranging from 0.4 to 8 solar masses/year. In the second case the accretion rate is around 10^2 solar masses/year for all AGN and the mass ranges from 3*10^6 to 10^8 solar masses. Unlike the disks with a low accretion rate...

Slim accretion disks around black holes

Sadowski, A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 01/08/2011
Relevância na Pesquisa
46.73%
In this thesis, I study hydrodynamical models of slim accretion disks --- advective, optically thick disks which generalize the standard models of radiatively efficient thin disks to all accretion rates. I start with a general introduction to the theory of accretion onto compact objects. It is followed by a derivation of the commonly-used standard models of thin disks. In the subsequent section I introduce the equations describing slim disks, explain the numerical methods I used to solve them and discuss properties of such solutions. I also give a general derivation of non-stationary equations and present the time evolution of thermally unstable accretion disks. I introduce a state-of-the-art approach coupling the radial and vertical structures of an advective accretion disk and discuss the improvements it brings to vertically-averaged solutions. I also present a numerical model of self-illuminated slim accretion disks. Finally, I present and discuss applications of slim accretion disks: estimating of spin of the central black hole in LMC X-3 through X-ray continuum fitting basing on high-luminosity data, spinning-up of black holes by super-critical accretion flows and normalizing of magnetohydrodynamical global simulations.; Comment: Ph.D. thesis written under the supervision of Prof. Marek Abramowicz and defended on July 1...

Accretion Disks around Young Stars: Lifetimes, Disk Locking and Variability

Jayawardhana, Ray; Coffey, Jaime; Scholz, Alexander; Brandeker, Alexis; van Kerkwijk, Marten H.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 23/05/2006
Relevância na Pesquisa
46.66%
We report the findings of a comprehensive study of disk accretion and related phenomena in four of the nearest young stellar associations spanning 6-30 million years in age, an epoch that may coincide with the late stages of planet formation. We have obtained ~650 multi-epoch high-resolution optical spectra of 100 low-mass stars that are likely members of the eta Chamaeleontis (~6 Myr), TW Hydrae (~8 Myr), beta Pictoris (~12 Myr) and Tucanae-Horologium (~30 Myr) groups. Our data were collected over 12 nights between 2004 December - 2005 July on the Magellan Clay 6.5m telescope. Based on H$\alpha$ line profiles, along with a variety of other emission lines, we find clear evidence of on-going accretion in three out of 11 eta Cha stars and two out of 32 TW Hydrae members. None of the 57 beta Pic or Tuc-Hor members shows measurable signs of accretion. Together, these results imply significant evolution of the disk accretion process within the first several Myr of a low-mass star's life. While a few disks can continue to accrete for up to ~10 Myr, our findings suggest that disks accreting for beyond that timescale are rather rare. This result provides an indirect constraint on the timescale for gas dissipation in inner disks and, in turn...

Wind-accretion disks in wide binaries, second generation protoplanetary disks and accretion onto white dwarfs

Perets, Hagai B.; Kenyon, Scott J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.68%
Mass transfer from an evolved donor star to its binary companion is a standard feature of stellar evolution in binaries. In wide binaries, the companion star captures some of the mass ejected in a wind by the primary star. The captured material forms an accretion disk. Here, we study the evolution of wind-accretion disks, using a numerical approach which allows us to follow the long term evolution. For a broad range of initial conditions, we derive the radial density and temperature profiles of the disk. In most cases, wind-accretion leads to long-lived stable disks over the lifetime of the AGB donor star. The disks have masses of a few times 10^{-5}-10^{-3} M_sun, with surface density and temperature profiles that follow broken power-laws. The total mass in the disk scales approximately linearly with the viscosity parameter used. Roughly 50% to 80% of the mass falling into the disk accretes onto the central star; the rest flows out through the outer edge of the disk into the stellar wind of the primary. For systems with large accretion rates, the secondary accretes as much as 0.1 M_sun. When the secondary is a white dwarf, accretion naturally leads to nova and supernova eruptions. For all types of secondary star, the surface density and temperature profiles of massive disks resemble structures observed in protoplanetary disks...

