Mass accretion to young stars triggered by flaring activity in circumstellar discs (original) (raw)
Related papers
Publications of the Astronomical Society of Japan, 2003
Many young stellar objects, such as protostars and T-Tauri stars, show strong flare activity. In this paper we present a hydrodynamic simulation of a flare loop that connects the central star and the accretion disk, and discuss the evaporation of the chromosphere of the central star and the disk. We assumed a long (> 10 R) loop length, and that the flare energy is deposited near the halfway point between the disk and the stellar surface. We found that in some cases all of the plasma in the accretion disk is heated to the flare temperature and spreads over the flare loop. The condition for this "disk disappearance" was examined. The X-ray spectrum expected when we observe the simulation result was synthesized by taking into account the instrumental response of ASCA/GIS. However, we could not find any clear observational signature of the existence of the disk, because the bulk properties of a flare loop are determined by the flare heating flux and loop length, and not by the involvement of the disk. We found that the synthesized spectrum is reasonably fitted with a two-temperature model, and that the temperature of the hotter component is several factors lower than the maximum temperature of the simulation result.
Outflows and particle acceleration in the accretion disks of young stars
EPJ Web of Conferences, 2019
Magneto-gas-dynamic (MGD) outflows from the accretion disks of T Tauri stars with fossil large-scale magnetic field are investigated. We consider two mechanisms of the outflows: rise of the magnetic flux tubes (MFT) formed in the regions of efficient generation of the toroidal magnetic field in the disk due to Parker instability, and acceleration of particles in the current layer formed near the boundary between stellar magnetosphere and the accretion disk. Structure of the disk is calculated using our MGD model of the accretion disks. We simulate dynamics of the MFT in frame of slender flux tube approximation taking into account aerodynamic and turbulent drags, and radiative heat exchange with external gas. Particle acceleration in the current layer is investigated on the basis of Sweet-Parker model of magnetic reconnection. Our calculations show that the MFT can accelerate to velocities up to 50 km s^−1 causing periodic outflows from the accretion disks. Estimations of the particle acceleration in the current layer are applied to interpret high-speed jets and X-rays observed in T Tauri stars with the accretion disks.
The role of accretion disks in the formation of massive stars
Proceedings of the International Astronomical Union, 2010
We present radiation hydrodynamics simulations of the collapse of massive pre-stellar cores. We treat frequency dependent radiative feedback from stellar evolution and accretion luminosity at a numerical resolution down to 1.27 AU. In the 2D approximation of axially symmetric simulations, it is possible for the first time to simulate the whole accretion phase of several 10 5 yr for the forming massive star and to perform a comprehensive scan of the parameter space. Our simulation series show evidently the necessity to incorporate the dust sublimation front to preserve the high shielding property of massive accretion disks. Our disk accretion models show a persistent high anisotropy of the corresponding thermal radiation field, yielding to the growth of the highest-mass stars ever formed in multi-dimensional radiation hydrodynamics simulations. Non-axially symmetric effects are not necessary to sustain accretion. The radiation pressure launches a stable bipolar outflow, which grows in angle with time as presumed from observations. For an initial mass of the pre-stellar host core of 60, 120, 240, and 480 M⊙ the masses of the final stars formed in our simulations add up to 28.2, 56.5, 92.6, and at least 137.2 M⊙ respectively.
Dynamics of magnetized accretion disks of young stars
Cornell University - arXiv, 2022
We investigate the dynamics of the accretion disks of young stars with fossil large-scale magnetic field. The author's magnetohydrodynamic (MHD) model of the accretion disks is generalized to take into account the dynamical influence of the magnetic field on gas rotation speed and the vertical structure of the disks. With the help of the developed dynamical MHD model, the structure of an accretion disk of a solar mass T Tauri star is simulated for different values of the accretion rate and sizes of dust grains d. The simulations of the radial structure of the disk show that the magnetic field in the disk is kinematic, and the electromagnetic force does not affect the rotation speed of the gas for typical values = 10 −8 yr −1 and d = 0.1 m. In the case of large dust grains, d ≥ 1 mm, the magnetic field is frozen into the gas and a dynamically strong magnetic field is generated at radial distances from the star 30 au, the tensions of which slow down the rotation speed by 1.5 % of the Keplerian velocity. This effect is comparable to the contribution of the radial gradient of gas pressure and can lead to the increase in the radial drift velocity of dust grains in the accretion disks. In the case of high accretion rate, ≥ 10 −7 yr −1 , the magnetic field is also dynamically strong in the inner region of the disk, < 0.2 au. The simulations of the vertical structure of the disk show that, depending on the conditions on the surface of the disk, the vertical gradient of magnetic pressure can lead to both decrease and increase in the characteristic thickness of the disk as compared to the hydrostatic one by 5-20 %. The change in the thickness of the disk occurs outside the region of low ionization fraction and effective magnetic diffusion ('dead' zone), which extends from = 0.3 to 20 au at typical parameters.
Formation and Evolution of Disks Around Young Stellar Objects
Space Science Reviews, 2020
Recent observations have suggested that circumstellar disks may commonly form around young stellar objects. Although the formation of circumstellar disks can be a natural result of the conservation of angular momentum in the parent cloud, theoretical studies instead show disk formation to be difficult from dense molecular cores magnetized to a realistic level, owing to efficient magnetic braking that transports a large fraction of the angular momentum away from the circumstellar region. We review recent progress in the formation and early evolution of disks around young stellar objects of both low-mass and high-mass, with an emphasis on mechanisms that may bridge the gap between observation and theory, including non-ideal MHD effects and asymmetric perturbations in the collapsing core (e.g., magnetic field misalignment and turbulence). We also address the associated processes of outflow launching and the formation of multiple systems, and discuss possible implications in properties ...
