593 research outputs found
On shocks driven by high-mass planets in radiatively inefficient disks. I. Two-dimensional global disk simulations
Recent observations of gaps and non-axisymmetric features in the dust
distributions of transition disks have been interpreted as evidence of embedded
massive protoplanets. However, comparing the predictions of planet-disk
interaction models to the observed features has shown far from perfect
agreement. This may be due to the strong approximations used for the
predictions. For example, spiral arm fitting typically uses results that are
based on low-mass planets in an isothermal gas. In this work, we describe
two-dimensional, global, hydrodynamical simulations of disks with embedded
protoplanets, with and without the assumption of local isothermality, for a
range of planet-to-star mass ratios 1-10 M_jup for a 1 M_sun star. We use the
Pencil Code in polar coordinates for our models. We find that the inner and
outer spiral wakes of massive protoplanets (M>5 M_jup) produce significant
shock heating that can trigger buoyant instabilities. These drive sustained
turbulence throughout the disk when they occur. The strength of this effect
depends strongly on the mass of the planet and the thermal relaxation
timescale; for a 10 M_jup planet embedded in a thin, purely adiabatic disk, the
spirals, gaps, and vortices typically associated with planet-disk interactions
are disrupted. We find that the effect is only weakly dependent on the initial
radial temperature profile. The spirals that form in disks heated by the
effects we have described may fit the spiral structures observed in transition
disks better than the spirals predicted by linear isothermal theory.Comment: 10 pages, 8 figures. ApJ, accepte
Chemistry in a gravitationally unstable protoplanetary disc
Until now, axisymmetric, alpha-disc models have been adopted for calculations
of the chemical composition of protoplanetary discs. While this approach is
reasonable for many discs, it is not appropriate when self-gravity is
important. In this case, spiral waves and shocks cause temperature and density
variations that affect the chemistry. We have adopted a dynamical model of a
solar-mass star surrounded by a massive (0.39 Msun), self-gravitating disc,
similar to those that may be found around Class 0 and early Class I protostars,
in a study of disc chemistry. We find that for each of a number of species,
e.g. H2O, adsorption and desorption dominate the changes in the gas-phase
fractional abundance; because the desorption rates are very sensitive to
temperature, maps of the emissions from such species should reveal the
locations of shocks of varying strengths. The gas-phase fractional abundances
of some other species, e.g. CS, are also affected by gas-phase reactions,
particularly in warm shocked regions. We conclude that the dynamics of massive
discs have a strong impact on how they appear when imaged in the emission lines
of various molecular species.Comment: 10 figures and 3 tables, accepted for publication in MNRA
Simulated Observations of Young Gravitationally Unstable Protoplanetary Discs
The formation and earliest stages of protoplanetary discs remain poorly
constrained by observations. ALMA will soon revolutionise this field.
Therefore, it is important to provide predictions which will be valuable for
the interpretation of future high sensitivity and high angular resolution
observations. Here we present simulated ALMA observations based on radiative
transfer modelling of a relatively massive (0.39 M_solar) self-gravitating disc
embedded in a 10 M_solar dense core, with structure similar to the pre-stellar
core L1544. We focus on simple species and conclude that C17O 3-2, HCO+ 3-2,
OCS 26-25 and H2CO 404-303 lines can be used to probe the disc structure and
kinematics at all scales.Comment: 12 pages, 15 figures, Accepted by MNRA
Stresses in and general instability of monocoque cylinders with cutouts VIII : calculation of the buckling load of cylinders with long symmetric cutout subjected to pure bending
Gravitational instabilities in a protosolar-like disc - I. Dynamics and chemistry
MGE gratefully acknowledges a studentship from the European Research Council (ERC; project PALs 320620). JDI gratefully acknowledges funding from the European Union FP7-2011 under grant agreement no. 284405. ACB's contribution was supported, in part, by The University of British Columbia and the Canada Research Chairs program. PC and TWH acknowledge the financial support of the European Research Council (ERC; project PALs 320620).To date, most simulations of the chemistry in protoplanetary discs have used 1 + 1D or 2D axisymmetric α-disc models to determine chemical compositions within young systems. This assumption is inappropriate for non-axisymmetric, gravitationally unstable discs, which may be a significant stage in early protoplanetary disc evolution. Using 3D radiative hydrodynamics, we have modelled the physical and chemical evolution of a 0.17 M⊙ self-gravitating disc over a period of 2000 yr. The 0.8 M⊙ central protostar is likely to evolve into a solar-like star, and hence this Class 0 or early Class I young stellar object may be analogous to our early Solar system. Shocks driven by gravitational instabilities enhance the desorption rates, which dominate the changes in gas-phase fractional abundances for most species. We find that at the end of the simulation, a number of species distinctly trace the spiral structure of our relatively low-mass disc, particularly CN. We compare our simulation to that of a more massive disc, and conclude that mass differences between gravitationally unstable discs may not have a strong impact on the chemical composition. We find that over the duration of our simulation, successive shock heating has a permanent effect on the abundances of HNO, CN and NH3, which may have significant implications for both simulations and observations. We also find that HCO+ may be a useful tracer of disc mass. We conclude that gravitational instabilities induced in lower mass discs can significantly, and permanently, affect the chemical evolution, and that observations with high-resolution instruments such as Atacama Large Millimeter/submillimeter Array (ALMA) offer a promising means of characterizing gravitational instabilities in protosolar discs.Publisher PDFPeer reviewe
On Shocks Driven by High-mass Planets in Radiatively Inefficient Disks. II. Three-dimensional Global Disk Simulations
