[en] We present an unified model of hard X-ray flares and bipolar plasma outflows observed in protostars. Assuming that the dipole magnetic field of the protostar threads the protostellar disk, we carried out 2.5-dimensional MHD simulations of the disk-star interaction. The closed magnetic loops connecting the central star and the disk are twisted by the rotation of the disk. In the presence of resistivity, the helicity injection from the disk leads to the magnetic reconnection in the current sheet formed inside the expanding loops. The released magnetic energy heats the flaring plasma up to 10⁸ K. The speed of the ejected hot plasmoid (rotating spheromak) is consistent with that of the optical jets (200 - 400 km s^-1). (author)

[en] The interaction of a bipolar molecular flow with interstellar condensations is examined (using the sweeping-magnetic-twist model) by performing 2.5-dimensional MHD numerical simulations. A torsional Alfven wave propagating with a hollow cylindrical bipolar flow interacts with the condensation, and the magnetic twist accumulates in the region between the flow and the condensation, since the Alfven velocity in the condensation is smaller than that in the ambient medium. The stored magnetic twist increases with time, causing various nonlinear effects, such as a pinching of the gas in the near-axis region, or an upward-acceleration of the gas in the condensation. Two cases for the condensation configuration are examined: spherical shape and ring-like shape. In the spherical case, the condensation, itself, is squeezed by the pinch effect. In the ring-shaped case, the hot gas near the axis is pinched and flow is generated along the axis due to an enhanced gas pressure resulting from the pinch. The velocity of the hot flow is comparable to that of the original cold bipolar flow. The results in the ring-shaped case may explain the observed characteristics in the velocity field around the blob observed in the L1551 flow, supporting the interpretation (Uchida et al. 1987; AAA 44.131.270) that the mass in the dense blob lying in the L1551 flow did not come from the source of the flow, but pre-existed there in the molecular cloud. (author)

[en] We present a nonsteady magnetodynamic mechanism for the formation of astrophysical jets in a magnetized accretion disk system. The dynamical processes in the contraction of a rotating disk, which is penetrated by a magnetic field parallel to the rotation axis, are investigated by using axially symmetric 2.5-dimensional MHD numerical simulations. As the rotating disk contracts, it pulls the magnetic field towards the center as well as to the azimuthal direction, producing a helically twisted magnetic field, and as the magnetic twist is accumulated and begins to relax along the poloidal field, the gas in the surface layers of the disk is pushed out to the polar directions by the J x B force with the relaxing magnetic twist. It is shown that the accelerated gas is collimated by the magnetic field and forms a supersonic bipolar jet which has a hollow cylindrical shell structure with helical motion in it. A considerable fraction of the gravitational potential energy released in the contraction of the disk is transformed to the kinetic energy of the jet through the action of the magnetic field. Also, angular momentum is carried away from the disk by the magnetic torque especially in the phase of the jet formation, and this allows the disk to keep contracting towards the gravitating center and can continue the ejection of the jet. (author)

[en] We present an MHD model for the galactic center lobes (GCL) by using an axisymmetric 2.5-dimensional MHD simulation. According to our model, GCL is a low-energy jet emanating from the H_II gas disk extending beyond r ∼ 100 pc from the galactic center. The model is based on the 'sweeping-magnetic-twist' mechanism developed by the authors for the production of cosmic jets, where the gas in the surface layer of the contracting disk is lifted up by the J x B force in the relaxing magnetic twist, which is generated by the interaction of the rotation of the contracting disk with the poloidal magnetic field. We incorporate the realistic gravitational potential suitable for the galactic center region, in which the rotational velocities are approximately constant for r = 20 - 100 pc. The difference between the models with this realistic potential and those with the potential due to a point mass is examined in detail. On the basis of the numerical results, we present a scenario for the formation of the GCL. (author)

[en] The accretion of magnetized nebular mass to a young star after the formation may take a form in which the accreted magnetized mass is first buffered by the stellar magnetic field, and then leaks into the stellar field region through magnetic reconnection at the magnetically neutral ring formed in the equatorial plane. The nebular mass is condensed in the buffering process, and the potential energy of this condensed mass is converted into the kinetic energy in the infall after the leakage. A high temperature region is produced when the infalling material crashes at the stellar surface and a shock produced in the crash propagates back upwards along the tail of the infalling material. The strength of the shock increases rapidly as the shock propagates into the tail of the inflow which has a large gradient in density, and the material in the tail is blown off by the shock along the magnetic field. The rate of mass loss in this mechanism, in which the flow is bent towards the polar directions by the effect of the magnetic field, can be as high as 10^-8 Msub(solar mass)yr^-1, and this, together with the morphology, suggests that the present mechanism may explain the relation of Herbig-Haro objects and/or of bipolar nebulae observed in millimetric CO lines to young stars. (author)

[en] An M dwarf’s atmosphere and wind are expected to be highly magnetized. The nonlinear propagation of Alfvén waves could play a key role in both heating the stellar atmosphere and driving the stellar wind. Using this Alfvén wave scenario, we carried out a one-dimensional compressive magnetohydrodynamic simulation to examine the nonlinear propagation of Alfvén waves from the M dwarf’s photosphere, chromosphere to the corona, and interplanetary space. Based on the simulation results, we developed a semi-empirical method describing the solar and M dwarf’s coronal temperature, stellar wind velocity, and wind’s mass-loss rate. We find that M dwarfs’ coronae tend to be cooler than the solar corona, and that M dwarfs’ stellar winds can be characterized as having a faster velocity and much smaller mass-loss rate compared to those of the solar wind.

