Mhd waves in stellar winds and accretion disks (original) (raw)
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The Role of Damped Alfven Waves in Magnetospheric Accretion Models of Young Stars
The Astrophysical Journal, 2002
We examine the role of Alfvén wave damping in heating the plasma in the magnetic funnels of magnetospheric accretion models of young stars. We study four different damping mechanisms of the Alfvén waves: nonlinear, turbulent, viscous-resistive and collisional. Two different possible origins for the Alfvén waves are discussed: 1) Alfvén waves generated at the surface of the star by the shock produced by the infalling matter; and 2) Alfvén waves generated locally in the funnel by the Kelvin-Helmholtz instability. We find that, in general, the damping lengths are smaller than the tube length. Since thermal conduction in the tube is not efficient, Alfvén waves generated only at the star's surface cannot heat the tube to the temperatures necessary to fit the observations. Only for very low frequency Alfvén waves ∼ 10 −5 the ion cyclotron frequency, is the viscous-resistive damping length greater than the tube length. In this case, the Alfvén waves produced at the surface of the star are able to heat the whole tube. Otherwise, local production of Alfvén waves is required to explain the observations. The turbulence level is calculated for different frequencies for optically thin and thick media. We find that turbulent velocities varies greatly for different damping mechanisms, reaching ∼ 100 km s −1 for the collisional damping of small frequency waves.
Toward A Self Consistent MHD Model of Chromospheres and Winds From Late Type Evolved Stars
2014
We present the first magnetohydrodynamic model of the stellar chromospheric heating and acceleration of the outer atmospheres of cool evolved stars, using alpha Tau as a case study. We used a 1.5D MHD code with a generalized Ohm's law that accounts for the effects of partial ionization in the stellar atmosphere to study Alfven wave dissipation and wave reflection. We have demonstrated that due to inclusion of the effects of ion-neutral collisions in magnetized weakly ionized chromospheric plasma on resistivity and the appropriate grid resolution, the numerical resistivity becomes 1-2 orders of magnitude smaller than the physical resistivity. The motions introduced by non-linear transverse Alfven waves can explain non-thermally broadened and non-Gaussian profiles of optically thin UV lines forming in the stellar chromosphere of alpha Tau and other late-type giant and supergiant stars. The calculated heating rates in the stellar chromosphere due to resistive (Joule) dissipation of...
WINDS FROM LUMINOUS LATE-TYPE STARS. II. BROADBAND FREQUENCY DISTRIBUTION OF ALFVÉN WAVES
The Astrophysical Journal, 2010
We present the numerical simulations of winds from evolved giant stars using a fully non-linear, time dependent 2.5-dimensional magnetohydrodynamic (MHD) code. This study extends our previous fully non-linear MHD wind simulations to include a broadband frequency spectrum of Alfvén waves that drive winds from red giant stars. We calculated four Alfvén wind models that cover the whole range of Alfvén wave frequency spectrum to characterize the role of freely propagated and reflected Alfvén waves in the gravitationally stratified atmosphere of a late-type giant star. Our simulations demonstrate that, unlike linear Alfvén wave-driven wind models, a stellar wind model based on plasma acceleration due to broadband non-linear Alfvén waves, can consistently reproduce the wide range of observed radial velocity profiles of the winds, their terminal velocities and the observed mass loss rates. Comparison of the calculated mass loss rates with the empirically determined mass loss rate for α Tau suggests an anisotropic and time-dependent nature of stellar winds from evolved giants. Danchi et al. 1994). Second, there is evidence for strong turbulence within the chromospheres of both giant and supergiant stars. The C II] λ2325 line is formed at temperatures between 5,000 and 10,000 K and is optically thin. It has an intrinsically narrow profile, so the line width primarily reflects the