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

Dynamics of magnetic flux tubes and IR-variability of young stellar objects

We simulate the dynamics of slender magnetic flux tubes (MFTs) in the accretion disks of T Tauri stars. The dynamical equations of our model take into account aerodynamic and turbulent drag forces, and the radiative heat exchange between the MFT and ambient gas. The structure of the disk is calculated with the help of our MHD model of the accretion disks. We consider the MFTs formed at distances of 0.027-0.8 au from the star with various initial radii and plasma betas β 0. The simulations show that MFTs with a weak magnetic field (β 0 = 10) rise slowly with speeds less than the speed of sound. MFTs with β 0 = 1 form an outflowing magnetized corona above the disk. Strongly magnetized MFTs (β 0 = 0.1) can cause outflows with velocities 20 - 50 km s-1. The tubes rise periodically over times from several days to several months according to our simulations. We propose that periodically rising MFTs can absorb stellar radiation and contribute to the IR-variability of young stellar objects. © 2018 National Astronomical Observatories, CAS and IOP Publishing Ltd.

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

Modeling of Protostellar Clouds and their Observational Properties

A physical model and two-dimensional numerical method for computing the evolution and spectra of protostellar clouds are described. The physical model is based on a system of magneto-gasdynamical equations, including ohmic and ambipolar diffusion, and a scheme for calculating the thermal and ionization structure of a cloud. The dust and gas temperatures are determined during the calculations of the thermal structure of the cloud. The results of computing the dynamical and thermal structure of the cloud are used to model the radiative transfer in continuum and in molecular lines. We presented the results for clouds in hydrostatic and thermal equilibrium. The evolution of a rotating magnetic protostellar cloud starting from a quasi-static state is also considered. Spectral maps for optically thick lines of linear molecules are analyzed. We have shown that the influence of the magnetic field and rotation can lead to a redistribution of angular momentum in the cloud and the formation of a characteristic rotational velocity structure. As a result, the distribution of the velocity centroid of the molecular lines can acquire an hourglass shape. We plan to use the developed program package together with a model for the chemical evolution to interpret and model observed starless and protostellar cores.Comment: Accepted to Astronomy Report

Dynamics of magnetic flux tubes in accretion discs of T Tauri stars

Dynamics of slender magnetic flux tubes (MFTs) in the accretion discs of T Tauri stars is investigated. We perform simulations taking into account buoyant, aerodynamic, and turbulent drag forces, radiative heat exchange between MFT and ambient gas, and magnetic field of the disc. The equations of MFT dynamics are solved using Runge-Kutta method of the fourth order. The simulations show that there are two regimes of MFT motion in absence of external magnetic field. In the region r < 0.2 au, the MFTs of radii 0.05 ≤ a0 ≤ 0.16, H (H is the scale height of the disc) with initial plasma beta of 1 experience thermal oscillations above the disc. The oscillations decay over some time, and MFTs continue upward motion afterwards. Thinner or thicker MFTs do not oscillate. MFT velocity increases with initial radius and magnetic field strength. MFTs rise periodically with velocities up to 5-15 km s-1 and periods of 0.5-10 yr determined by the toroidal magnetic field generation time. Approximately 20 per cent of disc mass and magnetic flux can escape to disc atmosphere via the magnetic buoyancy over characteristic time of disc evolution. MFTs dispersal forms expanding magnetized corona of the disc. External magnetic field causes MFT oscillations near the disc surface. These magnetic oscillations have periods from several days to 1-3 months at r < 0.6 au. The magnetic oscillations decay over few periods. We simulate MFT dynamics in accretion discs in the Chameleon I cluster. The simulations demonstrate that MFT oscillations can produce observed IR-variability of T Tauri stars. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society

Large-scale magnetic field in the accretion discs of young stars: The influence of magnetic diffusion, buoyancy and Hall effect

