12 research outputs found
Magnetic field buoyancy in accretion disks of young stars
Buoyancy of the fossil magnetic field in the accretion disks of young stars
is investigated. It is assumed that the Parker instability leads to the
formation of slender flux tubes of toroidal magnetic field in the regions of
effective magnetic field generation. Stationary solution of the induction
equation is written in the form in which buoyancy is treated as the additional
mechanism of the magnetic flux escape. We calculate the fossil magnetic field
intensity in the accretion disks of young T Tauri stars for the cases when
radius of the magnetic flux tubes , or ,
where is the accretion disk height scale. Calculations show that the
buoyancy limits toroidal magnetic field growth, so that its strength is
comparable with the vertical magnetic field strength for the case
.Comment: published in PEPAN Letter
Influence of Ohmic and ambipolar heating on thermal structure of accretion discs
We investigate dynamics of accretion discs of young stars with fossil
large-scale magnetic field. Our magneto-gas-dynamic (MHD) model of the
accretion discs includes equations of Shakura and Sunyaev, induction equation,
equations of thermal and collisional ionization. Induction equation takes into
account Ohmic and magnetic ambipolar diffusion, magnetic buoyancy. We also
consider the influence of Ohmic and ambipolar heating on thermal structure of
the accretion discs. We analyse the influence of considered dissipative MHD
effects on the temperature of the accretion discs around classical T Tauri
star. The simulations show that Ohmic and ambipolar heating operate near the
borders of the region with low ionization fraction (`dead' zone). Temperature
grows by K near the inner boundary of the `dead' zone, au, and by K near its outer boundary,
au.Comment: 8 pages, 3 figures, The Third Russian Conference on
Magnetohydrodynamics, accepted for publication in a Special Issue of the
Magnetohydrodynamics Journa
Magnetic ionization-thermal instability
Linear analysis of the stability of diffuse clouds in the cold neutral medium
with uniform magnetic field is performed. We consider that gas in equilibrium
state is heated by cosmic rays, X-rays and electronic photoeffect on the
surface of dust grains, and it is cooled by the collisional excitation of fine
levels of the CII. Ionization by cosmic rays and radiative recombinations is
taken into account. A dispersion equation is solved analytically in the
limiting cases of small and large wave numbers, as well as numerically in the
general case. In particular cases the dispersion equation describes thermal
instability of Field (1965) and ionization-coupled acoustic instability of
Flannery and Press (1979). We pay our attention to magnetosonic waves arising
in presence of magnetic field, in thermally stable region,
K and density n\lessapprox 10^3\,\mbox{cm}^{-3}. We have shown that these
modes can be unstable in the isobarically stable medium. The instability
mechanism is similar to the mechanism of ionization-coupled acoustic
instability. We determine maximum growth rates and critical wavelengths of the
instability of magnetosonic waves depending on gas temperature, magnetic field
strength and the direction of wave vector with respect to the magnetic field
lines. The minimum growth time of the unstable slow magnetosonic waves in
diffuse clouds is of Myr, minimum and the most unstable wavelengths lie
in ranges and pc, respectively. We discuss the application
of considered instability to the formation of small-scale structures and the
generation of MHD turbulence in the cold neutral medium.Comment: 11 pages, 9 figures, 2 tables, accepted for publication in MNRA
Dynamics of magnetized accretion disks of young stars
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 consider the dynamical influence of the
magnetic field on gas rotation speed and vertical structure of the disks. With
the help of the developed MHD model, the structure of an accretion disk of a
solar mass T Tauri star is simulated for different accretion rates
and dust grain sizes . 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
\dot{M}=10^{-8}\,M_\odot\,\mbox{yr}^{-1} and m. In the case of
large dust grains, mm, the magnetic field is frozen into the gas
and a dynamically strong magnetic field is generated at radial distances from
the star au, the tensions of which slow down the rotation speed
by % 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, \dot{M}\geq
10^{-7}\,M_\odot\,\mbox{yr}^{-1}, the magnetic field is also dynamically
strong in the inner region of the disk, 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 to au at
typical parameters.Comment: Accepted to Astronomy Reports, 12 pages, 5 figures, 1 tabl
Accretion bursts in magnetized gas-dust protoplanetary disks
Aims and Methods. Accretion bursts triggered by the magnetorotational
instability (MRI) in the innermost disk regions were studied for protoplanetary
gas-dust disks formed from prestellar cores of various mass and
mass-to-magnetic flux ratio . Numerical magnetohydrodynamics
simulations in the thin-disk limit were employed to study the long-term (~Myr) evolution of protoplanetary disks with an adaptive turbulent
-parameter, which depends explicitly on the strength of the magnetic
field and ionization fraction in the disk. The numerical models also feature
the co-evolution of gas and dust, including the back-reaction of dust on gas
and dust growth. Results. Dead zone with a low ionization fraction and temperature on the order of several hundred Kelvin forms in the
inner disk soon after its formation, extending from several to several tens of
astronomical units depending on the model. The dead zone features pronounced
dust rings that are formed due to the concentration of grown dust particles in
the local pressure maxima. Thermal ionization of alkaline metals in the dead
zone trigger the MRI and associated accretion burst, which is characterized by
a sharp rise, small-scale variability in the active phase, and fast decline
once the inner MRI-active region is depleted of matter. The burst occurrence
frequency is highest in the initial stages of disk formation, and is driven by
gravitational instability (GI), but declines with diminishing disk mass-loading
from the infalling envelope. There is a causal link between the initial burst
activity and the strength of GI in the disk fueled by mass infall from the
envelope. Abridged.Comment: Accepted for publication in Astronomy & Astrophysic
Recent updates on the Maser Monitoring Organisation
The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy researchers and maser theoreticians pursuing a joint goal of reaching a deeper understanding of maser emission and exploring its variety of uses as tracers of astrophysical events. These proceedings detail the origin, motivations and current status of the M2O, as was introduced at the 2021 EVN symposium
Outflows and particle acceleration in the accretion disks of young stars
Magneto-gas-dynamic (MGD) outflows from the accretion disks of T Tauri stars with fossil large-scale magnetic fileld 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 fileld 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
Hierarchical structure of the interstellar molecular clouds and star formation
Properties of the hierarchical structures of interstellar molecular clouds are discussed. Particular attention is paid to the statistical correlations between velocity dispersion and size, and between the magnetic field strength and gas density. We investigate the formation of some hierarchical structures with the help of numerical MHD simulations using the ENLIL code. The simulations show that the interstellar molecular filaments with parallel magnetic field and molecular cores can form via the collapse and fragmentation of cylindrical molecular clouds. The parallelmagnetic field halts the radial collapse of the cylindrical cloud maintaining its nearly constant radius ~0.1 pc. The observed filaments with perpendicularmagnetic field can form as a result of themagnetostatic contraction of oblate molecular clouds under the action of Alfvén and MHD turbulence. The theoretical density profiles are fitted with the Plummer-like function and agree with observed profiles of the filaments in Gould’s Belt. The characteristics of molecular cloud cores found in our simulations are in agreement with observations