325 research outputs found
Sub-Neptune Formation: The View from Resonant Planets
The orbital period ratios of neighbouring sub-Neptunes are distributed
asymmetrically near first-order resonances. There are deficits of
systems---"troughs" in the period ratio histogram---just short of
commensurability, and excesses---"peaks"---just wide of it. We reproduce
quantitatively the strongest peak-trough asymmetries, near the 3:2 and 2:1
resonances, using dissipative interactions between planets and their natal
discs. Disc eccentricity damping captures bodies into resonance and clears the
trough, and when combined with disc-driven convergent migration, draws planets
initially wide of commensurability into the peak. The migration implied by the
magnitude of the peak is modest; reductions in orbital period are 10\%,
supporting the view that sub-Neptunes complete their formation more-or-less in
situ. Once captured into resonance, sub-Neptunes of typical mass -- stay captured (contrary to an earlier claim), as they are immune to
the overstability that afflicts lower mass planets. Driving the limited,
short-scale migration is a gas disc depleted in mass relative to a
solar-composition disc by 3--5 orders of magnitude. Such gas-poor but not
gas-empty environments are quantitatively consistent with sub-Neptune core
formation by giant impacts (and not, e.g., pebble accretion). While disc-planet
interactions at the close of the planet formation era adequately explain the
3:2 and 2:1 asymmetries at periods -- days, subsequent
modification by stellar tides appears necessary at shorter periods,
particularly for the 2:1.Comment: Accepted to MNRA
The End of Runaway: How Gap Opening Limits the Final Masses of Gas Giants
Gas giants are thought to form by runaway accretion: an instability driven by
the self-gravity of growing atmospheres that causes accretion rates to rise
super-linearly with planet mass. Why runaway should stop at a Jupiter or any
other mass is unknown. We consider the proposal that final masses are
controlled by circumstellar disc gaps (cavities) opened by planetary
gravitational torques. We develop a fully time-dependent theory of gap
formation and couple it self-consistently to planetary growth rates. When gaps
first open, planetary torques overwhelm viscous torques, and gas depletes as if
it were inviscid. In low-viscosity discs, of the kind motivated by recent
observations and theory, gaps stay predominantly in this inviscid phase and
planet masses finalize at , with the host
stellar mass, the planet's orbital angular velocity,
the gas disc's lifetime, its aspect ratio, and its unperturbed
density. This final mass is independent of the dimensionless viscosity
and applies to large orbital distances, typically beyond 10 AU, where
disc scale heights exceed planet radii. It evaluates to a few Jupiter masses at
10-100 AU, increasing gradually with distance as gaps become harder to open.Comment: Accepted to MNRA
Gap Opening in 3D: Single Planet Gaps
Giant planets can clear deep gaps when embedded in 2D (razor-thin) viscous
circumstellar disks. We show by direct simulation that giant planets are just
as capable of carving out gaps in 3D. Surface density maps are similar between
2D and 3D, even in detail. In particular, the scaling of gap surface density with planet mass, derived from a global
"zero-dimensional" balance of Lindblad and viscous torques, applies equally
well to results obtained at higher dimensions. Our 3D simulations reveal
extensive, near-sonic, meridional flows both inside and outside the gaps; these
large-scale circulations might bear on disk compositional gradients, in dust or
other chemical species. At high planet mass, gap edges are mildly Rayleigh
unstable and intermittently shed streams of material into the gap - less so in
3D than in 2D.Comment: Accepted for publication in Ap
Stellar Winds and Dust Avalanches in the AU Mic Debris Disk
We explain the fast-moving, ripple-like features in the edge-on debris disk
orbiting the young M dwarf AU Mic. The bright features are clouds of sub-micron
dust repelled by the host star's wind. The clouds are produced by avalanches:
radial outflows of dust that gain exponentially more mass as they shatter
background disk particles in collisional chain reactions. The avalanches are
triggered from a region a few AU across -- the "avalanche zone" -- located on
AU Mic's primary "birth" ring, at a true distance of 35 AU from the star
but at a projected distance more than a factor of 10 smaller: the avalanche
zone sits directly along the line of sight to the star, on the side of the ring
nearest Earth, launching clouds that disk rotation sends wholly to the
southeast, as observed. The avalanche zone marks where the primary ring
intersects a secondary ring of debris left by the catastrophic disruption of a
progenitor up to Varuna in size, less than tens of thousands of years ago. Only
where the rings intersect are particle collisions sufficiently violent to spawn
the sub-micron dust needed to seed the avalanches. We show that this picture
works quantitatively, reproducing the masses, sizes, and velocities of the
observed escaping clouds. The Lorentz force exerted by the wind's magnetic
field, whose polarity reverses periodically according to the stellar magnetic
cycle, promises to explain the observed vertical undulations. The timescale
between avalanches, about 10 yr, might be set by time variability of the wind
mass-loss rate or, more speculatively, by some self-regulating limit cycle.Comment: Accepted to Ap
Eccentric Jupiters via Disk-Planet Interactions
Numerical hydrodynamics calculations are performed to determine conditions
under which giant planet eccentricities can be excited by parent gas disks.
