Direct imaging observations have revealed spiral structures in protoplanetary disks. Previous studies have suggested that planet-induced spiral arms cannot explain some of these spiral patterns, due to the large pitch angle and high contrast of the s
piral arms in observations. We have carried out three dimensional (3-D) hydrodynamical simulations to study spiral wakes/shocks excited by young planets. We find that, in contrast with linear theory, the pitch angle of spiral arms does depend on the planet mass, which can be explained by the non-linear density wave theory. A secondary (or even a tertiary) spiral arm, especially for inner arms, is also excited by a massive planet. With a more massive planet in the disk, the excited spiral arms have larger pitch angle and the separation between the primary and secondary arms in the azimuthal direction is also larger. We also find that although the arms in the outer disk do not exhibit much vertical motion, the inner arms have significant vertical motion, which boosts the density perturbation at the disk atmosphere. Combining hydrodynamical models with Monte-Carlo radiative transfer calculations, we find that the inner spiral arms are considerably more prominent in synthetic near-IR images using full 3-D hydrodynamical models than images based on 2-D models assuming vertical hydrostatic equilibrium, indicating the need to model observations with full 3-D hydrodynamics. Overall, companion-induced spiral arms not only pinpoint the companions position but also provide three independent ways (pitch angle, separation between two arms, and contrast of arms) to constrain the companions mass.
I calculate the spectral energy distributions (SEDs) of accreting circumplanetary disks using atmospheric radiative transfer models. Circumplanetary disks only accreting at $10^{-10} M_{odot} yr^{-1}$ around a 1 M$_{J}$ planet can be brighter than th
e planet itself. A moderately accreting circumplanetary disk ($dot{M}sim 10^{-8}M_{odot} yr^{-1}$; enough to form a 10 M$_{J}$ planet within 1 Myr) around a 1 M$_{J}$ planet has a maximum temperature of $sim$2000 K, and at near-infrared wavelengths ($J$, $H$, $K$ bands), this disk is as bright as a late M-type brown dwarf or a 10 M$_{J}$ planet with a hot start. To use direct imaging to find the accretion disks around low mass planets (e.g., 1 M$_{J}$) and distinguish them from brown dwarfs or hot high mass planets, it is crucial to obtain photometry at mid-infrared bands ($L$, $M$, $N$ bands) because the emission from circumplanetary disks falls off more slowly towards longer wavelengths than those of brown dwarfs or planets. If young planets have strong magnetic fields ($gtrsim$100 G), fields may truncate slowly accreting circumplanetary disks ($dot{M}lesssim10^{-9} M_{odot} yr^{-1}$) and lead to magnetospheric accretion, which can provide additional accretion signatures, such as UV/optical excess from the accretion shock and line emission.
We study dust transport in turbulent protoplanetary disks using three-dimensional global unstratified magnetohydrodynamic (MHD) simulations including Lagrangian dust particles. The turbulence is driven by the magnetorotational instability (MRI) with
either ideal or non-ideal MHD that includes ambipolar diffusion (AD). In ideal MHD simulations, the surface density evolution (except for dust that drifts fastest), turbulent diffusion, and vertical scale height of dust can all be reproduced by simple one-dimensoinal and/or analytical models. However, in AD dominated simulations which simulate protoplanetary disks beyond 10s of AU, the vertical scale height of dust is larger than previously predicted. To understand this anomaly in more detail, we carry out both unstratified and stratified local shearing box simulations with Lagrangian particles, and find that turbulence in AD dominated disks has very different properties (e.g., temporal autocorrelation functions and power spectra) than turbulence in ideal MHD disks, which leads to quite different particle diffusion efficiency. For example, MRI turbulence with AD has a longer correlation time for the vertical velocity, which causes significant vertical particle diffusion and large dust scale height. In ideal MHD the Schmidt numbers ($Sc$) for radial and vertical turbulent diffusion are $Sc_{r}sim 1$ and $Sc_{z}gtrsim 3$, but in the AD dominated regime both $Sc_{r}$ and $Sc_{z}$ are $lesssim 1$. Particle concentration in pressure bumps induced by MRI turbulence has also been studied. Since non-ideal MHD effects dominate most regions in protoplanetary disks, our study suggests that modeling dust transport in turbulence driven by MRI with non-ideal MHD effects is important for understanding dust transport in realistic protoplanetary disks.
