Magnetorotational Instability Research Articles

Enhanced extra mixing in low-mass stars approaching the RGB tip and the problem of Li-rich red-clump stars

Abstract A few percent of red giants are enriched in Lithium with A(Li) > 1.5. Their evolutionary status has remained uncertain because these Li-rich giants can be placed both on the red-giant branch (RGB) near the bump luminosity and in the red clump (RC) region. However, thanks to asteroseismology, it has been found that most of them are actually RC stars. Starting at the bump luminosity, RGB progenitors of the RC stars experience extra mixing in the radiative zone separating the H-burning shell from the convective envelope followed by a series of convective He-shell flashes at the RGB tip, known as the He-core flash. The He-core flash was proposed to cause fast extra mixing in the stars at the RGB tip that is needed for the Cameron-Fowler mechanism to produce Li. We propose that the RGB stars are getting enriched in Li by the RGB extra mixing that is getting enhanced and begins to produce Li, instead of destroying it, when the stars are approaching the RGB tip. After a discussion of several mechanisms of the RGB extra mixing, including the joint operation of rotation-driven meridional circulation and turbulent diffusion, the Azimuthal Magneto-Rotational Instability (AMRI), thermohaline convection, buoyancy of magnetic flux tubes, and internal gravity waves, and based on results of (magneto-) hydrodynamics simulations and asteroseismology observations, we are inclined to conclude that it is the mechanism of the AMRI or magnetically-enhanced thermohaline convection, that is most likely to support our hypothesis.

Local stability of differential rotation in magnetized radiation zones and the solar tachocline

ABSTRACT We study local magnetohydrodynamical instabilities of differential rotation in magnetized, stably stratified regions of stars and planets using a Cartesian Boussinesq model. We consider arbitrary latitudes and general shears (with gravity direction misaligned from this by an angle $\phi$), to model radial ($\phi =0$), latitudinal ($\phi =\pm 90^\circ$), and mixed differential rotations, and study both non-diffusive [including magnetorotational instability (MRI) and Solberg–Høiland instability] and diffusive instabilities [including Goldreich–Schubert–Fricke (GSF) and MRI with diffusion]. These instabilities could drive turbulent transport and mixing in radiative regions, including the solar tachocline and the cores of red giant stars, but their dynamics are incompletely understood. We revisit linear axisymmetric instabilities with and without diffusion and analyse their properties in the presence of magnetic fields, including deriving stability criteria and computing growth rates, wave vectors, and energetics, both analytically and numerically. We present a more comprehensive analysis of axisymmetric local instabilities than prior work, exploring arbitrary differential rotations and diffusive processes. The presence of a magnetic field leads to stability criteria depending upon angular velocity rather than angular momentum gradients. We find MRI operates for much weaker differential rotations than the hydrodynamic GSF instability, and that it typically prefers much larger length-scales, while the GSF instability is impeded by realistic strength magnetic fields. We anticipate MRI to be more important for turbulent transport in the solar tachocline than the GSF instability when $\phi \gt 0$ in the Northern (and vice versa in the Southern) hemisphere, though the latter could operate just below the convection zone when MRI is absent for $\phi \lt 0$.

A simple model of globally magnetized accretion discs

ABSTRACT We present an analytic, quasi-local model for accretion discs threaded by net, vertical magnetic flux. In a simple slab geometry and ignoring stochastic mean-field dynamo effects, we calculate the large-scale field resulting from the balance between kinematic field amplification and turbulent diffusion. The ability of the disc to accumulate magnetic flux is sensitive to a single parameter dependent on the ratio of the vertical diffusion time to the Alfvén crossing time, and we show how the saturation levels of magnetorotational and other instabilities can govern disc structure and evolution. Under wide-ranging conditions, inflow is governed by large-scale magnetic stresses rather than internal viscous stress. We present models of such ‘magnetically boosted’ discs and show that they lack a radiation pressure-dominated zone. Our model can account for ‘magnetically elevated’ discs as well as instances of midplane outflow and field reversals with height that have been seen in some global simulations. Using the time-dependent features of our model, we find that the incorporation of global transport effects into disc structure can lead to steady or episodic ‘magnetically arrested discs’ that maximize the concentration of magnetic flux in their central regions.

