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 $a_{\mathrm{mft}} = 0.1H$, $0.5 H$ or $1H$, where $H$ 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 $a_{\mathrm{mft}}=0.1H$.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 $\sim 1000$ K near the inner boundary of the `dead' zone, $r\approx (0.5-1)$ au, and by $\sim 100$ K near its outer boundary, $r\approx (30-50)$ 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, $35 \leq T \leq 95$ 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 $4-60$ Myr, minimum and the most unstable wavelengths lie in ranges $0.05-0.5$ and $0.5-5$ 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 $\dot{M}$ and dust grain sizes $a_d$. 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 \dot{M}=10^{-8}\,M_\odot\,\mbox{yr}^{-1} and $a_d=0.1 \mu$m. In the case of large dust grains, $a_d\geq 1$ mm, the magnetic field is frozen into the gas and a dynamically strong magnetic field is generated at radial distances from the star $r\gtrsim 30$ au, the tensions of which slow down the rotation speed by $\lesssim 1.5$ % 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, \dot{M}\geq 10^{-7}\,M_\odot\,\mbox{yr}^{-1}, the magnetic field is also dynamically strong in the inner region of the disk, $r<0.2$ 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 $r=0.3$ to $20$ au at typical parameters.Comment: Accepted to Astronomy Reports, 12 pages, 5 figures, 1 tabl

Dynamics of magnetic flux tubes in accretion disks of Herbig Ae/Be stars

The dynamics of magnetic flux tubes (MFTs) in the accretion disk of typical Herbig Ae/Be star (HAeBeS) with fossil large-scale magnetic field is modeled taking into account the buoyant and drag forces, radiative heat exchange with the surrounding gas, and the magnetic field of the disk. The structure of the disk is simulated using our magnetohydrodynamic model, taking into account the heating of the surface layers of the disk with the stellar radiation. The simulations show that MFTs periodically rise from the innermost region of the disk with speeds up to 10-12 km s - 1 {{\rm{s}}}^{-1}. MFTs experience decaying magnetic oscillations under the action of the external magnetic field near the disk's surface. The oscillation period increases with distance from the star and initial plasma beta of the MFT, ranging from several hours at r = 0.012 au r=0.012\hspace{0.33em}{\rm{au}} up to several months at r = 1 au r=1\hspace{0.33em}{\rm{au}}. The oscillations are characterized by pulsations of the MFT's characteristics including its temperature. We argue that the oscillations can produce observed IR-variability of HAeBeSs, which would be more intense than in the case of T Tauri stars, since the disks of HAeBeSs are hotter, denser, and have stronger magnetic field. © 2022 Sergey A. Khaibrakhmanov and Alexander E. Dudorov, published by De Gruyter.Russian Science Foundation, RSF: 19-72-10012Funding information : This work was supported by the Russian Science Foundation (project 19-72-10012

Evolution of protostellar disks around massive stars

С помощью численного МГД-моделирования исследуются образование и эволюция протозвездных дисков с остаточным магнитным полем. Расчеты проводятся с помощью двумерного кода Enlil, предназначенного для моделирования осесимметричных самогравитирующих МГД-течений. Рассматриваются образование и эволюция дисков вокруг звезд с массами 5 10M⊙ при различных значениях начальной угловой скорости и интенсивности полоидального магнитного поля. Изучается влияние омической диффузии и магнитной амбиполярной диффузии на эволюцию магнитного потока протозвездных дисков. Результаты расчетов используются для интерпретации наблюдаемых свойств протозвездных дисков в областях образования массивных звезд.We investigate formation and evolution of protostellar disks with fossil magnetic field using numerical MHD simulations. The simulations are carried out with the help of two-dimensional code Enlil developed for modelling of the axysymmetric self-graviting MHD flows. We consider the formation and evolution of the disks around stars with masses 5—10M⊙ for various initial angular velocities and magnetic field strengths. The influence of Ohmic diffusion and magnetic ambipolar diffusion on the evolution of the magnetic flux of the protostellar disks is studied. The results of the simulations are applied to interpret the observational properties of the protostellar disks in the regions of high mass star formation.Работа выполнена при поддержке гранта РНФ 15-12-10017