In this paper we investigate the influence of radiative transport on the growth of the magnetorotational instability (MRI) in accretion discs. The analysis is performed by use of analytical and numerical means. We provide a general dispersion relation together with the corresponding eigenfunctions describing the growth rates of small disturbances on a homogeneous background shear flow. The dispersion relation includes compressibility and radiative effects in the flux-limited diffusion approximation. By introducing an effective speed of sound, all the effects of radiation transport can be subsumed into one single parameter. It can be shown that the growth rates of the vertical modes -- which are the fastest growing ones -- are reduced by radiative transport. For the case of non-vertical modes, the growth rates may instead be enhanced. We quantify the effects of compressibility and radiative diffusion on the growth rates for the gas-pressure dominated case. The analytical discussion is supplemented by numerical simulations, which are also used for a first investigation of the non-linear stage of the MRI in gas-pressure dominated accretion discs with radiation transport included.
[abridged] We present a 1+1D global magnetically-driven disk accretion model that captures the essence of the MRI-driven accretion, without resorting to 3D global non-ideal MHD simulations. The gas dynamics is assumed to be solely controlled by the MRI and hydrodynamic instabilities. For given stellar and disk parameters, the Shakura-Sunyaev viscosity parameter $α$ is determined self-consistently under the framework of viscously-driven accretion from detailed considerations of the MRI with non-ideal MHD effects (Ohmic resistivity and ambipolar diffusion), accounting for disk heating by stellar…
Close-in super-Earths are the most abundant exoplanets known. It has been hypothesized that they form in the inner regions of protoplanetary discs, out of the dust that may accumulate at the boundary between the inner region susceptible to the magneto-rotational instability (MRI) and an MRI-dead zone further out. In Paper I we presented a model for the viscous inner disc which includes heating due to both irradiation and MRI-driven accretion; thermal and non-thermal ionization; dust opacities; and dust effects on ionization. Here we examine how the inner disc structure varies with stellar, dis…
Convection has been discussed in the field of accretion discs for several decades, both as a means of angular momentum transport and also because of its role in controlling discs' vertical structure via heat transport. If the gas is sufficiently ionized and threaded by a weak magnetic field, convection might interact in non-trivial ways with the magnetorotational instability (MRI). Recently, vertically stratified local simulations of the MRI have reported considerable variation in the angular momentum transport, as measured by the stress to thermal pressure ratio $α$, when convection is though…
Global simulations of accretion flows onto perturbers embedded in magnetized disks -I. MRI and jet formation in ideal MHD
Charles University（查理大学）；Instituto de Astronomía, Universidad Nacional Autónoma de México（墨西哥国立自治大学天文研究所）；Niels Bohr International Academy, Niels Bohr Institute（尼尔斯·玻尔国际学院，尼尔斯·玻尔研究所）
The classical alpha-disc model assumes that the turbulent stress scales linearly with -- and responds instantaneously to -- the pressure. It is likely, however, that the stress possesses a non-negligible relaxation time and will lag behind the pressure on some timescale. To measure the size of this lag we carry out unstratified 3D magnetohydrodynamic shearing box simulations with zero-net-magnetic-flux using the finite-volume code PLUTO. We impose thermal oscillations of varying periods via a cooling term, which in turn drives oscillations in the turbulent stress. Our simulations reveal that t…
The baroclinic instability in the context of layered accretion. Self-sustained vortices and their magnetic stability in local compressible unstratified models of protoplanetary disks
The generation of spiral density waves by MRI in accretion discs
Helmholtz-Zentrum Dresden-Rossendorf（德累斯顿-罗斯多夫亥姆霍兹中心）；Abastumani Astrophysical Observatory（阿巴斯图马尼天文台）；Faculty of Exact and Natural Sciences, Tbilisi State University（第比利斯国立大学理学院）；LyRIDS, ECE-Paris Engineering School（巴黎ECE工程学院LyRIDS）；LUTH, UMR 8102 CNRS, Observatoire de Paris-Meudon（巴黎-默东天文台CNRS联合研究实验室）