Magnetic field morphology and evolution in the Central Molecular Zone and its effect on gas dynamics

The interstellar medium in the Milky Way's Central Molecular Zone (CMZ) is known to be strongly magnetised, but its large-scale morphology and impact on the gas dynamics are not well understood. We explore the impact and properties of magnetic fields in the CMZ using three-dimensional non-self gravitating magnetohydrodynamical simulations of gas flow in an external Milky Way barred potential. We find that: (1) The magnetic field is conveniently decomposed into a regular time-averaged component and an irregular turbulent component. The regular component aligns well with the velocity vectors of the gas everywhere, including within the bar lanes. (2) The field geometry transitions from parallel to the Galactic plane near $z=0$ to poloidal away from the plane. (3) The magneto-rotational instability (MRI) causes an in-plane inflow of matter from the CMZ gas ring towards the central few parsecs of $0.01 that is absent in the unmagnetised simulations. However, the magnetic fields have no significant effect on the larger-scale bar-driven inflow that brings the gas from the Galactic disc into the CMZ. (4) A combination of bar inflow and MRI-driven turbulence can sustain a turbulent vertical velocity dispersion of $ on scales of $20 in the CMZ ring. The MRI alone sustains a velocity dispersion of $ Both these numbers are lower than the observed velocity dispersion of gas in the CMZ, suggesting that other processes such as stellar feedback are necessary to explain the observations. (5) Dynamo action driven by differential rotation and the MRI amplifies the magnetic fields in the CMZ ring until they saturate at a value that scales with the average local density as $B 102 \, (n/10^3 \ G $. Finally, we discuss the implications of our results within the observational context in the CMZ.

Large-scale ordered magnetic fields generated in mergers of helium white dwarfs

Stellar mergers are one important path to highly magnetised stars. Mergers of two low-mass white dwarfs may create up to every third hot subdwarf star. The merging process is usually assumed to dramatically amplify magnetic fields. However, so far only four highly magnetised hot subdwarf stars have been found, suggesting a fraction of less than $1<!PCT!>$. We present two high-resolution magnetohydrodynamical (MHD) simulations of the merger of two helium white dwarfs in a binary system with the same total mass of $0.6\,M_ We analysed an equal-mass merger with two $0.3\,M_ white dwarfs, and an unequal-mass merger with white dwarfs of $0.25\,M_ and $0.35\,M_ We simulated the inspiral, merger, and further evolution of the merger remnant for about $50$ rotations. We found efficient magnetic field amplification in both mergers via a small-scale dynamo, reproducing previous results of stellar merger simulations. The magnetic field saturates at a similar strength for both simulations. We then identified a second phase of magnetic field amplification in both merger remnants that happens on a timescale of several tens of rotational periods of the merger remnant. This phase generates a large-scale ordered azimuthal field via a large-scale dynamo driven by the magneto-rotational instability. Finally, we speculate that in the unequal-mass merger remnant, helium burning will initially start in a shell around a cold core, rather than in the centre. This forms a convection zone that coincides with the region that contains most of the magnetic energy, and likely destroys the strong, ordered field. Ohmic resistivity might then quickly erase the remaining small-scale field. Therefore, the mass ratio of the initial merger could be the selecting factor that decides if a merger remnant will stay highly magnetised long after the merger.

Inertial range of magnetorotational turbulence.

Accretion disks around compact stars are formed due to turbulence driven by magnetorotational instability. Despite over 30 years of numerous computational studies on magnetorotational turbulence, the properties of fluctuations in the inertial range-where cross-scale energy transfer dominates over energy injection-have remained elusive, primarily due to insufficient numerical resolution. Here, we report the highest-resolution simulation of magnetorotational turbulence ever conducted. Our simulations reveal a constant cross-scale energy flux, a hallmark of the inertial range. We found that as the cascade proceeds to smaller scales in the inertial range, the kinetic and magnetic energies tend toward equipartitioning with the same spectral slope, and slow magnetosonic fluctuations dominate over Alfvénic fluctuations, having twice the energy. These findings align remarkably with the theoretical expectations from the reduced magnetohydrodynamic model, which assumes a near-azimuthal mean magnetic field. Our results provide important implications for interpreting the radio observations by the Event Horizon Telescope.

Plasma centrifugation method for separation of the nuclear waste

The high-throughput, plasma-based, mass separation techniques of nuclear waste are still an active research topic due to the theoretical and practical complexity of processing such waste. The nuclear waste can be separated according to its masses by plasma centrifugation technique. Because of this, the present research has been performed by setting up E → × B → rotation in cold cylindrical plasma surrounded by an ideally conducting homogenous shell inside a plasma centrifuge device. The results obtained are useful in analyzing the magneto-rotational instability (MRI) and trajectories of the plasma particles which are beneficial for improving the throughput and separation factor of the device.