Doppler broadening caused by atmospheric turbulence. The deduced turbulent velocities range from 24 km/s for the K5 giant α Tau to 28 km/s for the K5 hybrid giant γ Dra to about 30 km/s for the M3.4 giant γ Cru. The fact that the chromospheric sound speed in these stars is typically less than 10 km/s implies that this turbulence is highly supersonic. This constraint suggests that sound waves driven by convection and/or pulsation do not contribute significantly to the initiation of the stellar wind (e. g. Castor 1981;. While this mechanism can be important in depositing energy in the lower parts of the atmospheres of giants and supergiants (for instance, Lobel & Dupree 2001), it also fails to reproduce observed terminal velocities and mass loss rates from late-type giants (Sutmann and Cuntz 1995). Third, observations suggests that winds from cool evolved stars are at least two orders of magnitude cooler than that observed from the Sun and coronal giants . This suggests that the thermal conduction cannot play any 18 − = τ ω
Damping of Alfven waves as a heating source in protostellar accretion discs
Monthly Notices of the Royal Astronomical Society, 2013
The need of a minimum amount of ionization in protostellar accretion discs is necessary for the magnetorotational instability to take place. This instability is believed to be the mechanism responsible for a magnetohydrodynamic (MHD) turbulence that could lead to the accretion observed. In this work, we study the role of MHD waves as a source of heating in discs. We analyse if Alfvén waves, when damped during their propagation through the disc, can transfer enough energy in order to raise its temperature. As the discs are composed of dust, we suggest here that the Alfvén waves are damped by the dust-cyclotron mechanism of damping. In this mechanism when charged dust particles acquire the same (cyclotron) frequency as the waves, a resonance occurs that leads to the damping of the waves. Here, we present a disc model with two heating mechanisms: the 'anomalous' viscosity considered in terms of the α parametrization and the damping of Alfvén waves. We vary the space parameters in order to study the second mechanism's behaviour. We show that the waves can increase the temperature of the disc and flatten the traditional r 3/4 effective temperature profile of the disc.
Alfvén Waves in the Context of Solar-Like Star Formation: Accretion Columns and Disks
Advances in Space Environment Research - Volume I, 2003
In this work we examine the damping of Alfvén waves as a source of plasma heating in disks and magnetic funnels of young solar like stars, the T Tauri stars. We apply four different damping mechanisms in this study: viscous-resistive, collisional, nonlinear and turbulent, exploring a wide range of wave frequencies, from 10 −5 i to 10 −1 i (where i is the ion-cyclotron frequency). The results show that Alfvénic heating can increase the ionization rate of accretion disks and elevate the temperature of magnetic funnels of T Tauri stars opening possibilities to explain some observational features of these objects.
MHD Structures, Waves and Turbulence in the Solar Wind: Observations and Theories
Physics Today, 1996
A comprehensive overview is presented of recent observational and theoretical results on solar wind structures and fluctuations and magnetohydrodynamic waves and turbulence, with preference given to phenomena in the inner heliosphere. Emphasis is placed on the progress made in the past decade in the understanding of the nature and origin of especially small-scale, compressible and incompressible fluctuations. Turbulence models to describe the spatial transport and spectral transfer of the fluctuations in the inner heliosphere are discussed, and results from direct numerical simulations are dealt with. Intermittency of solar wind fluctuations and their statistical distributions are briefly investigated. Studies of the heating and acceleration effects of the turbulence on the background wind are critically surveyed. Finally, open questions concerning the origin, nature and evolution of the fluctuations are listed, and possible avenues and perspectives for future research are outlined.