Two-component magnetohydrodynamical outflows around young stellar objects
Astronomy and Astrophysics, 2006
Context. We present the first-ever simulations of non-ideal magnetohydrodynamical (MHD) stellar magnetospheric winds coupled with discdriven jets where the resistive and viscous accretion disc is self-consistently described. Aims. These innovative MHD simulations are devoted to the study of the interplay between a stellar wind (having different ejection mass rates) and an MHD disc-driven jet embedding the stellar wind. Methods. The transmagnetosonic, collimated MHD outflows are investigated numerically using the VAC code. We first investigate the various angular momentum transports occurring in the magneto-viscous accretion disc. We then analyze the modifications induced by the interaction between the two components of the outflow. Results. Our simulations show that the inner outflow is accelerated from the central object's hot corona thanks to both the thermal pressure and the Lorentz force. In our framework, the thermal acceleration is sustained by the heating produced by the dissipated magnetic energy due to the turbulence. Conversely, the outflow launched from the resistive accretion disc is mainly accelerated by the magneto-centrifugal force. Conclusions. The simulations show that the MHD disc-driven outflow extracts angular momentum more efficiently than do viscous effects in near-equipartition, thin-magnetized discs where turbulence is fully developed. We also show that, when a dense inner stellar wind occurs, the resulting disc-driven jet has a different structure, namely a magnetic structure where poloidal magnetic field lines are more inclined because of the pressure caused by the stellar wind. This modification leads to both an enhanced mass-ejection rate in the disc-driven jet and a larger radial extension that is in better agreement with the observations, besides being more consistent.
Accretion funnels onto weakly magnetized young stars
Astronomy & Astrophysics, 2008
Aims : We re-examine the conditions required to steadily deviate an accretion flow from a circumstellar disc into a magnetospheric funnel flow onto a slow rotating young forming star. Methods : New analytical constraints on the formation of accretion funnels flows due to the presence of a dipolar stellar magnetic field disrupting the disc are derived. The Versatile Advection Code is used to confirm these constraints numerically. Axisymmetric MHD simulations are performed, where a stellar dipole field enters the resistive accretion disc, whose structure is self-consistently computed. Results : The analytical criterion derived allows to predict a priori the position of the truncation radius from a non perturbative accretion disc model. Accretion funnels are found to be robust features which occur below the co-rotation radius, where the stellar poloidal magnetic pressure becomes both at equipartition with the disc thermal pressure and is comparable to the disc poloidal ram pressure. We confirm the results of Romanova et al. 2002 and find accretion funnels for stellar dipole fields as low as 140 G in the low accretion rate limit of 10−9Modot.yr−110^{-9} M_\odot.yr^{-1}10−9Modot.yr−1. With our present numerical setup with no disc magnetic field, we found no evidence of winds, neither disc driven nor X-winds, and the star is only spun up by its interaction with the disc. Conclusions : Weak dipole fields, similar in magnitude to those observed, lead to the development of accretion funnel flows in weakly accreting T Tauri stars. However, the higher accretion observed for most T Tauri stars (${\dot M} \sim 10^{-8} M_\odot.yr^{-1}$) requires either larger stellar field strength and/or different magnetic topologies to allow for magnetospheric accretion.
Outflows from Young Stars: Theory and Observation
Symposium - International Astronomical Union, 2004
Recent observations have revealed that young stellar objects are associated with jet-like structures and Herbig-Haro objects emitting at wavelengths ranging from optical lines to radio continua. These phenomena are similar in morphologies, and have mostly comparable energetics, dynamics, and kinematics. Probing such phenomena observed at various wavelengths with self-consistent physical and radiative processes arising within an inner disk-wind driven magnetocentrifugally from the circumstellar accretion disk is a challenge for confronting theory and observation of outflows. How such early outflow phase may play a role in forming planetary materials may help solve puzzles posed by meteorites. We will discuss the relevant observations, theoretical foundations for modelling approaches, magnetic structures and dynamical effects, and the connection to the early solar system.
Magnetized Disks Around Young Stars
Revista mexicana de astronomía y astrofísica
We discuss the structure and evolution of a magnetized accretion disks around young stars that have dragged their magnetic �eld in the process of gravitational collapse. The disk evolves due to two di�usive processes: viscous stresses that redistribute mass and angular momentum, and the resistive di�usion of mass across magnetic �eld lines due to imperfect conduction. In steady-state there is an analytic model of the structure of these magnetized disks. We discuss the application of this model to disks around low and high mass young stars and recent results of time dependent models.
Modeling of Disk-Star Interaction: Different Regimes of Accretion and Variability
AIP Conference Proceedings, 2008
The appearance and time variability of accreting millisecond X-ray pulsars (hereafter AMXPs, e.g. Wijnands & van der Klis 1998) depends strongly on the accretion rate, the effective viscosity and the effective magnetic diffusivity of the disk-magnetosphere boundary. The accretion rate is the main parameter which determines the location of the magnetospheric radius of the star for a given stellar magnetic field. We introduce a classification of accreting neutron stars as a function of the accretion rate and show the corresponding stages obtained from our global 3D magnetohydrodynamic (MHD) simulations and from our axisymmetric MHD simulations. We discuss the expected variability features in each stage of accretion, both periodic and quasi-periodic (QPOs). We conclude that the periodicity may be suppressed at both very high and very low accretion rates. In addition the periodicity may disappear when ordered funnel flow accretion is replaced by disordered accretion through the interchange instability.