Spiral magnetic fields and their role on accretion dynamics in the circumnuclear disk of Sagittarius A*: Insight from λ = 850 μm polarization imaging

Abstract We showcase a study on the physical properties of the circumnuclear disk (CND) surrounding the supermassive black hole (SMBH) Sgr A* of the Galactic Center, emphasizing the role of magnetic field ($\boldsymbol {B}$ field) with $0.50\,$pc spatial resolution. Based on the sensitive $\lambda = 850\, \mu$m polarization data taken with the JCMT SCUBA2/POL2 (James Clerk Maxwell Telescope Submillimetre Common-User Bolometer Array 2), we analyzed ancillary datasets: CS $J = 2$–1 emission taken with ALMA (Atacama Large Millimeter/submillimeter Array), continuum emissions taken at $\lambda = 6\,$cm and at $\lambda = 37\, \mu$m taken with the VLA (Very Large Array) and SOFIA (the Stratospheric Observatory for Infrared Astronomy telescope). The $\boldsymbol {B}$ field within the CND exhibits a coherent spiral pattern. Applying the model described by Wardle and Königl (1990, ApJ, 362, 12; the WK model) to the observed $\boldsymbol {B}$ field pattern, it favors gas-pressure-dominant models without dismissing a gas-and-$\boldsymbol {B}$ field comparable model, leading us to estimate the $\boldsymbol {B}$-field strength in the ionized cavity around Sgr A* as $0.24^{+0.06}_{-0.04}\,$mG. Analysis based on the WK model further allows us to derive representative $\boldsymbol {B}$-field strengths for the radial, azimuthal, and vertical components as $(B_r, B_\phi , B_z) = (0.4 \pm 0.1, -0.7 \pm 0.2, 0.2 \pm 0.05)\,$mG, respectively. A key finding is that the $|B_\phi |$ component is dominant over $B_r$ and $B_z$ components, consistent with the spiral morphology, indicating that the CND’s $\boldsymbol {B}$-field is predominantly toroidal, possibly shaped by accretion dynamics. Considering the turbulent pressure, estimated plasma $\beta$ values indicate that the effective gas pressure should surpass the magnetic pressure. Assessing the CND of our MWG in the toroidal-and-vertical stability parameter space, we propose that such an “effective” magneto-rotational instability (MRI) may likely be active. The estimated maximum unstable wavelength, $\lambda _{\rm max} = 0.1 \pm 0.1\,$pc, is smaller than the CND’s scale height ($0.2 \pm 0.1\,$pc), which indicates the potential for the effective MRI intermittent cycles of $\sim 10^6\,$yr, which should profoundly affect the CND’s evolution, considering the estimated mass accretion rate of $10^{-2} M_{\odot }\,$yr$^{-1}$ to the SMBH.

One-winged butterflies: mode selection for azimuthal magnetorotational instability by thermal convection

The effects of thermal convection on turbulence in accretion discs, and particularly its interplay with the magnetorotational instability (MRI), are of significant astrophysical interest. Despite extensive theoretical and numerical studies, such an interplay has not been explored experimentally. We conduct linear analysis of the azimuthal version of MRI (AMRI) in the presence of thermal convection and compare the results with our experimental data published before. We show that the critical Hartmann number ( $Ha$ ) for the onset of AMRI is reduced by convection. Importantly, convection breaks symmetry between $m = \pm 1$ instability modes ( $m$ is the azimuthal wavenumber). This preference for one mode over the other makes the AMRI wave appear as a ‘one-winged butterfly’.

Dynamics and emission properties of flux ropes from two-temperature GRMHD simulations with multiple magnetic loops

Context. Flux ropes erupting in the vicinity of a black hole are thought to be a potential model for the flares observed in Sagittarius A*. Aims. In this study, we examine the radiative properties of flux ropes that emerged in the vicinity of a black hole. Methods. We performed three-dimensional two-temperature general relativistic magnetohydrodynamic (GRMHD) simulations of magnetized accretion flows with alternating multiple magnetic loops and general relativistic radiation transfer (GRRT) calculations. In the GRMHD simulations, we implemented two different sizes of initial magnetic loops. Results. In the small loop case, magnetic dissipation leads to a weaker excitement of magneto-rotational instability inside the torus, which generates a lower accretion rate compared to the large loop case. However, the small loop case generates more flux ropes due to frequent reconnection by magnetic loops with different polarities. By calculating the thermal synchrotron emission, we found that the variability of light curves and the emitting region are tightly related. At 230 GHz and higher, the emission from the flux ropes is relatively stronger compared to the background, which is responsible for the filamentary structure in the images. At lower frequencies (e.g., 43 GHz), emission comes from more extended regions, which have a less filamentary structure in the image. Conclusions. Our study shows that self-consistent electron temperature models are essential for the calculation of thermal synchrotron radiation and the morphology of the GRRT images. Flux ropes contribute considerable emission at frequencies ≳230 GHz.