Recent high-resolution, near-infrared images of protoplanetary disks have shown that these disks often present spiral features. Spiral arms are among the structures predicted by models of disk–planet interaction and thus it is tempting to suspect that planetary perturbers are responsible for these signatures. However, such interpretation is not free of problems. The observed spirals have large pitch angles, and in at least one case (HD 100546) it appears effectively unpolarized, implying thermal emission of the order of 1000 K (465 ± 40 K at closer inspection). We have recently shown in two-dimensional models that shock dissipation in the supersonic wake of high-mass planets can lead to significant heating if the disk is sufficiently adiabatic. Here we extend this analysis to three dimensions in thermodynamically evolving disks. We use the Pencil Code in spherical coordinates for our models, with a prescription for thermal cooling based on the optical depth of the local vertical gas column. We use a 5M_J planet, and show that shocks in the region around the planet where the Lindblad resonances occur heat the gas to substantially higher temperatures than the ambient gas. The gas is accelerated vertically away from the midplane to form shock bores, and the gas falling back toward the midplane breaks up into a turbulent surf. This turbulence, although localized, has high α values, reaching 0.05 in the inner Lindblad resonance, and 0.1 in the outer one. We find evidence that the disk regions heated up by the shocks become superadiabatic, generating convection far from the planet's orbit
The Formation of Fragments at Corotation in Isothermal Protoplanetary Disks
Numerical hydrodynamics simulations have established that disks which are
evolved under the condition of local isothermality will fragment into small
dense clumps due to gravitational instabilities when the Toomre stability
parameter is sufficiently low. Because fragmentation through disk
instability has been suggested as a gas giant planet formation mechanism, it is
important to understand the physics underlying this process as thoroughly as
possible. In this paper, we offer analytic arguments for why, at low ,
fragments are most likely to form first at the corotation radii of growing
spiral modes, and we support these arguments with results from 3D hydrodynamics
simulations.Comment: 21 pages, 1 figur
Gravitational Instabilities, Chondrule Formation, and the FU Orionis Phenomenon
Using analytic arguments and numerical simulations, we examine whether
chondrule formation and the FU Orionis phenomenon can be caused by the
burst-like onset of gravitational instabilities (GIs) in dead zones. At least
two scenarios for bursting dead zones can work, in principle. If the disk is on
the verge of fragmention, GI activation near to 5 AU can produce
chondrule-forming shocks, at least under extreme conditions. Mass fluxes are
also high enough during the onset of GIs to suggest that the outburst is
related to an FU Orionis phenomenon. This situation is demonstrated by
numerical simulations. In contrast, as supported by analytic arguments, if the
burst takes place close to AU, then even low pitch angle spiral waves
can create chondrule-producing shocks and outbursts. We also study the
stability of the massive disks in our simulations against fragmentation and
find that although disk evolution is sensitive to changes in opacity, the disks
we study do not fragment, even at high resolution and even for extreme
assumptions.Comment: To appear in Ap
The Internal Energy for Molecular Hydrogen in Gravitationally Unstable Protoplanetary Disks
The gas equation of state may be one of the critical factors for the disk
instability theory of gas giant planet formation. This letter addresses the
treatment of H in hydrodynamical simulations of gravitationally unstable
disks. In our discussion, we point out possible consequences of erroneous
specific internal energy relations, approximate specific internal energy
relations with discontinuities, and assumptions of constant . In
addition, we consider whether the ortho/para ratio for H in protoplanetary
disks should be treated dynamically as if the species are in equilibrium.
Preliminary simulations indicate that the correct treatment is particularly
critical for the study of gravitational instability when -50 K.Comment: 13 pages, 3 figures. To appear in ApJ
Convergence of SPH simulations of self-gravitating accretion discs: Sensitivity to the implementation of radiative cooling
Recent simulations of self-gravitating accretion discs, carried out using a
three-dimensional Smoothed Particle Hydrodynamics (SPH) code by Meru and Bate,
have been interpreted as implying that three-dimensional global discs fragment
much more easily than would be expected from a two-dimensional local model.
Subsequently, global and local two-dimensional models have been shown to
display similar fragmentation properties, leaving it unclear whether the
three-dimensional results reflect a physical effect or a numerical problem
associated with the treatment of cooling or artificial viscosity in SPH. Here,
we study how fragmentation of self-gravitating disc flows in SPH depends upon
the implementation of cooling. We run disc simulations that compare a simple
cooling scheme, in which each particle loses energy based upon its internal
energy per unit mass, with a method in which the cooling is derived from a
smoothed internal energy density field. For the simple per particle cooling
scheme, we find a significant increase in the minimum cooling time scale for
fragmentation with increasing resolution, matching previous results. Switching
to smoothed cooling, however, results in lower critical cooling time scales,
and tentative evidence for convergence at the highest spatial resolution
tested. We conclude that precision studies of fragmentation using SPH require
careful consideration of how cooling (and, probably, artificial viscosity) is
implemented, and that the apparent non-convergence of the fragmentation
boundary seen in prior simulations is likely a numerical effect. In real discs,
where cooling is physically smoothed by radiative transfer effects, the
fragmentation boundary is probably displaced from the two-dimensional value by
a factor that is only of the order of unity.Comment: 9 pages, 11 figures, MNRAS in pres
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