249 research outputs found
Non-linear evolution of the tidal elliptical instability in gaseous planets and stars
Tidally distorted rotating stars and gaseous planets are subject to a well-known linear fluid instability – the elliptical instability. It has been proposed that this instability might drive enough energy dissipation to solve the long-standing problem of the origin of tidal dissipation in stars and planets. But the non-linear outcome of the elliptical instability has yet to be investigated in the parameter regime of interest, and the resulting turbulent energy dissipation has not yet been quantified. We do so by performing three-dimensional hydrodynamical simulations of a small patch of a tidally deformed fluid planet or star subject to the elliptical instability. We show that when the tidal deformation is weak, the non-linear outcome of the instability leads to the formation of long-lived columnar vortices aligned with the axis of rotation. These vortices shut off the elliptical instability, and the net result is insufficient energy dissipation to account for tidal dissipation. However, further work is required to account for effects neglected here, including magnetic fields, turbulent convection and realistic boundary conditions
Theory and Simulations of Rotating Convection
We study thermal convection in a rotating fluid in order to better understand the properties of convection zones in rotating stars and planets. We first derive a mixing-length theory for rapidly rotating convection, arriving at the results of Stevenson via simple physical arguments. The theory predicts the properties of convection as a function of the imposed heat flux and rotation rate, independent of microscopic diffusivities. In particular, it predicts the mean temperature gradient, the rms velocity and temperature fluctuations, and the size of the eddies that dominate heat transport. We test all of these predictions with high resolution three-dimensional hydrodynamical simulations of Boussinesq convection in a Cartesian box. The results agree remarkably well with the theory across more than two orders of magnitude in rotation rate. For example, the temperature gradient is predicted to scale as the rotation rate to the four-fifths power at fixed flux, and the simulations yield 0.75 ± 0.06. We conclude that the mixing-length theory is a solid foundation for understanding the properties of convection zones in rotating stars and planets
A Brief History of Trans-Neptunian Space
The Edgeworth-Kuiper belt encodes the dynamical history of the outer solar
system. Kuiper belt objects (KBOs) bear witness to coagulation physics, the
evolution of planetary orbits, and external perturbations from the solar
neighborhood. We critically review the present-day belt's observed properties
and the theories designed to explain them. Theories are organized according to
a possible time-line of events. In chronological order, epochs described
include (1) coagulation of KBOs in a dynamically cold disk, (2) formation of
binary KBOs by fragmentary collisions and gravitational captures, (3) stirring
of KBOs by Neptune-mass planets (``oligarchs''), (4) eviction of excess
oligarchs, (5) continued stirring of KBOs by remaining planets whose orbits
circularize by dynamical friction, (6) planetary migration and capture of
Resonant KBOs, (7) creation of the inner Oort cloud by passing stars in an open
stellar cluster, and (8) collisional comminution of the smallest KBOs. Recent
work underscores how small, collisional, primordial planetesimals having low
velocity dispersion permit the rapid assembly of ~5 Neptune-mass oligarchs at
distances of 15-25 AU. We explore the consequences of such a picture. We
propose that Neptune-mass planets whose orbits cross into the Kuiper belt for
up to ~20 Myr help generate the high-perihelion members of the hot Classical
disk and Scattered belt. By contrast, raising perihelia by sweeping secular
resonances during Neptune's migration might fill these reservoirs too
inefficiently when account is made of how little primordial mass might reside
in bodies having sizes of order 100 km. These and other frontier issues in
trans-Neptunian space are discussed quantitatively.Comment: Final proofed version for Protostars and Planets V; some numbers
adjusted by factors of 2; references update
Imbalanced Strong MHD Turbulence
We present a phenomenological model of imbalanced MHD turbulence in an
incompressible magnetofluid. The steady-state cascades, of waves traveling in
opposite directions along the mean magnetic field, carry unequal energy fluxes
to small length scales, where they decay due to viscous and resistive
dissipation. The inertial-range scalings are well-understood when both cascades
are weak. We study the case when both cascades are, in a sense, strong. The
inertial-range of this imbalanced cascade has the following properties: (i) the
ratio of the r.m.s. Elsasser amplitudes is independent of scale, and is equal
to the ratio of the corresponding energy fluxes; (ii) in common with the
balanced strong cascade, the energy spectra of both Elsasser waves are of the
anisotropic Kolmogorov form, with their parallel correlation lengths equal to
each other on all scales, and proportional to the two-thirds power of the
transverse correlation length; (iii) the equality of cascade time and
waveperiod (critical balance) that characterizes the strong balanced cascade
does not apply to the Elsasser field with the larger amplitude. Instead, the
more general criterion that always applies to both Elsasser fields is that the
cascade time is equal to the correlation time of the straining imposed by
oppositely-directed waves. Our results are particularly relevant for turbulence
in the solar wind. Spacecraft measurements have established that, in the
inertial range of solar wind turbulence, waves travelling away from the sun