The Astrophysical Journal, 1999
The maser connected with the young stellar object in the globule IC 1396N has been mapped H 2 O with the VLBA during its highest state of activity in 1996 June. The spectrum of the maser consist-H 2 O ed of a dense group of strong low-velocity features near the LSR velocity of the globule, and two highvelocity features : one redshifted to 9.3 km s~1 and the other blueshifted to [14.1 km s~1. The map of low-velocity features displays a remarkable chain of at least eight maser spots located very close to a straight line about 15 AU in extent, with LSR velocities varying linearly along the line. The two highvelocity features are o †set from the low-velocity group by 410 and 10,000 AU for the blue and red features, respectively. We discuss three models that can describe the observed distribution of maser spots : a Keplerian disk, a shock front, and a molecular outÑow. The Ðnal model that we propose incorporates all three of these models : the low-velocity features arise in the Keplerian disk with maser emission excited by shock waves traveling in the disk, while the high-velocity features arise at the root of the molecular outÑow originating from the central 4 young star or a protostar. The mass of the disk and its M _ angular momentum are similar to those of the solar system planets. It is suggested that it is a circumstellar accretion disk accumulating the excess angular momentum of the collapsing molecular core, which may give rise to the formation of a planetary system. This model can be tested by measurements of the proper motion and radial velocity variations of the maser spots. Subject headings : ISM : individual (IC 1396N) È ISM : molecules È masers È radio lines : ISM È stars : formation O MASERS IN IC 1396N 237 3. RESULTS
Enhanced MHD Transport in Astrophysical Accretion Flows: Turbulence, Winds and Jets
Plasma and Fusion Research, 2009
Astrophysical accretion is arguably the most prevalent physical process in the Universe; it occurs during the birth and death of individual stars and plays a pivotal role in the evolution of entire galaxies. Accretion onto a black hole, in particular, is also the most efficient mechanism known in nature, converting up to 40% of accreting rest mass energy into spectacular forms such as high-energy (X-ray and gamma-ray) emission and relativistic jets. Whilst magnetic fields are thought to be ultimately responsible for these phenomena, our understanding of the microphysics of MHD turbulence in accretion flows as well as large-scale MHD outflows remains far from complete. We present a new theoretical model for astrophysical disk accretion which considers enhanced vertical transport of momentum and energy by MHD winds and jets, as well as transport resulting from MHD turbulence. We also describe new global, 3D simulations that we are currently developing to investigate the extent to which non-ideal MHD effects may explain how small-scale, turbulent fields (generated by the magnetorotational instability-MRI) might evolve into large-scale, ordered fields that produce a magnetized corona and/or jets where the highest energy phenomena necessarily originate.
2005
ABSTRACT We show that the coronal heating and the acceleration of the fast solar wind in the coronal holes are natural consequence of the footpoint fluctuations of the magnetic fields at the photosphere by one-dimensional, time-dependent, and nonlinear magnetohydrodynamical simulation with radiative cooling and thermal conduction. We impose low-frequency (<0.05Hz) transverse photospheric motions, corresponding to the granulations, with velocity = 0.7$km/s. In spite of the attenuation in the chromosphere by the reflection, the sufficient energy of the generated outgoing Alfven waves transmit into the corona to heat and accelerate of the plasma by nonlinear dissipation. Our result clearly shows that the initial cool (10^4K) and static atmosphere is naturally heated up to 10^6K and accelerated to 800km/s, and explain recent SoHO observations and Interplanetary Scintillation measurements.
Three-dimensional simulations of MHD disk winds to hundred AU scale from the protostar
EPJ Web of Conferences, 2014
We present the results of four, large scale, three-dimensional magnetohydrodynamics simulations of jets launched from a Keplerian accretion disk. The jets are followed from the source out to 90 AU, a scale that covers several pixels of HST images of nearby protostellar jets. The four simulations analyzed are for four different initial magnetic field configuration threading the surface of the accretion disk with varying degree of openness of the field lines. Our simulations show that jets are heated along their length by many shocks and we compute the line emission that is produced. We find excellent agreement with the observations and use these diagnostics to discriminate between different magnetic field configurations. A two-component jet emerges in simulations with less open field lines along the disk surface. The two-components are physically and dynamically separated with an inner fast and rotating jet and an outer slow jet. The second component weakens and eventually only one-component jet (i.e. only the inner jet) is obtained for the most open field configurations. In all of our simulations we find that the faster inner component inherits the Keplerian profile and preserves it to large distances from the source. On the other hand, the outer component is associated with velocity gradients mimicking rotation.