scholarly journals A new viscous instability in weakly ionised protoplanetary discs

2010 ◽  
Vol 6 (S274) ◽  
pp. 50-55
Author(s):  
Anders Johansen ◽  
Mariko Kato ◽  
Takayoshi Sano
Keyword(s):  
Transition Region ◽  
Turbulent Viscosity ◽  
Dead Zone ◽  
High Amplitude ◽  
Linear Instability ◽  
Outer Edge ◽  
Free Electrons ◽  
Viscous Instability ◽  
Pile Up ◽  
Protoplanetary Discs

AbstractLarge regions of protoplanetary discs are believed to be too weakly ionised to support magnetorotational instabilities, because abundant tiny dust grains soak up free electrons and reduce the conductivity of the gas. At the outer edge of this “dead zone”, the ionisation fraction increases gradually and the resistivity drops until the magnetorotational instability can develop turbulence. We identify a new viscous instability which operates in the semi-turbulent transition region between “dead” and “alive” zones. The strength of the saturated turbulence depends strongly on the local resistivity in this transition region. A slight increase (decrease) in dust density leads to a slight increase (decrease) in resistivity and a slight decrease (increase) in turbulent viscosity. Such spatial variation in the turbulence strength causes a mass pile-up where the turbulence is weak, leading to a run-away process where turbulence is weakened and mass continues to pile up. The final result is the appearance of high-amplitude pressure bumps and deep pressure valleys. Here we present a local linear stability analysis of weakly ionised accretion discs and identify the linear instability responsible for the pressure bumps. A paper in preparation concerns numerical results which confirm and expand the existence of the linear instability.

scholarly journals On the vortex evolution in non-isothermal protoplanetary discs

2020 ◽  
Vol 493 (2) ◽  
pp. 3014-3025
Author(s):  
D Tarczay-Nehéz ◽  
Zs Regály ◽  
E Vorobyov
Keyword(s):  
Large Scale ◽  
Planet Formation ◽  
Dead Zone ◽  
Giant Planet ◽  
Building Blocks ◽  
Outer Edge ◽  
Wave Instability ◽  
Large Scale Vortex ◽  
Self Gravity ◽  
Protoplanetary Discs

ABSTRACT It is believed that large-scale horseshoe-like brightness asymmetries found in dozens of transitional protoplanetary discs are caused by anticyclonic vortices. These vortices can play a key role in planet formation, as mm-sized dust – the building blocks of planets – can be accumulated inside them. Anticyclonic vortices are formed by the Rossby wave instability, which can be excited at the gap edges opened by a giant planet or at sharp viscosity transitions of accretionally inactive regions. It is known that vortices are prone to stretching and subsequent dissolution due to disc self-gravity for canonical disc masses in the isothermal approximation. To improve the hydrodynamic model of protoplanetary discs, we include the disc thermodynamics in our model. In this paper, we present our results on the evolution of the vortices formed at the outer edge of an accretionally inactive region (dead zone) assuming an ideal equation of state and taking PdV work, disc cooling in the β-approximation, and disc self-gravity into account. Thermodynamics affects the offset and the mode number (referring to the number of small vortices at the early phase) of the RWI excitation, as well as the strength, shape, and lifetime of the large-scale vortex formed through merging of the initial small vortices. We found that the inclusion of gas thermodynamics results in stronger, however decreased lifetime vortices. Our results suggest that a hypothetical vortex-aided planet formation scenario favours effectively cooling discs.

scholarly journals 3D global simulations of proto-planetary disk with dynamically evolving outer edge of dead zone

2010 ◽  
Vol 6 (S276) ◽  
pp. 407-408
Author(s):  
Natalia Dzyurkevich ◽  
Neal J. Turner ◽  
Willy Kley ◽  
Hubert Klahr ◽  
Thomas Henning
Keyword(s):  
Turbulent Viscosity ◽  
Dead Zone ◽  
Cosmic Ray ◽  
Outer Edge ◽  
Smooth Transition ◽  
Dust Grains ◽  
X Ray ◽  
Hard Edge ◽  
Mhd Simulations ◽  
Pressure Bump