[en] The nonlinear evolution of the Parker instability in magnetized gas disks (e.g, accretion disks, galactic gas disks) is studied by performing two-dimensional MHD numerical simulations, taking account of the spatial variation in the vertical gravitational acceleration. Unperturbed states consist of isothermal, magnetostatic gas layers with the spatially constant α (=the ratio of magnetic to gas pressure). The modes with the most unstable wavenumbers are examined. As the instability develops, the gas slides down the expanding magnetic loop, forming a dense structure in the valley. The growth of the perturbation is saturated when the maximum horizontal velocity of the downflow becomes comparable to the initial Alfven speed. The maximum velocity of the rising motion of the magnetic loop is 0.3 - 0.5 times that of the initial Alfven speed. When α > approx 1, (1) shock waves are formed in the downflow near the footpoint of the loop, (2) the dense spur or sheet, whose maximum compression rate is 2 - 80 for 1 ≤ α ≤ 10, is created in the valley, and (3) the outward gas motion occurs just above dense spurs. When α < 1, the shock wave is not formed and the system shows an oscillatory motion. (author)

[en] The linear stability of a magnetized gas layer is studied for accretion disks and galactic gas disks. The gas layer is assumed to be located at some distance from a point mass which is the origin of gravity. The magnetic fields are assumed to run parallel to the disk in a magnetohydrostatic equilibrium state. In this equilibrium, the sound speed and the Alfven speed are taken to be uniform. The medium is an ideal gas and perfectly conducting. The eigenfunctions and the growth rates are computed for isothermal perturbations. It is found that the most unstable mode breaks the inversion symmetry of the disk. In the perturbed state, some magnetic field lines run across the original disk plane, and the gas disk is warped. The two-dimensional configuration of the disk in the perturbed state is displayed graphically. In the unstable modes, the disturbance has a large amplitude in the region where the gravitational acceleration is large. In cold and-or magnetic pressure dominated disks, the minimum growth time is 2-5H-v_A, where v_A is the Alfved speed and H is the pressure scale height defined where the gravitational acceleration is maximum. (author)

[en] An M dwarf’s atmosphere is expected to be highly magnetized. The magnetic energy can be responsible for heating the stellar chromosphere and corona and driving the stellar wind. The nonlinear propagation of Alfvén waves is a promising mechanism for both heating the stellar atmosphere and driving the stellar wind. Based on this Alfvén wave scenario, we carried out a 1D compressive magnetohydrodynamic simulation to reproduce the stellar atmospheres and winds of TRAPPIST-1, Proxima Centauri, YZ CMi, AD Leo, AX Mic, and the Sun. The nonlinear propagation of Alfvén waves from the stellar photosphere to the chromosphere, corona, and interplanetary space is directly resolved in our study. The simulation result particularly shows that the slow shock generated through the nonlinear mode coupling of Alfvén waves is crucially involved in both the dynamics of the stellar chromosphere (stellar spicule) and stellar wind acceleration. Our parameter survey further revealed the following general trends of the physical quantities of the stellar atmosphere and wind. (1) The M dwarf coronae tend to be cooler and denser than the solar corona. (2) The M dwarf stellar winds can be characterized by a relatively faster velocity and much smaller mass-loss rate compared to those of the solar wind. The physical mechanisms behind these tendencies are clarified in this paper, where the stronger stratification of the M dwarf’s atmosphere and relatively smaller Alfvén wave energy input from the M dwarf’s photosphere are remarkable.

[en] The detailed structure and velocity field in the L1551 CO bipolar flows were observed by using the 45-m millimetric-wave telescope at Nobeyama. The observations were made in January and April of 1985 and supplemented in January of 1986 in the 115 GHz ¹²CO J = 1 - 0 line with spatial and spectral resolutions of 18 ench and 250 kHz (0.65 km s^-1 in velocity), respectively. It was revealed as the result that the bipolar flow lobes have a clear hollow cylindrical structure and that both lobes are likely to be spinning with a velocity of 1 - 2 km s^-1 in the same direction as that of the disklike object claimed by Kaifu et al. (1984; AAA 37.131.128) in the CS 49-GHz line. The longitudinal velocity of the flow increases with distance along the axis up to 0.15 pc from IRS-5, the central object. These characteristics coincide well with those predicted by the magnetodynamic theory proposed by Uchida and Shibata (1985; AAA 40.131.174), and indicate the essential importance of the magnetic field in producing such flows. It is also suggested that the angular momentum loss due to the magnetodynamic process is important in the star formation itself. (author)