We explore the properties of cold gravitoturbulent accretion disks - non-fragmenting disks hovering on the verge of gravitational instability - using a realistic prescription for the effective viscosity caused by gravitational torques. This prescription is based on a direct relationship between the angular momentum transport in a thin accretion disk and the disk cooling in a steady state. Assuming that opacity is dominated by dust we are able to self-consistently derive disk properties for a given $\dot M$ assuming marginal gravitational stability. We also allow external irradiation of the disk and account for a non-zero background viscosity which can be due to the MRI. Spatial transitions between different co-existing disk states (e.g. between irradiated and self-luminous or between gravitoturbulent and viscous) are described and the location of the boundary at which disk must fragment is determined in a variety of situations. We demonstrate in particular that at low enough $\dot M$ external irradiation stabilizes gravitoturbulent disk against fragmentation all the way to infinity thus providing means of steady mass transport to the central object. Implications of our results for the possibility of planet formation by gravitational instability in protoplanetary disks and star formation in the Galactic Center and for the problem of feeding supermassive black holes in galactic nuclei are discussed.
The turbulence driven by gravitational instabilities (GIs) can amplify magnetic fields in massive gaseous disks. This GI dynamo may appear in young circumstellar disks, whose weak ionization challenges other amplification routes, as well as in active galactic nuclei. Although regarded as a large-scale dynamo, only local simulations have so far described its kinematic regime. We study the GI dynamo in global magnetohydrodynamic (MHD) models of accretion disks, focusing on its kinematic phase. We perform resistive MHD simulations with the Pluto code for different radiative cooling times and elec…
Though usually treated in isolation, the magnetorotational and gravitational instabilities (MRI and GI) may coincide at certain radii and evolutionary stages of protoplanetary discs and active galactic nuclei. Their mutual interactions could profoundly influence several important processes, such as accretion variability and outbursts, fragmentation and disc truncation, or large-scale magnetic field production. Direct numerical simulations of both instabilities are computationally challenging and remain relatively unexplored. In this paper, we aim to redress this neglect via a set of 3D vertica…
In the early stages of a protoplanetary disk, when its mass is a significant fraction of its star's, turbulence generated by gravitational instability (GI) should feature significantly in the disk's evolution. At the same time, the disk may be sufficiently ionised for magnetic fields to play some role in the dynamics. Though usually neglected, the impact of magnetism on the GI may be critical, with consequences for several processes: the efficiency of accretion, spiral structure formation, fragmentation, and the dynamics of solids. In this paper, we report on global three-dimensional magnetohy…
The origin of large-scale and coherent magnetic fields in astrophysical discs is an important and long standing problem. Researchers commonly appeal to a turbulent dynamo, sustained by the magneto-rotational instability (MRI), to supply the large-scale field. But research over the last decade in particular has demonstrated that various non-ideal MHD effects can impede or extinguish the MRI, especially in protoplanetary disks. In this paper we propose a new scenario, by which the magnetic field is generated and sustained via the gravitational instability (GI). We use 3D stratified shearing box …
Observations suggest that protoplanetary disks have moderate accretion rates onto the central young star, especially at early stages (e.g. HL Tau), indicating moderate disk turbulence. However, recent ALMA observations suggest that dust is highly settled, implying weak turbulence. Motivated by such tension, we carry out 3D stratified local simulations of self-gravitating disks, focusing on settling of dust particles in actively accreting disks. We find that gravitationally unstable disks can have moderately high accretion rates while maintaining a relatively thin dust disk for two reasons. Fir…
Protoplanetary disks around young stars are the birth sights of planetary systems like our own. Disks represent the gaseous dusty matter left after the formation of their central stars. The mass and luminosity of the star, initial disk mass and angular momentum, and gas viscosity govern disk evolution and accretion. Protoplanetary disks are the cosmic nurseries where microscopic dust grains grow into pebbles, planetesimals, and planets.
Determining the mechanisms that drive the evolution of protoplanetary disks is a necessary step to understand how planets form. Here we measured the mass accretion rate for young stellar objects at age >5 Myr, a critical test for the current models of disk evolution. We present the analysis of the spectra of 36 targets in the ~5-10 Myr old Upper Scorpius region for which disk masses were measured with ALMA. We find that the mass accretion rates in this sample of old but still survived disks are similarly high as those of the younger (<3 Myr old) star-forming regions of Lupus and Cha I, when co…
The development of perturbations in the circumstellar disks of pre-main-sequence stars caused by clumpy accretion was investigated. Here we perform 3D hydrodynamical smoothed particle hydrodynamics simulations of disks perturbed by a recent clump accretion event. These simulations are further explored by radiative transfer calculations to quantify the observational appearance of such disks. It was shown that the density waves in the disks were formed at the fall of the clump. After several revolutions they can transform into spirals and ring structures. Their images in millimeter wavelengths a…
The core-accretion model predicts that planetary cores as massive as super-Earths undergo runaway gas accretion to become gas giants. However, the exoplanet census revealed the prevalence of super-Earths close to their host stars, which should have avoided runaway gas accretion. In fact, mass-radius relationships of transiting planets suggest that some close-in super-Earths possess H$_2$/He atmospheres of ~ 0.1-10% by mass. Previous studies indicated that properties of a disk gas such as metallicity and the inflow/outflow cycle of a disk gas around a super-Earth can regulate accumulation of a …
Global Simulations of Gravitational Instability in Protostellar Disks with Full Radiation Transport II. Locality of Gravitoturbulence, Clumpy Spirals, and Implications for Observable Substructure
Radial Stresses and Energy Transport in Accretion Disks
在研究粘性吸积盘的早期，人们认识到来自遥远来源的能量转移一定很重要，尤其是因为在盘的大部分中，盘中平面处的流动很可能是向外的，离开引力势阱。如果粘性的来源是由吸积提供动力的，例如在磁旋转不稳定性的情况下，那么这种遥远的能源必须位于盘的最内部区域，在那里吸积甚至发生在中平面。我们在这里认为，调制在这个能源供应可以改变吸积率的动态，而不是更长的粘性，时间尺度。这意味着内盘吸积率的稳态值和波动（取决于内边界条件的细节并发生在内盘的快速演化时间）都会影响外盘。这是特别有趣的，因为观测表明，磁盘吸积是不稳定的（例如~ EX Lupi类型对象）。我们还注意到，提供给剪切盒的功率是由剪切盒本身设定的，而不是由全球盘中的物理能量通量设定的。也就是说，它们的饱和磁场不受实际磁盘中存在的全套能量约束的影响。我们的分析表明，大尺度径向能量输运对吸积盘的演化和变化有着重要的影响。
FU Ori is the prototype of FU Orionis systems which are outbursting protoplanetary disks. Magnetic fields in FU Ori's accretion disks have previously been detected using spectropolarimetry observations for Zeeman effects. We carry out global radiation ideal MHD simulations to study FU Ori's inner accretion disk. We find that (1) when the disk is threaded by vertical magnetic fields, most accretion occurs in the magnetically dominated atmosphere at z$\sim$R, similar to the "surface accretion" mechanism in previous locally-isothermal MHD simulations. (2) A moderate disk wind is launched in the v…
We present the results of a global, three-dimensional magnetohydrodynamics simulation of an accretion disk with a rotating, weakly magnetized central star. The disk is threaded by a weak, large-scale poloidal magnetic field, and the central star has no strong stellar magnetosphere initially. Our simulation investigates the structure of the accretion flows from a turbulent accretion disk onto the star. The simulation reveals that fast accretion onto the star at high latitudes occurs even without a stellar magnetosphere. We find that the failed disk wind becomes the fast, high-latitude accretion…