Abstract3D global MHD simulations of magneto-driven turbulence are performed for the disk of 100 AU with reduced amount of 10μm fluffy dust grains. We use X-ray and cosmic ray ionization, as well as simplified treatment of recombination on dust grains. The ionization of gas and charging of dust grains are dynamically evolving during the simulation, making the zone of high magnetic dissipation (’dead’ zone) variable. In our simulations, the jump in MRI-driven turbulent viscosity inside and outside of dead zone is insignificant. We find no hard edge, but rather a smooth transition between active and dead zone. Subsequently, there is no visible pressure bump at outer edge of the dead zone.

scholarly journals Toward Consistent Models of Protoplanetary Discs

2004 ◽  
Vol 202 ◽  
pp. 359-361
Author(s):  
Mauricio Reyes-Ruiz
Keyword(s):  
Dead Zone ◽  
Linear Analysis ◽  
Vertical Stratification ◽  
Magnetorotational Instability ◽  
Vertical Diffusivity ◽  
Magnetic Diffusivity ◽  
Protoplanetary Discs

In this paper we present results on the effect of the vertical stratification of magnetic diffusivity, expected in current models of protoplanetary discs, on the development of the magnetorotational instability. Specifically, on the basis of a quasi-global, linear analysis we study the operation of the magnetorotational instability across the so-called dead zone of protoplanetary discs. Our results indicate that the predicted strong vertical diffusivity gradients can damp the instability in such regions. This suggests the necessity of a revision of current models for the structure and evolution of protoplanetary discs.

scholarly journals Testing viscous disc theory using the balance between stellar accretion and external photoevaporation of protoplanetary discs

2020 ◽  
Vol 497 (1) ◽  
pp. L40-L45
Author(s):  
Andrew J Winter ◽  
Megan Ansdell ◽  
Thomas J Haworth ◽  
J M Diederik Kruijssen
Keyword(s):  
Angular Momentum ◽  
Mass Loss ◽  
Central Star ◽  
Outer Edge ◽  
Momentum Transport ◽  
Angular Momentum Transport ◽  
Accretion Rates ◽  
Novel Method ◽  
Viscous Models ◽  
Protoplanetary Discs

ABSTRACT The nature and rate of (viscous) angular momentum transport in protoplanetary discs (PPDs) have important consequences for the formation process of planetary systems. While accretion rates on to the central star yield constraints on such transport in the inner regions of a PPD, empirical constraints on viscous spreading in the outer regions remain challenging to obtain. Here, we demonstrate a novel method to probe the angular momentum transport at the outer edge of the disc. This method applies to PPDs that have lost a significant fraction of their mass due to thermal winds driven by UV irradiation from a neighbouring OB star. We demonstrate that this external photoevaporation can explain the observed depletion of discs in the 3–5 Myr old σ Orionis region, and use our model to make predictions motivating future empirical investigations of disc winds. For populations of intermediate-age PPDs, in viscous models we show that the mass flux outwards due to angular momentum redistribution is balanced by the mass-loss in the photoevaporative wind. A comparison between wind mass-loss and stellar accretion rates therefore offers an independent constraint on viscous models in the outer regions of PPDs.

scholarly journals Thermal-viscous instability in tilted accretion disks: A possible application to IW Andromeda-type dwarf novae

2020 ◽  
Vol 72 (2) ◽  
Author(s):  
Mariko Kimura ◽  
Yoji Osaki ◽  
Taichi Kato ◽  
Shin Mineshige
Keyword(s):  
Accretion Disks ◽  
Outer Edge ◽  
Light Curves ◽  
Dwarf Novae ◽  
Transfer Rates ◽  
Secondary Star ◽  
Viscous Instability ◽  
Simplifying Assumptions ◽  
Nova Outbursts

Abstract IW And stars are a subgroup of dwarf novae characterized by repetition of the intermediate brightness state with oscillatory variations terminated by brightening. This group of dwarf novae is also known to exhibit a wide variety even within one system in long-term light curves, including the usual dwarf-nova outbursts, Z Cam-type standstills, and so on, besides the typical IW And-type variations mentioned above. Following recent observations suggesting that some IW And stars seem to have tilted disks, we have investigated how the thermal-viscous instability works in tilted accretion disks in dwarf novae and whether it could reproduce the essential features of the light curves in IW And stars. By adopting various simplifying assumptions for tilted disks, we have performed time-dependent one-dimensional numerical simulations of a viscous disk by taking into account various mass supply patterns to the disk; that is, the gas stream from the secondary star flows not only to the outer edge of the disk but also to the inner portions of the disk. We find that tilted disks can achieve a new kind of accretion cycle, in which the inner disk almost always stays in the hot state while the outer disk repeats outbursts, thereby reproducing alternating mid-brightness intervals with dips and brightening, which are quite reminiscent of the most characteristic observational light variations of IW And stars. Further, we have found that our simulations produce diverse light variations, depending on different mass supply patterns even without time variations in mass transfer rates. This could explain the wide variety in long-term light curves of IW And stars.