We use 12000 stars from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) spectroscopic data to show that the metallicities of Kepler field stars as given in the Kepler Input Catalog (KIC) systematically underestimate both the true m
etallicity and the dynamic range of the Kepler sample. Specifically, to the first order approximation, we find [Fe/H]_KIC = -0.20 + 0.43 [Fe/H]_LAMOST, with a scatter of ~0.25 dex, due almost entirely to errors in KIC. This relation is most secure for -0.3<[Fe/H]_LAMOST<+0.4 where we have >200 comparison stars per 0.1 dex bin and good consistency is shown between metallicities determined by LAMOST and high-resolution spectra. It remains approximately valid in a slightly broader range. When the relation is inverted, the error in true metallicity as derived from KIC is (0.25 dex)/0.43~0.6 dex. We thereby quantitatively confirm the cautionary note by Brown et al. (2011) that KIC estimates of [Fe/H] should not be used by anyone with a particular interest in stellar metallicities. Fortunately, many more LAMOST spectroscopic metallicities will be available in the near future.
We perform a systematic study of the dynamics of dust particles in protoplanetary disks with embedded planets using global 2-D and 3-D inviscid hydrodynamic simulations. Lagrangian particles have been implemented into magnetohydrodynamic code Athena
with cylindrical coordinates. We find two distinct outcomes depending on the mass of the embedded planet. In the presence of a low mass planet ($8 M_{oplus}$), two narrow gaps start to open in the gas on each side of the planet where the density waves shock. These shallow gaps can dramatically affect particle drift speed and cause significant, roughly axisymmetric dust depletion. On the other hand, a more massive planet ($>0.1 M_{J}$) carves out a deeper gap with sharp edges, which are unstable to the vortex formation. Particles with a wide range of sizes ($0.02<Omega t_{s}<20$) are trapped and settle to the midplane in the vortex, with the strongest concentration for particles with $Omega t_{s}sim 1$. The dust concentration is highly elongated in the $phi$ direction, and can be as wide as 4 disk scale heights in the radial direction. Dust surface density inside the vortex can be increased by more than a factor of 10$^2$ in a very non-axisymmetric fashion. For very big particles ($Omega t_{s}gg 1$) we find strong eccentricity excitation, in particular around the planet and in the vicinity of the mean motion resonances, facilitating gap opening there. Our results imply that in weakly turbulent protoplanetary disk regions (e.g. the dead zone) dust particles with a very wide range of sizes can be trapped at gap edges and inside vortices induced by planets with $M_{p}<M_{J}$, potentially accelerating planetesimal and planet formation there, and giving rise to distinctive features that can be probed by ALMA and EVLA.
We perform calculations of our one-dimensional, two-zone disk model to study the long-term evolution of the circumstellar disk. In particular, we adopt published photoevaporation prescriptions and examine whether the photoevaporative loss alone, coup
led with a range of initial angular momenta of the protostellar cloud, can explain the observed decline of the frequency of optically-thick dusty disks with increasing age. In the parameter space we explore, disks have accreting and/or non-accreting transitional phases lasting of $lesssim20 %$ of their lifetime, which is in reasonable agreement with observed statistics. Assuming that photoevaporation controls disk clearing, we find that initial angular momentum distribution of clouds needs to be weighted in favor of slowly rotating protostellar cloud cores. Again, assuming inner disk dispersal by photoevaporation, we conjecture that this skewed angular momentum distribution is a result of fragmentation into binary or multiple stellar systems in rapidly-rotating cores. Accreting and non-accreting transitional disks show different evolutionary paths on the $dot{M}-R_{rm wall}$ plane, which possibly explains the different observed properties between the two populations. However, we further find that scaling the photoevaporation rates downward by a factor of 10 makes it difficult to clear the disks on the observed timescales, showing that the precise value of the photoevaporative loss is crucial to setting the clearing times. While our results apply only to pure photoevaporative loss (plus disk accretion), there may be implications for models in which planets clear disks preferentially at radii of order 10 AU.