We investigate the fossil magnetic field in the accretion and protoplanetary discs using the Shakura and Sunyaev approach. The distinguishing feature of this study is the accurate solution of the ionization balance equations and the induction equation with Ohmic diffusion, magnetic ambipolar diffusion, buoyancy and the Hall effect.We consider the ionization by cosmic rays, X-rays and radionuclides, radiative recombinations, recombinations on dust grains and also thermal ionization. The buoyancy appears as the additional mechanism of magnetic flux escape in the steady-state solution of the induction equation. Calculations show that Ohmic diffusion and magnetic ambipolar diffusion constraint the generation of the magnetic field inside the 'dead' zones. The magnetic field in these regions is quasi-vertical. The buoyancy constraints the toroidal magnetic field strength close to the disc inner edge. As a result, the toroidal and vertical magnetic fields become comparable. The Hall effect is important in the regions close to the borders of the 'dead' zones because electrons are magnetized there. The magnetic field in these regions is quasi-radial. We calculate the magnetic field strength and geometry for the discs with accretion rates (10-8-10-6)M⊙ yr-1. The fossil magnetic field geometry does not change significantly during the disc evolution while the accretion rate decreases.We construct the synthetic maps of dust emission polarized due to the dust grain alignment by the magnetic field. In the polarization maps, the 'dead' zones appear as the regions with the reduced values of polarization degree in comparison to those in the adjacent regions. © 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society

Cosmic Magnetic Fields

The lecture is dedicated to observational and theoretical aspects of investigations of cosmic magnetic fields. Main methods for determination of the strength and geometry of the magnetic field are discussed. Observational data on the magnetic fields in astrophysical objects are given. We consider main theories of the cosmic magnetic field origin. Special attention is paid to the role of the magnetic field in star formation.Лекция посвящена наблюдательным и теоретическим аспектам исследований магнитных полей в космосе. Обсуждаются основные методы определения интенсивности и геометрии магнитного поля. Приводятся данные о магнитных полях астрофизических объектов. Рассматриваются основные теории происхождения магнитного поля космических объектов. Особое внимание уделяется роли магнитного поля в процессе звездообразования.Работа выполнена при поддержке Российского научного фонда (проект 18–12–00193), Российского фонда фундаментальных исследований (проект 18–02–01067) и Министерства науки и высшего образования Российской Федерации (базовая часть госзадания, РК № AAAA–A17–117030310283–7)

Magnetic Ionization-Thermal Instability in HII Regions

Magnetic ionization-thermal instability (MITI) in H II regions is investigated. Non-stationary ionization, cooling and heating processes are taken into account. With normal modes method the dispersion relation was obtained. The dispersion equation is solved numerically with Bairstow method. Complex roots of dispersion equations for some cooling and heating functions, different parameters of temperature, magnetic field and ionization rates are determined. It was shown, that condensations in H II regions can be consequence of growing unstable mods of MITI. Other applications of MITI are discussed.Исследуется магнитная ионизационно-тепловая неустойчивость (МИТН) в областях ионизованного водорода. В модели учитываются нестационарная ионизация и объемные процессы нагрева и охлаждения. Методом нормальных мод получено дисперсионное соотношение, которое решается численно методом Берстоу. Комплексные корни дисперсионных уравнений получены для некоторых функций нагрева и охлаждения, а также различных параметров температуры, магнитного поля и скорости ионизации. Показано, что развитие неустойчивых мод МИТН может привести к образованию конденсаций в областях ионизованного водорода H II. Обсуждаются другие приложения МИТН.Работа выполнена при поддержке гранта РФФИ 1802–01067/18

Influence of the magnetic field on the formation of protostellar disks

We numerically model the collapse of magnetic rotating protostellar clouds with mass of 10 M The simulations are carried out with the help of 2D MHD code Enlil. The structure of the cloud at the isothermal stage of the collapse is investigated for the cases of weak, moderate, and strong initial magnetic field. Simulations reveal the universal hierarchical structure of collapsing protostellar clouds, consisting of the flattened envelope with the qausi-magnetostatc disk inside and the first core in its center. The size of the primary disk increases with the initial magnetic energy of the cloud. The magnetic braking efficiently transports the angular momentum from the primary disk into the envelope in the case, when the initial magnetic energy of the cloud is more than 20% of its gravitational energy. The intensity of the outflows launched from the region near the boundary of the first core increases with initial magnetic energy. The "dead"zone with small ionization fraction, x < 1 0-11 x< 1{0){-11}, forms inside the first hydrostatic core and at the base of the outflow. Ohmic dissipation and ambipolar diffusion determine conditions for further formation of the protostellar disk in this region. © 2022 Natalya S. Kargaltseva et al., published by De Gruyter.Russian Science Foundation, RSF: 19-72-10012Funding information: This work is financially supported by the Russian Science Foundation (project 19-72-10012)