scholarly journals Snow lines can be thermally unstable

2020 ◽  
Vol 495 (3) ◽  
pp. 3160-3174 ◽  
Author(s):  
James E Owen
Keyword(s):  
Temperature Profile ◽  
Thermal Instability ◽  
Solid Phase ◽  
Turbulent Viscosity ◽  
Vapour Phase ◽  
Snow Line ◽  
Total Opacity ◽  
Planetary Migration ◽  
Solid Distribution ◽  
Protoplanetary Discs

ABSTRACT Volatile species in protoplanetary discs can undergo a phase change from vapour to solid. These ‘snow lines’ can play vital roles in planet formation at all scales, from dust coagulation to planetary migration. In the outer regions of protoplanetary discs, the temperature profile is set by the absorption of reprocessed stellar light by the solids. Further, the temperature profile sets the distribution of solids through sublimation and condensation at various snow lines. Hence, the snow line position depends on the temperature profile and vice versa. We show that this coupling can be thermally unstable, such that a patch of the disc at a snow line will produce either runaway sublimation or condensation. This thermal instability arises at moderate optical depths, where heating by absorption of reprocessed stellar light from the disc’s atmosphere is optically thick, yet cooling is optically thin. Since volatiles in the solid phase drift much faster than volatiles in the vapour phase, this thermal instability results in a limit cycle. The snow line progressively moves in, condensing volatiles, before receding, as the volatiles sublimate. Using numerical simulations, we study the evolution of the carbon monoxide (CO) snow line. We find the CO snow line is thermally unstable under typical disc conditions and evolves inwards from ∼50 to ∼30 au on time-scales from 1000 to 10 000 yr. The CO snow line spends between ${\sim}10{{\ \rm per\ cent}}\,\mathrm{ and}\,50{{\ \rm per\ cent}}$ of its time at smaller separations, where the exact value is sensitive to the total opacity and turbulent viscosity. The evolving snow line also creates ring-like structures in the solid distribution interior to the snow line. Multiple ring-like structures created by moving snow lines could potentially explain the substructures seen in many ALMA images.

scholarly journals Experiments on the dark space in vacuum tubes

1907 ◽  
Vol 79 (528) ◽  
pp. 98-117 ◽  
Keyword(s):  
Positive Column ◽  
Outer Boundary ◽  
Mean Free Path ◽  
Outer Edge ◽  
Free Electrons ◽  
Positive Ions ◽  
Negative Pole ◽  
Dark Space ◽  
Narrow Space ◽  
Layer Coating

1. When an induction spark passes through an exhausted vacuum tube we see, firstly, a luminous layer coating the cathode, next a dark space; beyond the outer edge of this dark space comes a luminous envelope, then another blank, sometimes called “Faraday’s dark space,” and, lastly, the positive column. Between the second dark space and the positive column, if the exhaustion is suitable, stratifications occur. In the present paper I speak of the first dark space extending from the luminous layer on the cathode to a more or less sharply defined luminous boundary. The luminous coating on the cathode is produced by the ionisation of the atoms of residual gas and the union of the electrons from the metal with the positive ions, with liberation of a further jet of electrons starting from the neighbourhood of the cathode with velocities of the order of that of light. 2. The dark space is a measure of the mean free path of the electrons, and its outer luminous margin is the scene of the collisions between free electrons and the column of positive ions. It varies in size with the degree of exhaustion. At a pressure of about 4 mm. it begins to appear as a narrow space, a fraction of a millimetre removed from the negative pole, and grows larger as the exhaustion increases. At a pressure of about 3 mm. the margin of the dark space is about 4 mm. from the negative pole. At an exhaustion of 0·25 mm. it is about the best size for such work as I now describe. When the exhaustion is pushed, further, the outer boundary becomes indistinct and soon fades away, the dark space now filling the tube, the walls of which glow with a phosphorescent light. It is to the phenomena occurring within this dark space that I have devoted years of work, and I now have the honour of presenting to the Society some account of the results of my prolonged investigations; in parts they lead to conclusions which have been already made public by other observers.

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