Horizon-Penetrating Transonic Accretion Disks around Rotating Black Holes

Takahashi, Rohta
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 30/04/2007
Relevância na Pesquisa
46.7%
The stationary hydrodynamic equations for the transonic accretion disks and flows around rotating black holes are presented by using the Kerr-Schild coordinate where there is no coordinate singularity at the event horizon. We use two types of the causal viscosity prescription, and the boundary conditions for the transonic accretion flows are given at the sonic point. For one type of the causal viscosity prescription we also add the boundary conditions at the viscous point where the accreting radial velocity is nearly equal to the viscous diffusion velocity. Based on the formalism for the transonic accretion disks, after we present the calculation method of the transonic solutions, the horizon-penetrating transonic solutions which smoothly pass the event horizon are calculated for several types of the accretion flow models: the ideal isothermal flows, the ideal and the viscous polytropic flows, the advection dominated accretion flows (ADAFs) with the relativistic equation of state, the adiabatic accretion disks, the standard accretion disks, the supercritical accretion disks. These solutions are obtained for both non-rotating and rotating black holes. The calculated accretion flows plunge into black hole with finite three velocity smaller than the speed of light even at the event horizon or inside the horizon...

On the structure of Accretion Disks with Outflows

Jiao, Cheng-Liang; Wu, Xue-Bing
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 22/03/2011
Relevância na Pesquisa
46.66%
In order to study the outflows from accretion disks, we solve the set of hydrodynamic equations for accretion disks in the spherical coordinates ($r\theta\phi$) to obtain the explicit structure along the $\theta$ direction. Using self-similar assumptions in the radial direction, we change the equations to a set of ordinary differential equations (ODEs) about the $\theta$-coordinate, which are then solved with symmetrical boundary conditions in the equatorial plane, and the velocity field is obtained. The $\alpha$ viscosity prescription is applied and an advective factor $f$ is used to simplify the energy equation.The results display thinner, quasi-Keplerian disks for Shakura-Sunyaev Disks (SSDs) and thicker, sub-Keplerian disks for Advection Dominated Accretion Flows (ADAFs) and slim disks, which are consistent with previous popular analytical models. However, an inflow region and an outflow region always exist, except when the viscosity parameter $\alpha$ is too large, which supports the results of some recent numerical simulation works. Our results indicate that the outflows should be common in various accretion disks and may be stronger in slim disks, where both advection and radiation pressure are dominant. We also present the structure dependence on the input parameters and discuss their physical meanings. The caveats of this work and possible improvements in the future are discussed.; Comment: 24 pages...

Instability of Population III Black Hole Accretion Disks

Ohsuga, Ken; Susa, Hajime; Uchiyama, Yosuke
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 19/11/2007
Relevância na Pesquisa
46.66%
We investigate the stability of black hole accretion disks in a primordial environment (POP III disks for short), by solving the vertical structure of optically thick disks, including convective energy transport, and by employing a one-zone model for optically thin isothermal disks. Because of the absence of metals in POP III disks, we find significant differences in stability associated with ionization between POP III disks and the disks of solar metallicity. An unstable branch in S-shaped equilibrium curves on the Mdot-Sigma (mass accretion rate - surface density) plane extends to a larger surface density compared with the case of disks of solar metallicity. The resulting equilibrium loci indicate that quasi-periodic oscillations in luminosity can also be driven in POP III disks, and their maximal luminosity is typically by an order of magnitude larger than that of the disks of solar metallicity. Such a strong outburst of POP III disks can be observed by future huge telescopes, in case that the mass is supplied onto the disks at the Bondi accretion rates in typical virialized small dark halos.; Comment: 7 pages, 4 figures, accepted for publication in PASJ

