Formation Of Protoplanetary Disks Research Articles

Gravitational Collapse of Magnetized Clouds. I. Ideal Magnetohydrodynamic Accretion Flow

We study the self-similar collapse of an isothermal magnetized rotating cloud in the ideal magnetohydrodynamic (MHD) regime. In the limit of small distance from the accreting protostar, we find an analytic solution that corresponds to free fall onto a central mass point. The density distribution is not spherically symmetric but depends on the mass loading of magnetic field lines, which can be obtained by matching our inner solution to an outer collapse solution previously computed by Allen et al. The concentration of magnetic field trapped by the central mass point under field freezing, independent on the details of the starting state, creates a split-monopole configuration in which the magnetic field strength increases as the inverse square of the distance from the center. Under such conditions, the inflow eventually becomes sub-Alfvenic and the outward transfer of angular momentum by magnetic braking very efficient, thus preventing the formation of a centrifugally supported disk. Instead, the azimuthal velocity of the infalling gas decreases to zero at the center, and the gas spirals into the star. Therefore, the dissipation of dynamically important levels of magnetic field is a fundamental requisite for the formation of protoplanetary disks around young stars.

The Onset of Planet Formation in Brown Dwarf Disks

The onset of planet formation in protoplanetary disks is marked by the growth and crystallization of sub-micrometer-sized dust grains accompanied by dust settling toward the disk mid-plane. Here, we present infrared spectra of disks around brown dwarfs and brown dwarf candidates. We show that all three processes occur in such cool disks in a way similar or identical to that in disks around low- and intermediate-mass stars. These results indicate that the onset of planet formation extends to disks around brown dwarfs, suggesting that planet formation is a robust process occurring in most young circumstellar disks.

Evolution of protoplanetary disks: constraints from DM Tauri and GM Aurigae

We present a one-dimensional model of the formation and viscous evolution of protoplanetary disks. The formation of the early disk is modeled as the result of the gravitational collapse of an isothermal molecular cloud. The disk's viscous evolution is integrated according to two parameterizations of turbulence: the classical α representation and a β parameterization, representative of non-linear turbulence driven by the keplerian shear. We apply the model to DM Tau and GM Aur, two classical T-Tauri stars with relatively well-characterized disks, retrieving the evolution of their surface density with time. We perform a systematic Monte-Carlo exploration of the parameter space (i.e. values of the α-β parameters, and of the temperature and rotation rate in the molecular cloud) to find the values that are compatible with the observed disk surface density distribution, star and disk mass, age and present accretion rate. We find that the observations for DM Tau require or . For GM Aur, we find that the turbulent viscosity is such that or . These relatively large values show that an efficient turbulent diffusion mechanism is present at distances larger than ~AU. This is to be compared to studies of the variations of accretion rates of T-Tauri stars versus age that mostly probe the inner disks, but also yield values of . We show that the mechanism responsible for turbulent diffusion at large orbital distances most probably cannot be convection because of its suppression at low optical depths.

Formation of giant planets in disks with different metallicities

We present the first results from simulations of processes leading to planet formation in protoplanetary disks with different metallicities. For a given metallicity, we construct a two-dimensional grid of disk models with different initial masses and radii (M0, R0). For each disk, we follow the evolution of gas and solids from an early evolutionary stage, when all solids are in the form of small dust grains, to the stage when most solids have condensed into planetesimals. Then, based on the core accretion - gas capture scenario, we estimate the planet-bearing capability of the environment defined by the final planetesimal swarm and the still evolving gaseous component of the disk. We define the probability of planet-formation, Pp, as the normalized fractional area in the (M0 ,l ogR0) plane populated by disks that have formed planets inside 5 AU. With such a definition, and under the assumption that the population of planets discovered at R 5 AU, our results agree fairly well with the observed dependence between the probability that a star harbors a planet and the star's metal content. The agreement holds for the disk viscosity parameter α ranging from 10 −3 to 10 −2 , and it becomes much poorer when the redistribution of solids relative to the gas is not allowed for during the evolution of model disks.