Viscous torque in turbulent magnetized active galactic nucleus accretion disks and its effects on the gravitational waves of extreme mass ratio inspirals

The merger of supermassive black holes produces millihertz gravitational waves (GWs), which are potentially detectable by the future Laser Interferometer Space Antenna (LISA). Such binary systems are usually embedded in an accretion disk environment at the center of an active galactic nucleus (AGN). Recent studies suggest the plasma environment imposes measurable imprints on the GW signal if the mass ratio of the binary is around q ∼ 10−4 − 10−3. The effect of the gaseous environment on the GW signal is strongly dependent on the disk’s parameters; therefore, it is believed that future low-frequency GW detections will provide us with precious information about the physics of AGN accretion disks. We investigated this effect by measuring the viscous torque via modeling of the evolution of magnetized tori around the primary massive black hole. Using the general relativistic magnetohydrodynamic HARM-COOL code, we performed 2D and 3D simulations of weakly magnetized, thin accretion disks, with a possible truncation and transition to advection-dominated accretion flow. We studied the angular momentum transport and turbulence generated by the magnetorotational instability. We quantified the disk’s effective alpha viscosity and its evolution over time. We applied our numerical results to quantify the relativistic viscous torque on a hypothetical low-mass secondary black hole via a 1D analytical approach, and we estimated the GW phase shift due to the gas environment.

The Origin of the Slow-to-Alfvén Wave Cascade Power Ratio and Its Implications for Particle Heating in Accretion Flows

The partition of turbulent heating between ions and electrons in radiatively inefficient accretion flows plays a crucial role in determining the observational appearance of accreting black holes. Modeling this partition is, however, a challenging problem because of the large scale-separation between the macroscopic scales at which energy is injected by turbulence and the microscopic ones at which it is dissipated into heat. Recent studies of particle heating from collisionless damping of turbulent energy have shown that the partition of energy between ions and electrons is dictated by the ratio of the energy injected into the slow and Alfvén wave cascades as well as the plasma β parameter. In this paper, we study the mechanism of the injection of turbulent energy into slow- and Alfvén-wave cascades in magnetized shear flows. We show that this ratio depends on the particular (r ϕ) components of the Maxwell and Reynolds stress tensors that cause the transport of angular momentum, the shearing rate, and the orientation of the mean magnetic field with respect to the shear. We then use numerical magnetohydrodynamic shearing-box simulations with background conditions relevant to black hole accretion disks to compute the magnitudes of the stress tensors for turbulence driven by the magneto-rotational instability and derive the injection power ratio between slow and Alfvén wave cascades. We use these results to formulate a local subgrid model for the ion-to-electron heating ratio that depends on the macroscopic characteristics of the accretion flow.

Magnetorotational dynamo can generate large-scale vertical magnetic fields in 3D GRMHD simulations of accreting black holes

ABSTRACT Jetted astrophysical phenomena with black hole engines, including binary mergers, jetted tidal disruption events, and X-ray binaries, require a large-scale vertical magnetic field for efficient jet formation. However, a dynamo mechanism that could generate these crucial large-scale magnetic fields has not been identified and characterized. We have employed three-dimensional global general relativistic magnetohydrodynamical simulations of accretion discs to quantify, for the first time, a dynamo mechanism that generates large-scale magnetic fields. This dynamo mechanism primarily arises from the non-linear evolution of the magnetorotational instability (MRI). In this mechanism, large non-axisymmetric MRI-amplified shearing wave modes, mediated by the axisymmetric azimuthal magnetic field, generate and sustain the large-scale vertical magnetic field through their non-linear interactions. We identify the advection of magnetic loops as a crucial feature, transporting the large-scale vertical magnetic field from the outer regions to the inner regions of the accretion disc. This leads to a larger characteristic size of the, now advected, magnetic field when compared to the local disc height. We characterize the complete dynamo mechanism with two time-scales: one for the local magnetic field generation, $t_{\rm gen}$, and one for the large-scale scale advection, $t_{\rm adv}$. Whereas the dynamo we describe is non-linear, we explore the potential of linear mean field models to replicate its core features. Our findings indicate that traditional $\alpha$-dynamo models, often computed in stratified shearing box simulations, are inadequate and that the effective large-scale dynamics is better described by the shear current effects or stochastic $\alpha$-dynamos.

