138 research outputs found
Efficiency of thermal relaxation by radiative processes in protoplanetary discs: constraints on hydrodynamic turbulence
Hydrodynamic, non-magnetic instabilities can provide turbulent stress in the
regions of protoplanetary discs, where the MRI can not develop. The induced
motions influence the grain growth, from which formation of planetesimals
begins. Thermal relaxation of the gas constrains origins of the identified
hydrodynamic sources of turbulence in discs.
We estimate the radiative relaxation timescale of temperature perturbations
and study the dependence of this timescale on the perturbation wavelength, the
location within the disc, the disc mass, and the dust-to-gas mass ratio. We
then apply thermal relaxation criteria to localise modes of the convective
overstability, the vertical shear instability, and the zombie vortex
instability.
Our calculations employed the latest tabulated dust and gas mean opacities
and we account for the collisional coupling to the emitting species.
The relaxation criterion defines the bulk of a typical T Tauri disc as
unstable to the development of linear hydrodynamic instabilities. The midplane
is unstable to the convective overstability from at most 2\mbox{ au} and up
to 40\mbox{ au}, as well as beyond 140\mbox{ au}. The vertical shear
instability can develop between 15\mbox{ au} and 180\mbox{ au}. The
successive generation of (zombie) vortices from a seeded noise can work within
the inner 0{.}8\mbox{ au}.
Dynamic disc modelling with the evolution of dust and gas opacities is
required to clearly localise the hydrodynamic turbulence, and especially its
non-linear phase.Comment: 13 pages, 8 figure
Magnetically driven accretion in protoplanetary discs
We characterize magnetically driven accretion at radii between 1 au and 100
au in protoplanetary discs, using a series of local non-ideal
magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass
Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of
specified strength. Confirming previous results, we find that the Hall effect
has only a modest impact on accretion at 30 au, and essentially none at 100 au.
At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion
process, with vertical magnetic fields aligned to the disc rotation supporting
a strong laminar Maxwell stress that is absent if the field is anti-aligned. In
the anti-aligned case, we instead find evidence for bursts of turbulent stress
at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear
instability. The presence or absence of these bursts depends upon the details
of the adopted chemical model, which suggests that appreciable regions of
actual protoplanetary discs might lie close to the borderline between laminar
and turbulent behaviour. Given the number of important control parameters that
have already been identified in MHD models, quantitative predictions for disc
structure in terms of only radius and accretion rate appear to be difficult.
Instead, we identify robust qualitative tests of magnetically driven accretion.
These include the presence of turbulence in the outer disc, independent of the
orientation of the vertical magnetic fields, and a Hall-mediated bi-modality in
turbulent properties extending from the region of thermal ionization to 10 au.Comment: accepted to MNRAS after very minor revision
Magnetic fields in gaps surrounding giant protoplanets
Giant protoplanets evacuate a gap in their host protoplanetary disc, which
gas must cross before it can be accreted. A magnetic field is likely carried
into the gap, potentially influencing the flow. Gap crossing has been simulated
with varying degrees of attention to field evolution (pure hydrodynamical,
ideal, and resistive MHD), but as yet there has been no detailed assessment of
the role of the field accounting for all three key non-ideal MHD effects: Ohmic
resistivity, ambipolar diffusion, and Hall drift. We present a detailed
investigation of gap magnetic field structure as determined by non-ideal
effects. We assess susceptibility to turbulence induced by the
magnetorotational instability, and angular momentum loss from large-scale
fields. As full non-ideal simulations are computationally expensive, we take an
a posteriori approach, estimating MHD quantities from the pure hydrodynamical
gap crossing simulation by Tanigawa et al. (2012). We calculate the ionisation
fraction and estimate field strength and geometry to determine the strength of
non-ideal effects. We find that the protoplanetary disc field would be easily
drawn into the gap and circumplanetary disc. Hall drift dominates, so that much
of the gap is conditionally MRI unstable depending on the alignment of the
field and disc rotation axes. Field alignment also influences the strong
toroidal field component permeating the gap. Large-scale magnetic forces are
small in the circumplanetary disc, indicating they cannot drive accretion
there. However, turbulence will be key during satellite growth as it affects
critical disc features, such as the location of the ice line.Comment: 14 pages, 6 figures, accepted for publication by MNRA
Magnetic self-organisation in Hall-dominated magnetorotational turbulence
The magnetorotational instability (MRI) is the most promising mechanism by
which angular momentum is efficiently transported outwards in astrophysical
discs. However, its application to protoplanetary discs remains problematic.