We study wakes and gap opening by low mass planets in gaseous protoplanetary disks threaded by net vertical magnetic fields which drive magnetohydrodynamical (MHD) turbulence through the magnetorotational instabilty (MRI), using three dimensional sim
ulations in the unstratified local shearing box approximation. The wakes, which are excited by the planets, are damped by shocks similar to the wake damping in inviscid hydrodynamic (HD) disks. Angular momentum deposition by shock damping opens gaps in both MHD turbulent disks and inviscid HD disks even for low mass planets, in contradiction to the thermal criterion for gap opening. To test the viscous criterion, we compared gap properties in MRI-turbulent disks to those in viscous HD disks having the same stress, and found that the same mass planet opens a significantly deeper and wider gap in net vertical flux MHD disks than in viscous HD disks. This difference arises due to the efficient magnetic field transport into the gap region in MRI disks, leading to a larger effective alpha within the gap. Thus, across the gap, the Maxwell stress profile is smoother than the gap density profile, and a deeper gap is needed for the Maxwell stress gradient to balance the planetary torque density. We also confirmed the large excess torque close to the planet in MHD disks, and found that long-lived density features (termed zonal flows) produced by the MRI can affect planet migration. The comparison with previous results from net toroidal flux/zero flux MHD simulations indicates that the magnetic field geometry plays an important role in the gap opening process. Overall, our results suggest that gaps can be commonly produced by low mass planets in realistic protoplanetary disks, and caution the use of a constant alpha-viscosity to model gaps in protoplanetary disks.
We extend the one-dimensional, two-zone models of long-term protostellar disk evolution with infall of Zhu et al. to consider the potential effects of a finite viscosity in regions where the ionization is too low for the magnetorotational instability
(MRI) to operate (the dead zone). We find that the presence of a small but finite dead zone viscosity, as suggested by simulations of stratified disks with MRI-active outer layers, can trigger inside-out bursts of accretion, starting at or near the inner edge of the disk, instead of the previously-found outside-in bursts with zero dead zone viscosity, which originate at a few AU in radius. These inside-out bursts of accretion bear a qualitative resemblance to the outburst behavior of one FU Ori object, V1515 Cyg, in contrast to the outside-in burst models which more closely resemble the accretion events in FU Ori and V1057 Cyg. Our results suggest that the type and frequency of outbursts are potentially a probe of transport efficiency in the dead zone. Simulations must treat the inner disk regions, $R lesssim 0.5$ AU, to show the detailed time evolution of accretion outbursts in general and to observe the inside-out bursts in particular.
We carry out local three dimensional (3D) hydrodynamic simulations of planet-disk interaction in stratified disks with varied thermodynamic properties. We find that whenever the Brunt-Vaisala frequency (N) in the disk is nonzero, the planet exerts a
strong torque on the disk in the vicinity of the planet, with a reduction in the traditional torque cutoff. In particular, this is true for adiabatic perturbations in disks with isothermal density structure, as should be typical for centrally irradiated protoplanetary disks. We identify this torque with buoyancy waves, which are excited (when N is non-zero) close to the planet, within one disk scale height from its orbit. These waves give rise to density perturbations with a characteristic 3D spatial pattern which is in close agreement with the linear dispersion relation for buoyancy waves. The torque due to these waves can amount to as much as several tens of per cent of the total planetary torque, which is not expected based on analytical calculations limited to axisymmetric or low-m modes. Buoyancy waves should be ubiquitous around planets in the inner, dense regions of protoplanetary disks, where they might possibly affect planet migration.
We use one-dimensional two-zone time-dependent accretion disk models to study the long-term evolution of protostellar disks subject to mass addition from the collapse of a rotating cloud core. Our model consists of a constant surface density magnetic
ally coupled active layer, with transport and dissipation in inactive regions only via gravitational instability. We start our simulations after a central protostar has formed, containing ~ 10% of the mass of the protostellar cloud. Subsequent evolution depends on the angular momentum of the accreting envelope. We find that disk accretion matches the infall rate early in the disk evolution because much of the inner disk is hot enough to couple to the magnetic field. Later infall reaches the disk beyond ~10 AU, and the disk undergoes outbursts of accretion in FU Ori-like events as described in Zhu et al. 2009c. If the initial cloud core is moderately rotating most of the central stars mass is built up by these outburst events. Our results suggest that the protostellar luminosity problem is eased by accretion during these FU Ori-like outbursts. After infall stops the disk enters the T Tauri phase. An outer, viscously evolving disk has structure that is in reasonable agreement with recent submillimeter studies and its surface density evolves from $Sigma propto R^{-1}$ to $R^{-1.5}$. An inner, massive belt of material-- the dead zone -- would not have been observed yet but should be seen in future high angular resolution observations by EVLA and ALMA. This high surface density belt is a generic consequence of low angular momentum transport efficiency at radii where the disk is magnetically decoupled, and would strongly affect planet formation and migration.
