3,130 research outputs found
The Distortion of the Cosmic Microwave Background by the Milky Way
The Milky Way can act as a large-scale weak gravitational lens of the cosmic
microwave background (CMB). We study this effect using a photon ray-tracing
code and a Galactic mass distribution with disk, bulge and halo components. For
an observer at the Sun's coordinates in the Galaxy, the bending of CMB photon
paths is limited to less than one arcsecond, and only for rays that pass within
a few degrees of the Galactic Center. However, the entire sky is affected,
resulting in global distortions of the CMB on large angular scales. These
distortions can cause the low-order multipoles of a spherical harmonic
expansion of the CMB sky temperature to leak into higher-order modes. Thus the
component of the CMB dipole that results from the Local Group's motion relative
to the local cosmic frame of rest contributes to higher-order moments for an
observer in the solar system. With our ray-tracing code we show that the
phenomenon is not sensitive to the specific choice of Galactic potential. We
also quantitatively rule it out as a contributor to CMB anomalies such as power
asymmetry or correlated alignment of low-order multipole moments.Comment: 4 pages, 3 Figures, Brief Report in Physical Review D, accepted for
publicatio
Evolution of a ring around the Pluto-Charon binary
We consider the formation of satellites around the Pluto-Charon binary. An
early collision between the two partners likely produced the binary and a
narrow ring of debris, out of which arose the moons Styx, Nix, Kerberos and
Hydra. How the satellites emerged from the compact ring is uncertain. Here we
show that a particle ring spreads from physical collisions and collective
gravitational scattering, similar to migration. Around a binary, these
processes take place in the reference frames of "most circular" orbits, akin to
circular ones in a Keplerian potential. Ring particles damp to these orbits and
avoid destructive collisions. Damping and diffusion also help particles survive
dynamical instabilities driven by resonances with the binary. In some
situations, particles become trapped near resonances that sweep outward with
the tidal evolution of the Pluto-Charon binary. With simple models and
numerical experiments, we show how the Pluto-Charon impact ring may have
expanded into a broad disk, out of which grew the circumbinary moons. In some
scenarios, the ring can spread well beyond the orbit of Hydra, the most distant
moon, to form a handful of smaller satellites. If these small moons exist, New
Horizons will find them.Comment: 44 pages of text, 10 figures, ApJ, accepted. (In this version,
Section 2 and Figure 1 are new, outlining our approach to the problem of
satellite formation around the Pluto-Charon binary.
Making Planet Nine: Pebble Accretion at 250--750 AU in a Gravitationally Unstable Ring
We investigate the formation of icy super-Earth mass planets within a
gravitationally unstable ring of solids orbiting at 250-750 AU around a 1 solar
mass star. Coagulation calculations demonstrate that a system of a few large
oligarchs and a swarm of pebbles generates a super-Earth within 100-200 Myr at
250 AU and within 1-2 Gyr at 750 AU. Systems with more than ten oligarchs fail
to yield super-Earths over the age of the solar system. As these systems
evolve, destructive collisions produce detectable debris disks with
luminosities of to relative to the central star.Comment: 24 pages of text, 1 table, 8 figures, ApJ submitted, comments welcom
Numerical Simulations of Collisional Cascades at the Roche Limits of White Dwarf Stars
We consider the long-term collisional and dynamical evolution of solid
material orbiting in a narrow annulus near the Roche limit of a white dwarf.
With orbital velocities of 300 km/sec, systems of solids with initial
eccentricity generate a collisional cascade where objects
with radii 100--300 km are ground to dust. This process converts
1-100 km asteroids into 1 m particles in yr. Throughout this
evolution, the swarm maintains an initially large vertical scale height .
Adding solids at a rate enables the system to find an equilibrium
where the mass in solids is roughly constant. This equilibrium depends on
and , the radius of the largest solid added to the swarm. When
10 km, this equilibrium is stable. For larger , the mass
oscillates between high and low states; the fraction of time spent in high
states ranges from 100% for large to much less than 1% for small
. During high states, the stellar luminosity reprocessed by the solids
is comparable to the excess infrared emission observed in many metallic line
white dwarfs.Comment: 37 pages of text, 12 figures, ApJ, accepte
Making Planet Nine: A Scattered Giant in the Outer Solar System
Correlations in the orbits of several minor planets in the outer solar system
suggest the presence of a remote, massive Planet Nine. With at least ten times
the mass of the Earth and a perihelion well beyond 100 AU, Planet Nine poses a
challenge to planet formation theory. Here we expand on a scenario in which the
planet formed closer to the Sun and was gravitationally scattered by Jupiter or
Saturn onto a very eccentric orbit in an extended gaseous disk. Dynamical
friction with the gas then allowed the planet to settle in the outer solar
system. We explore this possibility with a set of numerical simulations.