These discs are so poorly ionised that they may not support magnetorotational
turbulence in regions referred to as `dead zones'. It has recently been
suggested that the Hall effect, a non-ideal magnetohydrodynamic (MHD) effect,
could revive these dead zones by enhancing the magnetically active column
density by an order of magnitude or more. We investigate this idea by
performing local, three-dimensional, resistive Hall-MHD simulations of the MRI
in situations where the Hall effect dominates over Ohmic dissipation. As
expected from linear stability analysis, we find an exponentially growing
instability in regimes otherwise linearly stable in resistive MHD. However,
instead of vigorous and sustained magnetorotational turbulence, we find that
the MRI saturates by producing large-scale, long-lived, axisymmetric structures
in the magnetic and velocity fields. We refer to these structures as zonal
fields and zonal flows, respectively. Their emergence causes a steep reduction
in turbulent transport by at least two orders of magnitude from extrapolations
based upon resistive MHD, a result that calls into question contemporary models
of layered accretion. We construct a rigorous mean-field theory to explain this
new behaviour and to predict when it should occur. Implications for
protoplanetary disc structure and evolution, as well as for theories of planet
formation, are briefly discussed.Comment: 18 pages, 16 figures, accepted for publication in MNRA
Grain charging in protoplanetary discs
Recent work identified a growth barrier for dust coagulation that originates
in the electric repulsion between colliding particles. Depending on its charge
state, dust material may have the potential to control key processes towards
planet formation such as MHD (magnetohydrodynamic) turbulence and grain growth
which are coupled in a two-way process. We quantify the grain charging at
different stages of disc evolution and differentiate between two very extreme
cases: compact spherical grains and aggregates with fractal dimension D_f = 2.
Applying a simple chemical network that accounts for collisional charging of
grains, we provide a semi-analytical solution. This allowed us to calculate the
equilibrium population of grain charges and the ionisation fraction
efficiently. The grain charging was evaluated for different dynamical
environments ranging from static to non-stationary disc configurations. The
results show that the adsorption/desorption of neutral gas-phase heavy metals,
such as magnesium, effects the charging state of grains. The greater the
difference between the thermal velocities of the metal and the dominant
molecular ion, the greater the change in the mean grain charge. Agglomerates
have more negative excess charge on average than compact spherical particles of
the same mass. The rise in the mean grain charge is proportional to N**(1/6) in
the ion-dust limit. We find that grain charging in a non-stationary disc
environment is expected to lead to similar results. The results indicate that
the dust growth and settling in regions where the dust growth is limited by the
so-called "electro-static barrier" do not prevent the dust material from
remaining the dominant charge carrier.Comment: 18 pages, 10 figures, accepted for publication in Astronomy and
Astrophysic
Connecting Planetary Composition with Formation
The rapid advances in observations of the different populations of
exoplanets, the characterization of their host stars and the links to the
properties of their planetary systems, the detailed studies of protoplanetary
disks, and the experimental study of the interiors and composition of the
massive planets in our solar system provide a firm basis for the next big
question in planet formation theory. How do the elemental and chemical
compositions of planets connect with their formation? The answer to this
requires that the various pieces of planet formation theory be linked together
in an end-to-end picture that is capable of addressing these large data sets.
In this review, we discuss the critical elements of such a picture and how they
affect the chemical and elemental make up of forming planets. Important issues
here include the initial state of forming and evolving disks, chemical and dust
processes within them, the migration of planets and the importance of planet
traps, the nature of angular momentum transport processes involving turbulence
and/or MHD disk winds, planet formation theory, and advanced treatments of disk
astrochemistry. All of these issues affect, and are affected by the chemistry
of disks which is driven by X-ray ionization of the host stars. We discuss how
these processes lead to a coherent end-to-end model and how this may address
the basic question.Comment: Invited review, accepted for publication in the 'Handbook of
Exoplanets', eds. H.J. Deeg and J.A. Belmonte, Springer (2018). 46 pages, 10
figure
Ice Lines, Planetesimal Composition and Solid Surface Density in the Solar Nebula
To date, there is no core accretion simulation that can successfully account
for the formation of Uranus or Neptune within the observed 2-3 Myr lifetimes of
protoplanetary disks. Since solid accretion rate is directly proportional to
the available planetesimal surface density, one way to speed up planet
formation is to take a full accounting of all the planetesimal-forming solids
present in the solar nebula. By combining a viscously evolving protostellar
disk with a kinetic model of ice formation, we calculate the solid surface
density in the solar nebula as a function of heliocentric distance and time. We
find three effects that strongly favor giant planet formation: (1) a decretion
flow that brings mass from the inner solar nebula to the giant planet-forming
region, (2) recent lab results (Collings et al. 2004) showing that the ammonia
and water ice lines should coincide, and (3) the presence of a substantial
amount of methane ice in the trans-Saturnian region. Our results show higher
solid surface densities than assumed in the core accretion models of Pollack et
al. (1996) by a factor of 3 to 4 throughout the trans-Saturnian region. We also
discuss the location of ice lines and their movement through the solar nebula,
and provide new constraints on the possible initial disk configurations from
gravitational stability arguments.Comment: Version 2: reflects lead author's name and affiliation change,
contains minor changes to text from version 1. 12 figures, 7 tables, accepted
for publication in Icaru
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