Evidence for non-zero turbulence in the protoplanetary disc around IM Lup

ABSTRACT The amount of turbulence in protoplanetary discs around young stars is critical for determining the efficiency, timeline, and outcomes of planet formation. It is also difficult to measure. Observations are still limited, but direct measurements of the non-thermal, turbulent gas motion are possible with the Atacama Large Millimeter/submillimeter Array (ALMA). Using CO(2–1)/$^{13}$CO(2–1)/C$^{18}$O(2–1) ALMA observations of the disc around IM Lup at $\sim 0.4$ arcsec ($\sim$60 au) resolution we find evidence of significant turbulence, at the level of $\delta v_{\rm turb}=(0.18-0.30)$c$_\mathrm{ s}$. This result is robust against systematic uncertainties (e.g. amplitude flux calibration, mid-plane gas temperature, disc self-gravity). We find that gravito-turbulence as the source of the gas motion is unlikely based on the lack of an imprint on the rotation curve from a massive disc, while magneto-rotational instabilities and hydrodynamic instabilities are still possible, depending on the unknown magnetic field strength and the cooling time-scale in the outer disc.

Co-evolution of dust grains and protoplanetary disks. II. Structure and evolution of protoplanetary disks: An analytical approach

Abstract In our previous study (Tsukamoto et al. 2023b, PASJ, 75, 835), we investigated the formation and early evolution of protoplanetary disks with 3D non-ideal magnetohydrodynamics simulations considering dust growth, and found that the modified equations of the conventional steady accretion disk model that consider magnetic braking, dust growth, and ambipolar diffusion reproduce the disk structure (such as density and vertical magnetic field) obtained from simulations very well. In this paper, as a sequel to our previous study, we analytically investigate the structure and evolution of protoplanetary disks corresponding to Class 0/I young stellar objects using the modified steady accretion disk model combining an analytical model of envelope accretion. We estimate that the disk radius is several astronomical units at the disk formation epoch and increases to several hundred astronomical units at the end of the accretion phase. The disk mass is estimated to be $0.01 \lesssim M_{\rm disk} \lesssim 0.1 \, M_\odot$ for a disk with a radius of several tens of astronomical units and a mass accretion rate of $\dot{M}_{\rm disk} \sim 10^{-6} \, M_\odot \,\, {\rm yr^{-1}}$. These estimates seems to be consistent with recent observations. We also found that, with typical disk ionization rates (ζ ≳ 10−19 s−1) and a moderate mass accretion rate ($\dot{M}_{\rm disk}\gtrsim 10^{-8} \, M_\odot \,\, {\rm yr^{-1}}$), magnetorotational instability is suppressed in the disk because of low plasma β and efficient ambipolar diffusion. We argue that the radial profile of specific angular momentum (or rotational velocity) at the disk outer edge should be continuously connected to that of the envelope if the disk evolves by magnetic braking, and should be discontinuous if the disk evolves by an internal angular momentum transport process such as gravitational instability or magnetorotational instability. Future detailed observations of the specific angular momentum profile around the disk outer edge are important for understanding the angular momentum transport mechanism of protoplanetary disks.

Parametric Survey of Nonaxisymmetric Accretion Disk Instabilities: Magnetorotational Instability to Super-Alfvénic Rotational Instability

Accretion disks are highly unstable to magnetic instabilities driven by shear flow, where classically, the axisymmetric, weak-field magnetorotational instability (MRI) has received much attention through local WKB approximations. In contrast, discrete nonaxisymmetric counterparts require a more involved analysis through a full global approach to deal with the influence of the nearby magnetohydrodynamic (MHD) continua. Recently, rigorous MHD spectroscopy identified a new type of ultralocalized, nonaxisymmetric instability in global disks with super-Alfvénic flow. These super-Alfvénic rotational instabilities (SARIs) fill vast unstable regions in the complex eigenfrequency plane with (near eigen)modes that corotate at the local Doppler velocity and are radially localized between Alfvénic resonances. Unlike discrete modes, they are utterly insensitive to the radial disk boundaries. In this work, we independently confirm the existence of these unprecedented modes using our novel spectral MHD code Legolas, reproducing and extending our earlier study with detailed eigenspectra and eigenfunctions. We calculate the growth rates of SARIs and MRI in a variety of disk equilibria, highlighting the impact of field strength and orientation, and find correspondence with analytical predictions for thin, weakly magnetized disks. We show that nonaxisymmetric modes can significantly extend instability regimes at high mode numbers, with maximal growth rates comparable to those of the MRI. Furthermore, we explicitly show a region filled with quasi-modes whose eigenfunctions are extremely localized in all directions. These modes must be ubiquitous in accretion disks, and play a role in local shearing box simulations. Finally, we revisit recent dispersion relations in the appendix, highlighting their relation to our global framework.