The Desert Fireball Network

Meteorites contain a record of processes ranging from the pre-solar stellar evolution of our galactic neighbourhood, to protoplanetary disk formation and evolution, and the accretion and differentiation of planetesimals and protoplanets in the early solar system. But unlike geologists, planetary scientists must interpret this unique record with no spatial context to help them: with precise orbital data for only four samples, we don't really know where individual meteorites come from. Camera networks, designed to calculate orbits and fall positions of meteorites based on observations of fireballs, have been in operation for several decades but have recovered only a small number of samples - meteorites are very difficult to find in vegetated areas. We intend to remedy this problem by siting a network in a place where we already know we can recover meteorites - the Nullarbor desert of Western Australian. Our first camera was installed at a test site in Australia in October 2003 and has performed flawlessly since that date, taking images of fireballs, several large enough to have dropped meteorites.

On the Evolution and Stability of a Protoplanetary Disk Dust Layer

In this manuscript we perform detailed analytical and numerical studies of the physical processes that result from the interaction between the dust and gas components of a protostellar accretion disk. We consider the most favorable condition for dust sedimentation in a laminar gaseous background and look at the conditions that precede the onset of gravitational and shear instability. We adopt a two-fluid formalism to examine two issues of particular interest: (1) the slow sedimentation of a laminar dust layer, for which we are able to extract self-similar solutions, and (2) the rapid growth of perturbations to that slowly settling shear flow, for which we present a linear stability analysis. We also include two fundamental physical processes that have been ignored in previous work: the specificities of the two-fluid equations compared to the single-fluid approximation, and the potentially destabilizing effect of radiative cooling by the reduction of the buoyancy of fluid motions. From these results we are able to compare the conditions for onset of gravitational and shearing instabilities for various dust-to-gas surface density ratios and nebular structures. We confirm previous findings that during the dust sedimentation, the shearing instability occurs prior to the gravitational instability unless the disk is grossly enriched up to a level where the surface density of grains is comparable to that of the gas. Finally, we discuss the implications of these results with respect to theories of planetesimal formation and observations of protoplanetary disks.

Infall in Protostellar Envelopes

We summarize recent progress of observational studies of infall in protostellar envelopes, with great emphasis on results obtained using millimeter and submillimeter interferometers. Interferometric observations allow us to spatially resolve kinematical structures of protostellar envelopes, enabling us to detect infalling motions in the envelope directly. High angular resolution observations of infalling envelopes having compact disks sufficiently bright in continuum show inverse P-Cygni profiles, which are the least ambiguous evidence for infall. Observations of infalling envelopes using the Submillimeter Array (SMA) may allow us to study the innermost infalling envelopes, where infalling motions most probably transform to Keplerian motions, leading to formation of Keplerian or protoplanetary disks around protostars.

The effect of cooling on the global stability of self-gravitating protoplanetary discs

Using a local model, Gammie has shown that accretion discs with cooling times tcool ≤ 3Ω−1 fragment into gravitationally bound objects, while those with cooling times tcool > 3Ω−1 evolve into a quasi-steady state. We use three-dimensional smoothed particle hydrodynamic simulations of protoplanetary accretion discs to test if the local results hold globally. We find that for disc masses appropriate for T Tauri discs, the fragmentation boundary still occurs at a cooling time close to tcool= 3Ω−1. For more massive discs, which are likely to be present at an earlier stage of the star formation process, fragmentation occurs for longer cooling times, but still within a factor of 2 of that predicted using a local model. These results have implications not only for planet formation in protoplanetary discs and star formation in active galactic nucleus discs, but also for the redistribution of angular momentum which could be driven by the presence of relatively massive objects within the accretion disc.

Large-Scale Vortices in Protoplanetary Disks: On the Observability of Possible Early Stages of Planet Formation

We investigate the possibility of mapping large-scale anticyclonic vortices, resulting from a global baroclinic instability, as precursors of planet formation in protoplanetary disks with the planned Atacama Large Millimeter Array (ALMA). On the basis of three-dimensional radiative transfer simulations, images of a hydrodynamically calculated disk are derived that provide the basis for the simulation of ALMA. We find that ALMA will be able to trace the theoretically predicted large-scale anticyclonic vortex and will therefore allow testing of existing models of this very early stage of planet formation in circumstellar disks.