Vertical Structure of Magnetized Accretion Disks around Young Stars

Lizano, S.; Tapia, C.; Boehler, Y.; D'Alessio, P.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 03/12/2015
Relevância na Pesquisa
46.65%
We model the vertical structure of magnetized accretion disks subject to viscous and resistive heating, and irradiation by the central star. We apply our formalism to the radial structure of magnetized accretion disks threaded by a poloidal magnetic field dragged during the process of star formation developed by Shu and coworkers. We consider disks around low mass protostars, T Tauri, and FU Orionis stars. We consider two levels of disk magnetization, $\lambda_{sys} = 4$ (strongly magnetized disks), and $\lambda_{sys} = 12$ (weakly magnetized disks). The rotation rates of strongly magnetized disks have large deviations from Keplerian rotation. In these models, resistive heating dominates the thermal structure for the FU Ori disk. The T Tauri disk is very thin and cold because it is strongly compressed by magnetic pressure; it may be too thin compared with observations. Instead, in the weakly magnetized disks, rotation velocities are close to Keplerian, and resistive heating is always less than 7\% of the viscous heating. In these models, the T Tauri disk has a larger aspect ratio, consistent with that inferred from observations. All the disks have spatially extended hot atmospheres where the irradiation flux is absorbed, although most of the mass ($\sim 90-95$ \%) is in the disk midplane. With the advent of ALMA one expects direct measurements of magnetic fields and their morphology at disk scales. It will then be possible to determine the mass-to-flux ratio of magnetized accretion disks around young stars...

The radial structure of protostellar accretion disks: influence of jets

Combet, C.; Ferreira, J.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/12/2007
Relevância na Pesquisa
46.69%
The radial structure of accretion disks is a fundamental issue regarding star and planet formation. Many theoretical studies, focussing on different aspects such as e.g. disk emissivity or ionization, have been conducted in the context of the Standard Accretion Disk (SAD) model, where no jet is present. We wish to calculate the structure of YSO accretion disks in an approach that takes into account the presence of the protostellar jets. The radial structure of these Jet Emitting Disks (JED) should then be compared to that of standard accretion disks. The analytical treatment used in this work is very similar to that of standard accretion disks but is using the parameter space of Magnetised Accretion-Ejection Structures that include the jet torque on the underlying disk. In this framework, the analytical expressions of key quantities, such as mid-plane temperatures, surface densities or disk aspect ratio are derived. It is found that JEDs present a structure very different from the SADs and that can be observationally tested. The implications on planet formation in the inner regions of accretion disks are briefly discussed. We also supply sets of analytical formulae, valid in different opacity regimes, for the disk quantities. These expressions can be readily used for any work where the disk structure is needed as an input for the model.; Comment: 11 pages...

Photon Bubbles and the Vertical Structure of Accretion Disks

Begelman, Mitchell C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 01/02/2006
Relevância na Pesquisa
46.7%
We consider the effects of "photon bubble" shock trains on the vertical structure of radiation pressure-dominated accretion disks. These density inhomogeneities are expected to develop spontaneously in radiation-dominated accretion disks where magnetic pressure exceeds gas pressure, even in the presence of magnetorotational instability. They increase the rate at which radiation escapes from the disk, and may allow disks to exceed the Eddington limit by a substantial factor. We first generalize the theory of photon bubbles to include the effects of finite optical depths and radiation damping. Modifications to the diffusion law at low optical depth tend to fill in the low-density regions of photon bubbles, while radiation damping inhibits the formation of photon bubbles at large radii, small accretion rates, and small heights above the equatorial plane. Accretion disks dominated by photon bubble transport may reach luminosities of 10 to >100 times the Eddington limit (L_E), depending on the mass of the central object, while remaining geometrically thin. However, photon bubble-dominated disks with alpha-viscosity are subject to the same thermal and viscous instabilities that plague standard radiation pressure-dominated disks, suggesting that they may be intrinsically unsteady. Photon bubbles can lead to a "core-halo" vertical disk structure. In super-Eddington disks the halo forms the base of a wind...