have higher amplitudes than those travelling towards it. Result (i) allows us
to infer the turbulent flux ratios from the amplitude ratios, thus providing
insight into the origin of the turbulence
Neptune Trojans as a Testbed for Planet Formation
The problem of accretion in the Trojan 1:1 resonance is akin to the standard
problem of planet formation, transplanted from a star-centered disk to a disk
centered on the Lagrange point. The newly discovered class of Neptune Trojans
promises to test theories of planet formation by coagulation. Neptune Trojans
resembling the prototype 2001 QR322 (``QR'')--whose radius of ~100 km is
comparable to that of the largest Jupiter Trojan--may outnumber their Jovian
counterparts by a factor of ~10. We discover that seeding the 1:1 resonance
with debris from planetesimal collisions and having the seed particles accrete
in situ naturally reproduces the inferred number of QR-sized Trojans. We
analyze accretion in the Trojan sub-disk by applying the two-groups method,
accounting for kinematics specific to the resonance. We find that a Trojan
sub-disk comprising decimeter-sized seed particles and having a surface density
1e-3 that of the local minimum-mass disk produces ~10 QR-sized objects in ~1
Gyr, in accord with observation. Further growth is halted by collisional
diffusion of seed particles out of resonance. In our picture, the number and
sizes of the largest Neptune Trojans represent the unadulterated outcome of
dispersion-dominated, oligarchic accretion. Large Neptune Trojans, perhaps the
most newly accreted objects in our Solar System, may today have a dispersion in
orbital inclination of less than ~10 degrees, despite the existence of niches
of stability at higher inclinations. Such a vertically thin disk, born of a
dynamically cold environment necessary for accretion, and raised in minimal
contact with external perturbations, contrasts with the thick disks of other
minor body belts.Comment: Accepted to ApJ April 6, 200
Transformation of the Poynting flux into the kinetic energy in relativistic jets
The acceleration of relativistic jets from the Poynting to the matter
dominated stage is considered. The are generally two collimation regimes, which
we call equilibrium and non-equilibrium, correspondingly. In the first regime,
the jet is efficiently accelerated till the equipartition between the kinetic
and electro-magnetic energy. We show that after the equilibrium jet ceases to
be Poynting dominated, the ratio of the electro-magnetic to the kinetic energy
decreases only logarithmically so that such jets become truly matter dominated
only at extremely large distances. Non-equilibrium jets remain generally
Poynting dominated till the logarithmically large distances. In the only case
when a non-equilibrium jet is accelerated till the equipartition level, we
found that the flow is not continued to the infinity but is focused towards the
axis at a finite distance from the origin.Comment: Submitted to MNRAS Minor changes in the Conclusion
Nonlinear energy transfers in accretion discs MRI turbulence. I-Net vertical field case
The magnetorotational instability (MRI) is believed to be responsible for
most of the angular momentum transport in accretion discs. However, molecular
dissipation processes may drastically change the efficiency of MRI turbulence
in realistic astrophysical situations. The physical origin of this dependency
is still poorly understood as linear and quasi linear theories fail to explain
it. In this paper, we look for the link between molecular dissipation processes
and MRI transport of angular momentum in non stratified shearing box
simulations including a mean vertical field. We show that magnetic helicity is
unimportant in the model we consider. We perform a spectral analysis on the
simulations tracking energy exchanges in spectral space when turbulence is
fully developed. We find that the energy exchanges are essentially direct (from
large to small scale) whereas some non linear interactions appear to be non
local in spectral space. We speculate that these non local interactions are
responsible for the correlation between turbulent transport and molecular
dissipation. We argue that this correlation should then disappear when a
significant scale separation is achieved and we discuss several methods by
which one can test this hypothesis.Comment: 10 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Secular dynamics of multiplanet systems: implications for the formation of hot and warm Jupiters via high-eccentricity migration
Stars and planetary system
UV/Optical Emission Accompanying Gamma-ray Burst
We discuss the possible simultaneously UV/optical emission accompanying
Gamma-ray bursts (GRBs). We show that as long as the intrinsic spectrum of GRB
can extend to 10 GeV or higher, there is a large amount of relativistic
pairs generated due to the annihilation of the soft rays with
the very energetic photons, which dominates over the electrons/positrons
associated with the fireball, no matter the fireball is highly magnetized or
not (For the highly magnetized fireball, the magnetic field is ordered, the
high linear polarization of the multi-wavelength emission is expected). We find
that these pairs can power an UV flash with
magnitude, and the corresponding optical emission can be up to magnitude. Such bright UV emission can be detected by
the upcoming satellite Swift, planned for launch in early 2004. The behavior of
the optical-UV spectrum () differs significantly from
that of the reverse shock emission (, ), which is a signature of the emission accompanying with GRB. The
mild optical emission can be detected with the ROTSE-IIIa telescope system, if
the response to the GRB alert is fast enough.Comment: 5 pages, no figures. MNRAS in pres
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