scholarly journals Global axisymmetric simulations of photoevaporation and magnetically driven protoplanetary disk winds

2019 ◽  
Vol 633 ◽  
pp. A21 ◽  
Author(s):  
P. J. Rodenkirch ◽  
H. Klahr ◽  
C. Fendt ◽  
C. P. Dullemond

Context. Photoevaporation and magnetically driven winds are two independent mechanisms that remove mass from protoplanetary disks. In addition to accretion, the effect of these two principles acting concurrently could be significant, and the transition between them has not yet been extensively studied and quantified. Aims. In order to contribute to the understanding of disk winds, we present the phenomena emerging in the framework of two-dimensional axisymmetric, nonideal magnetohydrodynamic simulations including extreme-ultraviolet (EUV) and X-ray driven photoevaporation. Of particular interest are the examination of the transition region between photoevaporation and magnetically driven wind, the possibility of emerging magnetocentrifugal wind effects, and the morphology of the wind itself, which depends on the strength of the magnetic field. Methods. We used the PLUTO code in a two-dimensional axisymmetric configuration with additional treatment of EUV and X-ray heating and dynamic ohmic diffusion based on a semi-analytical chemical model. Results. We determine that the transition between the two outflow types occurs for values of the initial plasma beta β ≥ 107, while magnetically driven winds generally outperform photoevaporation for stronger fields. In our simulations we observe irregular and asymmetric outflows for stronger magnetic fields. In the weak-field regime, the photoevaporation rates are slightly lowered by perturbations of the gas density in the inner regions of the disk. Overall, our results predict a wind with a lever arm smaller than 1.5, consistent with a hot magnetothermal wind. Stronger accretion flows are present for values of β < 107.

2020 ◽  
Vol 902 (2) ◽  
pp. 103
Author(s):  
Taichi Igarashi ◽  
Yoshiaki Kato ◽  
Hiroyuki R. Takahashi ◽  
Ken Ohsuga ◽  
Yosuke Matsumoto ◽  
...  

2016 ◽  
Vol 11 (S322) ◽  
pp. 43-49
Author(s):  
Monika Mościbrodzka

AbstractThe Galactic center is a perfect laboratory for testing various theoretical models of accretion flows onto a supermassive black hole. Here, I review general relativistic magnetohydrodynamic simulations that were used to model emission from the central object - Sgr A*. These models predict dynamical and radiative properties of hot, magnetized, thick accretion disks with jets around a Kerr black hole. Models are compared to radio-VLBI, mm-VLBI, NIR, and X-ray observations of Sgr A*. I present the recent constrains on the free parameters of the model such as accretion rate onto the black hole, the black hole angular momentum, and orientation of the system with respect to our line of sight.


Activations and disruptions of dark Ha filaments are very common phenomena on the Sun. They precede the most powerful two-ribbon solar flares, but they also appear far from any active region without any chromospheric flaring. Therefore, until very recently, filament disruptions were considered as interesting, but physically insignificant, flare precursors. Only Skylab observations have shown that the filament disruptions actually represent one of the basic and most important mechanisms of solar activity. These observations have revealed (1) that many coronal transients originate in eruptive filaments without chromospheric flares, (2) that Bruzek’s slow-mode waves originate in disrupted filaments and not in flares themselves, and (3) that many coronal X-ray enhancements outside active regions are also tops of newly formed loops, similar to the post-flare loops observed after filament disruptions in active regions. An interpretation of these data stems from Kopp & Pneuman’s theory of postflare loops: the process that disrupts a filament opens the magnetic field and causes a greatly enhanced mass-flow along the field lines. The open field lines subsequently reconnect, starting from the bottom of the corona and proceeding upwards. This process can last for many hours. Hot loops are first seen in X-rays, later in extreme ultraviolet (e.u.v.) lines, and, after an appropriate cooling time, in Hx as the loop prominence systems. The visibility of loops depends on plasma density. Several observed properties of solar flares indicate that the primary acceleration occurs as the field lines reconnect. Thus the process of particle acceleration in two ribbon flares can last for hours. Because reconnection is accomplished after essentially all filament disruptions, ‘disparitions brusques’ outside active regions should also accelerate particles.


2004 ◽  
Vol 219 ◽  
pp. 541-545 ◽  
Author(s):  
P. S. Cally ◽  
M. Dikpati ◽  
P. A. Gilman

Two dimensional magneto-shear instabilities in the solar tachocline have been extensively explored in recent years. One of their most notable traits over a wide range of shear and magnetic profiles is a propensity for the magnetic field to tip substantially from its initial axisymmetric configuration, with possible implications for patterns of flux emergence. However, it is found that modifications of the standard models to include either kinetic and magnetic drag, or prograde toroidal velocity jets associated with magnetic bands, can suppress the instabilities, or considerably reduce their nonlinear development. In the case of tip reduction by jets, for a toroidal field of around 100kG in the tachocline (required for sunspots to emerge in sunspot latitudes), simulations indicate that jets capable of reducing tipping below the limits of detection from sunspot patterns at the surface are potentially detectable by helioseismic methods, and should be looked for. Establishing an upper limit to the jet may result in a lower limit for the amount of tipping to be expected.


Author(s):  
K. H. Downing ◽  
S. G. Wolf ◽  
E. Nogales

Microtubules are involved in a host of critical cell activities, many of which involve transport of organelles through the cell. Different sets of microtubules appear to form during the cell cycle for different functions. Knowledge of the structure of tubulin will be necessary in order to understand the various functional mechanisms of microtubule assemble, disassembly, and interaction with other molecules, but tubulin has so far resisted crystallization for x-ray diffraction studies. Fortuitously, in the presence of zinc ions, tubulin also forms two-dimensional, crystalline sheets that are ideally suited for study by electron microscopy. We have refined procedures for forming the sheets and preparing them for EM, and have been able to obtain high-resolution structural data that sheds light on the formation and stabilization of microtubules, and even the interaction with a therapeutic drug.Tubulin sheets had been extensively studied in negative stain, demonstrating that the same protofilament structure was formed in the sheets and microtubules. For high resolution studies, we have found that the sheets embedded in either glucose or tannin diffract to around 3 Å.


2006 ◽  
Vol 132 ◽  
pp. 87-90
Author(s):  
M. El Kazzi ◽  
G. Delhaye ◽  
S. Gaillard ◽  
E. Bergignat ◽  
G. Hollinger

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