The Formation and Role of Vortices in Protoplanetary Disks

We carry out a two-dimensional, compressible, simulation of a disk, including dust particles, to study the formation and role of vortices in protoplanetary disks. We find that anticyclonic vortices can form out of an initial random perturbation of the vorticity field. Vortices have a typical decay time of the order of 50 orbital periods (for a viscosity parameter alpha=0.0001 and a disk aspect ratio of H/r = 0.15). If vorticity is continuously generated at a constant rate in the flow (e.g. by convection), then a large vortex can form and be sustained (due to the merger of vortices). We find that dust concentrates in the cores of vortices within a few orbital periods, when the drag parameter is of the order of the orbital frequency. Also, the radial drift of the dust induces a significant increase in the surface density of dust particles in the inner region of the disk. Thus, vortices may represent the preferred location for planetesimal formation in protoplanetary disks. We show that it is very difficult for vortex mergers to sustain a relatively coherent outward flux of angular momentum.

On The Dynamics Of Dust Grains In A Hierarchical Environment

The star formation process usually leads to the formation of protoplanetary disks. Planets are thought to arise from the material of these disks. Amongst the stars in the solar neighbourhood, single systems like our own are a minority. Most stars are found in binaries or in systems of even higher multiplicity. In this paper, we extend the simulations presented in Paper I (de la Fuente Marcos and de la Fuente Marcos, 1998a) to hierarchical triple systems. As in Paper I, we study the stage of planetary formation during which the particulate material is still dispersed as centimetre-to-metre sized primordial aggregates. We investigate the response of the particles, in a protoplanetary disk with radius RD = 100 AU around a solar-like star, to the gravitational field of bound perturbing companions in a moderately wide (300–1600 AU) orbit. As for this problem no analytic description of the orbital evolution of the particles exists, we perform numerical integrations using a Bulirsch–Stoer integrator. For this purpose, we have carried out a series of simulations of coplanar hierarchical configurations with three stars using a direct integration code that models gravitational and viscous forces. The massive protoplanetary disk is around one of the components of the triple system. As in Paper I, the evolution in time of the dust sub-disk depends mainly on the nature (prograde or retrograde) of the relative revolution of the stellar companions, and on the temperature and mass of the circumstellar disk. The perturbation of prograde companions induces a trailing spiral structure across the protoplanetary sub-metric dust sub-disk. Metre-sized particles are affected by strong precession. Our results show that the lifetime of particles in a disk in a hierarchical triple system is slightly shorter relative to its valuein Paper I, although the actual value depends on the nature, prograde or retrograde, of the outer companion. The lifetime of particles in a hierarchical triple system including a prograde inner binary and a retrograde outer body is longer than in an equivalent triple with all the companions prograde. Dust disks in hierarchical triple systems with both companions in retrograde motion with respect to the particles in the disk show the shortest lifetimes. Our previous calculations suggest that disk luminosities in binary systems are several orders of magnitude higher than those for single stars. Circumstellar disks in triple systems may be 5–50% more luminous depending on the relative direction of rotation.

Mid-Infrared Imaging of the Young Binary Star Hen 3-600: Evidence for a Dust Disk around the Primary

We present high-resolution mid-infrared observations of the nearby late-type young binary system Hen 3-600. The binary, at a distance of ~50 pc, could be a member of the TW Hydrae association, the nearest known group of young stars, with an age of a few million years. Our images make it possible for the first time to determine which star in the pair, separated by 14, harbors the mid-infrared excess detected by IRAS. In the near-infrared, where the radiation is primarily photospheric, Hen 3-600A (M3) and Hen 3-600B (M3.5) have a flux ratio of 1.6. At 4.8, 10.8, and 18.2 μm, the primary becomes increasingly dominant over the secondary, suggesting that most of the circumstellar dust in the system resides around Hen 3-600A. Comparison of the spectral energy distribution (SED) of Hen 3-600A to the median SED of classical T Tauri stars suggests that its disk may be truncated by the secondary and provides tentative evidence for a central disk hole. The distribution of dust in the Hen 3-600 system may provide important clues to the formation and evolution of protoplanetary disks in close binaries.

Particle accumulation structures (PAS) in the toroidal thermocapillary vortex of a floating zone — Model for a step in planet-formation?

We report on experiments with small particles (tracers) in a ring vortex of thermocapillary flow (TF) in NaNO 3 melt which form for favorable conditions “clouds” if TF is oscillatory and ring structures if TF is time-independent or time-independent with small disturbances. We call this novel phenomenon particle accumulation structure (PAS). PAS is in our case a three-dimensional phenomenon, but we express the hope that modelling PAS numerically can lead to the experimentally tested basis for modelling cloud formation in proto-planetary accretion disks, where these clouds could become embryos for planets.