Depending on how the gas disk evolves, scattered super-Earths or small gas
giants settle on a range of orbits, with perihelion distances as large as 300
AU. Massive disks that clear from the inside out on million-year time scales
yield orbits that allow a super-Earth or gas giant to shepherd the minor
planets as observed. A massive planet can achieve a similar orbit in a
persistent, low-mass disk over the lifetime of the solar system.Comment: 14 pages of text, 2 tables, 5 figures, ApJ, submitte
Rapid Formation of Icy Super-Earths and the Cores of Gas Giant Planets
We describe a coagulation model that leads to the rapid formation of
super-Earths and the cores of gas giant planets. Interaction of collision
fragments with the gaseous disk is the crucial element of this model. The gas
entrains small collision fragments, which rapidly settle to the disk midplane.
Protoplanets accrete the fragments and grow to masses of at least 1 Earth mass
in roughly 1 Myr. Our model explains the mass distribution of planets in the
Solar System and predicts that super-Earths form more frequently than gas
giants in low mass disks.Comment: ApJLetters, accepted; 10 pages of text and 2 figure
Formation of Super-Earth Mass Planets at 125-250 AU from a Solar-type Star
We investigate pathways for the formation of icy super-Earth mass planets
orbiting at 125-250 AU around a 1 solar mass star. An extensive suite of
coagulation calculations demonstrates that swarms of 1 cm to 10 m planetesimals
can form super-Earth mass planets on time scales of 1-3 Gyr. Collisional
damping of 0.01-100 cm particles during oligarchic growth is a highlight of
these simulations. In some situations, damping initiates a second runaway
growth phase where 100-3000 km protoplanets grow to super-Earth sizes. Our
results establish the initial conditions and physical processes required for in
situ formation of super-Earth planets at large distances from the host star.
For nearby dusty disks in HD 107146, HD 202628, and HD 207129, ongoing
super-Earth formation at 80-150 AU could produce gaps and other structures in
the debris. In the solar system, forming a putative planet X at a
1000 AU) requires a modest (very massive) protosolar nebula.Comment: 48 pages of text, 23 figures, ApJ in press, revised version contains
new text on aspects of the calculations and a more comprehensive description
of the origin of the second phase of runaway growt
Variations on Debris Disks IV. An Improved Analytical Model for Collisional Cascades
We derive a new analytical model for the evolution of a collisional cascade
in a thin annulus around a single central star. In this model, the
size of the largest object declines with time (t); , with = 0.1-0.2. Compared to standard models where
is constant in time, this evolution results in a more rapid decline
of , the total mass of solids in the annulus and , the luminosity of
small particles in the annulus: and . We demonstrate that the analytical model
provides an excellent match to a comprehensive suite of numerical coagulation
simulations for annuli at 1 AU and at 25 AU. If the evolution of real debris
disks follows the predictions of the analytical or numerical models, the
observed luminosities for evolved stars require up to a factor of two more mass
than predicted by previous analytical models.Comment: ApJ, in press, 22 pages of text and 14 figure
Collisional Cascade Caclulations for Irregular Satellite Swarms in Fomalhaut b
We describe an extensive suite of numerical calculations for the collisional
evolution of irregular satellite swarms around 1--300 M-earth planets orbiting
at 120 AU in the Fomalhaut system. For 10--100 M-earth planets, swarms with
initial masses of roughly 1% of the planet mass have cross-sectional areas
comparable to the observed cross-sectional area of Fomalhaut b. Among 30--300
M-earth planets, our calculations yield optically thick swarms of satellites
for ages of 1-10 Myr. Observations with HST and ground-based AO instruments can
constrain the frequency of these systems around stars in the beta Pic moving
group and possibly other nearby associations of young stars.Comment: 46 pages, 30 figures, ApJ, accepte
Migration of small moons in Saturn's rings
The motions of small moons through Saturn's rings provide excellent tests of
radial migration models. In theory, torque exchange between these moons and
ring particles leads to radial drift. We predict that moons with Hill radii r_H
~ 2-24 km should migrate through the A ring in 1000 yr. In this size range,
moons orbiting in an empty gap or in a full ring eventually migrate at the same
rate. Smaller moons or moonlets -- such as the propellers (e.g., Tiscareno et
al. 2006) -- are trapped by diffusion of disk material into corotating orbits,
creating inertial drag. Larger moons -- such as Pan or Atlas -- do not migrate
because of their own inertia. Fast migration of 2-24 km moons should eliminate
intermediate-size bodies from the A ring and may be responsible for the
observed large-radius cutoff of r_H ~ 1-2 km in the size distribution of the A
ring's propeller moonlets. Although the presence of Daphnis (r_H ~ 5 km) inside
the Keeler gap challenges this scenario, numerical simulations demonstrate that
orbital resonances and stirring by distant, larger moons (e.g., Mimas) may be
important factors. For Daphnis, stirring by distant moons seems the most
promising mechanism to halt fast migration. Alternatively, Daphnis may be a
recent addition to the ring that is settling into a low inclination orbit in
~10^3 yr prior to a phase of rapid migration. We provide predictions of
observational constraints required to discriminate among possible scenarios for
Daphnis.Comment: ApJ, accepted; 47 pages, 14 figure
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