Unlike in other studies, Jupiter-mass planets are found to have their
eccentricities amplified --- provided their orbits start eccentric. We
disentangle the web of co-rotation, co-orbital, and external resonances to show
that this finite-amplitude instability is consistent with that predicted
analytically. Ellipticities can grow until they reach of order the disk's
aspect ratio, beyond which the external Lindblad resonances that excite
eccentricity are weakened by the planet's increasingly supersonic epicyclic
motion. Forcing the planet to still larger eccentricities causes catastrophic
eccentricity damping as the planet collides into gap walls. For standard
parameters, the range of eccentricities for instability is modest; the
threshold eccentricity for growth () is not much smaller than the
final eccentricity to which orbits grow (). If this threshold
eccentricity can be lowered (perhaps by non-barotropic effects), and if the
eccentricity driving documented here survives in 3D, it may robustly explain
the low-to-moderate eccentricities exhibited by many giant
planets (including Jupiter and Saturn), especially those without planetary or
stellar companions.Comment: Accepted to ApJ with added references and minor revision
Breeding Super-Earths and Birthing Super-Puffs in Transitional Disks
The riddle posed by super-Earths (1-4, 2-20) is that they
are not Jupiters: their core masses are large enough to trigger runaway gas
accretion, yet somehow super-Earths accreted atmospheres that weigh only a few
percent of their total mass. We show that this puzzle is solved if super-Earths
formed late, as the last vestiges of their parent gas disks were about to
clear. This scenario would seem to present fine-tuning problems, but we show
that there are none. Ambient gas densities can span many (up to 9) orders of
magnitude, and super-Earths can still robustly emerge after 0.1-1 Myr
with percent-by-weight atmospheres. Super-Earth cores are naturally bred in
gas-poor environments where gas dynamical friction has weakened sufficiently to
allow constituent protocores to merge. So little gas is present at the time of
core assembly that cores hardly migrate by disk torques: formation of
super-Earths can be in situ. The picture --- that close-in super-Earths form in
a gas-poor (but not gas-empty) inner disk, fed continuously by gas that bleeds
inward from a more massive outer disk --- recalls the largely evacuated but
still accreting inner cavities of transitional protoplanetary disks. We also
address the inverse problem presented by super-puffs: an uncommon class of
short-period planets seemingly too voluminous for their small masses
(4-10, 2-6). Super-puffs easily acquire their thick
atmospheres as dust-free, rapidly cooling worlds outside 1 AU where
nebular gas is colder, less dense, and therefore less opaque. Unlike
super-Earths which can form in situ, super-puffs migrated in to their current
orbits; they are expected to form the outer links of mean-motion resonant
chains, and to exhibit greater water content. We close by confronting
observations and itemizing remaining questions.Comment: Accepted to Ap
Strong Lefschetz property under reduction
Let n>1 and G be the group SU(n) or Sp(n). This paper constructs compact
symplectic manifolds whose symplectic quotient under a Hamiltonian G-action
does not inherit the strong Lefschetz property.Comment: 9 pages. Added some computation detail
To Cool is to Accrete: Analytic Scalings for Nebular Accretion of Planetary Atmospheres
Planets acquire atmospheres from their parent circumstellar disks. We derive
a general analytic expression for how the atmospheric mass grows with time ,
as a function of the underlying core mass and nebular
conditions, including the gas metallicity . Planets accrete as much gas as
can cool: an atmosphere's doubling time is given by its Kelvin-Helmholtz time.