Gap Formation in Protoplanetary Disks

view Abstract Citations (162) References (59) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Gap Formation in Protoplanetary Disks Takeuchi, Taku ; Miyama, Shoken M. ; Lin, D. N. C. Abstract Evolution of a protoplanetary disk under the tidal interaction between the disk and an embedded protoplanet is analyzed with a self-consistent WKB approximation. We assume that the protoplanetary disk is infinitesimally thin and non-self-gravitating and that the protoplanet's orbit is circular. The protoplanet excites density waves at its Lindblad resonances. As they propagate throughout the disk, these waves carry a flux of angular momentum that is eventually deposited into the gas at the locations where the waves are dissipated viscously. Protoplanets with a sufficiently large mass can induce the formation of a gap in the disk. The size of the gap and the structure of the disk are determined by the wave propagation length scale, which is a decreasing function of viscosity. For small effective viscosity, density waves propagate to inner regions near the protostellar surface. Using an α prescription, we find that a protoplanet with a mass of Jupiter can lead to the removal of the inner disk if α ≲ 3 × 10-4 . For larger values of α, the surface density in the disk surrounding the gap is adjusted in a manner such that the rapid orbital evolution of the protoplanet is prevented. We also inferred that α ∼ 1.7 × 10-2 in the disk around the binary T Tauri star GW Ori, based on the gap size derived from the observational data. Publication: The Astrophysical Journal Pub Date: April 1996 DOI: 10.1086/177013 Bibcode: 1996ApJ...460..832T Keywords: ACCRETION; ACCRETION DISKS; STARS: PLANETARY SYSTEMS; STARS: FORMATION full text sources ADS | data products SIMBAD (2)

Formation, early evolution, and gravitational stability of protoplanetary disks

The formation, viscous evolution, and gravitational stability of protoplanetary disks are investigated. The formation process is parameterized by the angular velocity of the molecular cloud core omega, while the viscous evolution is parameterized by the viscosity parameter alpha in the disk; in this study we consider a range of (0.4-6) x 10(exp -14)/s for omega and from 10(exp -5) to 10(exp -1) for alpha. The axisymmetric gravitational stabilities of the disks are checked using Toomre's criterion. The resulting disk surface temperature distribution, (d log T(sub s)/d log R) approximately = -0.6 (R is the cylindrical radius), can be attributed to two heating sources: the viscous heating dominant in the inner disk region, and the accretion shock heating dominant in the outer disk region. This surface temperature distribution matches that observed in many disks around young stellar objects. During the infall stage, disks with alpha less than 10(exp -1.5) become gravitationally unstable independent of omega. The gravitational instabilities occur at radii ranging from 5 to 40 AU. The ratio of the disk mass to the central star mass ranges from 0.2 to 0.5 at the times of instability, about 4 x 10(exp -5) x (omega/10(exp -14)/s)(exp -0.67) yr. Most disks with low alpha and high omega become gravitationally unstable during their formation phase.

Formation and Fragmentation of Protostellar and Protoplanetary Disksa

Formation and Fragmentation of Protostellar and Protoplanetary Disks^a

Formation of binary systems and protoplanetary discs: A unified approach

We develop a new thermodynamic approach to the problem of the last stages of star formation, when a collapsing fragment evolves adiabatically into its final state: single protostar, surrounded or not by protoplanetary disc, or binary system. In this context, we point out the crucial role of the angular momentum transfer: a very efficient mechanism tends to form double stars with small mass secondaries, while a total decoupling yields twin binaries. Intermediate assumptions allow the birth of both kinds of binary systems, as well as the formation of not very massive protoplanetary discs. Discs of larger mass, which would be required to produce protoplanetary systems as a consequence of dynamical instabilities, do not form under any circumstances. A representation of the outcomes as functions of the corresponding initial conditions on the usual α−β plane gives well definite regions for single stars, protoplanetary discs, unbalanced systems and twin binaries. On this ground, a preliminary estimate of the percentage of stars surrounded by planetary systems is possible. A particular numerical simulation confirms the bimodality of the mass ratio distribution as well as the main features of the α−β plane partition. A few suggestions about non-adiabatic effects are also given. Our thermodynamic approach, supported by the numerical one and by the analysis of the observational statistics, allow to define a first unitary sketch for the formation of binary systems and protoplanetary discs.