Short-period super-Earth-sized planets are common. Explaining how they form near their present orbits requires understanding the structure of the inner regions of protoplanetary discs. Previous studies have argued that the hot inner protoplanetary disc is unstable to the magneto-rotational instability (MRI) due to thermal ionization of potassium, and that a local gas pressure maximum forms at the outer edge of this MRI-active zone. Here we present a steady-state model for inner discs accreting viscously, primarily due to the MRI. The structure and MRI-viscosity of the inner disc are fully coup…
We explore in situ formation and subsequent evolution of close-in super-Earths and mini-Neptunes. We adopt a steady-state inner protoplanetary gas disc structure that arises from viscous accretion due to the magneto-rotational instability (MRI). We consider the evolution of dust in the inner disc, including growth, radial drift and fragmentation, and find that dust particles that radially drift into the inner disc fragment severely due to the MRI-induced turbulence. This result has two consequences: (1) radial drift of grains within the inner disc is quenched, leading to an enhancement of dust…
We present the results of local, vertically stratified, radiation magnetohydrodynamic shearing box simulations of magnetorotational instability (MRI) turbulence for a (hydrogen poor) composition applicable to accretion disks in AM CVn type systems. Many of these accreting white dwarf systems are helium analogues of dwarf novae (DNe). We utilize frequency-integrated opacity and equation of state tables appropriate for this regime to accurately portray the relevant thermodynamics. We find bistability of thermal equilibria in the effective temperature, surface mass density plane typically associa…
Aims and Methods. Accretion bursts triggered by the magnetorotational instability (MRI) in the innermost disk regions were studied for protoplanetary gas-dust disks formed from prestellar cores of various mass $M_{\rm core}$ and mass-to-magnetic flux ratio $λ$. Numerical magnetohydrodynamics simulations in the thin-disk limit were employed to study the long-term ($\sim 1.0$~Myr) evolution of protoplanetary disks with an adaptive turbulent $α$-parameter, which depends explicitly on the strength of the magnetic field and ionization fraction in the disk. The numerical models also feature the co-e…
While accreting through a circumstellar disk, young stellar objects are observed to undergo sudden and powerful accretion events known as FUor or EXor outbursts. Although such episodic accretion is considered to be an integral part of the star formation process, the triggers and mechanisms behind them remain uncertain. We conducted global numerical hydrodynamics simulations of protoplanetary disk formation and evolution in the thin-disk limit, assuming both magnetically layered and fully magnetorotational instability (MRI)-active disk structure. In this paper, we characterize the nature of the…
We derive expressions for the local ideal magnetohydrodynamic (MHD) equations for a warped astrophysical disc using a warped shearing box formalism. A perturbation expansion of these equations to first order in the warping amplitude leads to a linear theory for the internal local structure of magnetised warped discs in the absence of MRI turbulence. In the special case of an external magnetic field oriented normal to the disc surface these equations are solved semi-analytically via a spectral method. The relatively rapid warp propagation of low-viscosity Keplerian hydrodynamic warped discs is …
Accretion disc theory is less developed than stellar evolution theory although a similarly mature phenomenological picture is ultimately desired. While the interplay of theory and numerical simulations has amplified community awareness of the role of magnetic fields in angular momentum transport, there remains a long term challenge to incorporate insight gained from simulations back into improving practical models for comparison with observations. Here we emphasize the need to incorporate the role of non-local transport more precisely. To show where large scale transport would fit into the the…
Dwarf novae (DNe) are accreting white dwarfs that show eruptions due to a thermal-viscous instability in the accretion disk. The outburst timescales constrain $α$, the ratio of the viscous stress to the thermal pressure, and so the mechanism of angular momentum transport. The eruptive state has $α\approx0.1$ while the quiescent state has $α\approx0.03$. Turbulent transport due to the magneto-rotational instability (MRI) is generally considered to be the source of angular momentum transport in DNe. Here, we perform 3D local magnetohydrodynamic (MHD) shearing box simulations including vertical s…