Dusty atmospheres behave differently from atmospheres made dust-free by grain
growth and sedimentation. The gas-to-core mass ratio (GCR) of a dusty
atmosphere scales as GCR , where (for not too close to 1)
is the mean molecular weight at the innermost radiative-convective boundary.
This scaling applies across all orbital distances and nebular conditions for
dusty atmospheres; their radiative-convective boundaries, which regulate
cooling, are not set by the external environment, but rather by the internal
microphysics of dust sublimation, H dissociation, and the formation of
H. By contrast, dust-free atmospheres have their radiative boundaries at
temperatures close to nebular temperatures , and
grow faster at larger orbital distances where cooler temperatures, and by
extension lower opacities, prevail. At 0.1 AU in a gas-poor nebula, GCR
, while beyond 1 AU in a gas-rich nebula, GCR . We confirm our
analytic scalings against detailed numerical models for objects ranging in mass
from Mars (0.1 ) to the most extreme super-Earths (10-20 ),
and explain why heating from planetesimal accretion cannot prevent the latter
from undergoing runaway gas accretion.Comment: 9 pages, 6 figures, accepted to Ap
A Primer on Unifying Debris Disk Morphologies
A "minimum model" for debris disks consists of a narrow ring of parent
bodies, secularly forced by a single planet on a possibly eccentric orbit,
colliding to produce dust grains that are perturbed by stellar radiation
pressure. We demonstrate how this minimum model can reproduce a wide variety of
disk morphologies imaged in scattered starlight. Five broad categories of disk
shape can be captured: "rings," "needles," "ships-and-wakes," "bars," and
"moths (a.k.a. fans)," depending on the viewing geometry. Moths can also sport
"double wings." We explain the origin of morphological features from first
principles, exploring the dependence on planet eccentricity, disk inclination
dispersion, and the parent body orbital phases at which dust grains are born. A
key determinant in disk appearance is the degree to which dust grain orbits are
apsidally aligned. Our study of a simple steady-state (secularly relaxed) disk
should serve as a reference for more detailed models tailored to individual
systems. We use the intuition gained from our guidebook of disk morphologies to
interpret, informally, the images of a number of real-world debris disks. These
interpretations suggest that the farthest reaches of planetary systems are
perturbed by eccentric planets, possibly just a few Earth masses each.Comment: Accepted to ApJ; minor edits mad
Gravito-Turbulent Disks in 3D: Turbulent Velocities vs. Depth
Characterizing turbulence in protoplanetary disks is crucial for
understanding how they accrete and spawn planets. Recent measurements of
spectral line broadening promise to diagnose turbulence, with different lines
probing different depths. We use 3D local hydrodynamic simulations of cooling,
self-gravitating disks to resolve how motions driven by "gravito-turbulence"
vary with height. We find that gravito-turbulence is practically as vigorous at
altitude as at depth: even though gas at altitude is much too rarefied to be
itself self-gravitating, it is strongly forced by self-gravitating
overdensities at the midplane. The long-range nature of gravity means that
turbulent velocities are nearly uniform vertically, increasing by just a factor
of 2 from midplane to surface, even as the density ranges over nearly three
orders of magnitude. The insensitivity of gravito-turbulence to height
contrasts with the behavior of disks afflicted by the magnetorotational
instability (MRI); in the latter case, non-circular velocities increase by at
least a factor of 15 from midplane to surface, with various non-ideal effects
only magnifying this factor. The distinct vertical profiles of
gravito-turbulence vs. MRI turbulence may be used in conjunction with
measurements of non-thermal linewidths at various depths to identify the source
of transport in protoplanetary disks.Comment: Accepted to Ap
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