The solar dynamo begins near the surface

The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating region of sunspot emergence appears around 30° latitude and vanishes near the equator every 11 years (ref. 1). Moreover, longitudinal flows called torsional oscillations closely shadow sunspot migration, undoubtedly sharing a common cause2. Contrary to theories suggesting deep origins of these phenomena, helioseismology pinpoints low-latitude torsional oscillations to the outer 5–10% of the Sun, the near-surface shear layer3,4. Within this zone, inwardly increasing differential rotation coupled with a poloidal magnetic field strongly implicates the magneto-rotational instability5,6, prominent in accretion-disk theory and observed in laboratory experiments7. Together, these two facts prompt the general question: whether the solar dynamo is possibly a near-surface instability. Here we report strong affirmative evidence in stark contrast to traditional models8 focusing on the deeper tachocline. Simple analytic estimates show that the near-surface magneto-rotational instability better explains the spatiotemporal scales of the torsional oscillations and inferred subsurface magnetic field amplitudes9. State-of-the-art numerical simulations corroborate these estimates and reproduce hemispherical magnetic current helicity laws10. The dynamo resulting from a well-understood near-surface phenomenon improves prospects for accurate predictions of full magnetic cycles and space weather, affecting the electromagnetic infrastructure of Earth.

Magnetic amplification in premerger neutron stars through resonance-induced magnetorotational instabilities

Magnetic amplification in premerger neutron stars through resonance-induced magnetorotational instabilities

Large Fluctuations within 1 au in Protoplanetary Disks

Protoplanetary disks are often assumed to change slowly and smoothly during planet formation. Here, we investigate the time evolution of isolated disks subject to viscosity and a disk wind. The viscosity is assumed to increase rapidly at around 900 K due to thermal ionization of alkali metals, or thermionic and ion emission from dust, and the onset of magnetorotational instability (MRI). The disks generally undergo large, rapid fluctuations for a wide range of time-averaged mass accretion rates. Fluctuations involve coupled waves in temperature and surface density that move radially in either direction through the inner 1.5 au of the disk. Two types of waves are seen with radial speeds of roughly 50 and 1000 cm s−1, respectively. The pattern of waves repeats with a period of roughly 10,000 yr that depends weakly on the average mass accretion rate. Viscous transport due to MRI is confined to the inner disk. This region is resupplied by mass flux from the outer disk driven by the disk wind. Interior to 1 au, the temperature and surface density can vary by a factor of 2–10 on timescales of years to kiloyears. The stellar mass accretion rate varies by 3 orders of magnitude on a similar timescale. This behavior lasts for at least 1 Myr for initial disks comparable to the minimum-mass solar nebula.

The High Mass Accretion in the Innermost Regions of a Viscously Evolved Protoplanetary Disk

In this paper, we investigate the mass accretion properties in the innermost regions of a viscously evolved protoplanetary disk and try to find some clues to the outburst events. In our newly developed one-dimensional time-dependent disk model based on the diffusion equation for surface density, we take into account the following physical effects: the gravitational collapse of the parent molecular cloud core, the irradiation from the central star to the disk, the effect of the photoevaporation mechanism, the viscosity due to the magnetorotational instability (MRI) and the gravitational instability (GI), and the thermal ionization mechanism in the inner regions. We find that the mass accretion rate M·disk in the innermost regions is statistically high enough to generate outbursts, although there are regions where the accretion rate is low. Additionally, we find that there is a weak correlation between the high mass accretion rate M·disk and the molecular cloud core’s properties (angular velocity ω and mass Mcd), as well as a strong correlation with the minimum viscosity parameter αmin. In general, there are two regions of outburst, the inner Region I and outer Region II. The outburst of Region I is caused by the MRI mechanism and thermal instability, while neither the MRI, the GI, nor the thermal instability causes the outburst of Region II. Our analysis suggests that the outer Region II is dominated by, or largely related to, the Rosseland mean opacity κR and the αmin parameter.