182 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
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
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
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
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
A Pluto-Charon Sonata: The Dynamical Architecture of the Circumbinary Satellite System
Using a large suite of n-body simulations, we explore the discovery space for
new satellites in the Pluto-Charon system. For the adopted masses and orbits of
the known satellites, there are few stable prograde or polar orbits with
semimajor axes , where is the semimajor axis of the
outermost moon Hydra. Small moons with radii 2 km and are ejected on time scales ranging from several yr to more than 10
Myr. Orbits with are stable on time scales exceeding 100
Myr. Near-IR and mid-IR imaging with JWST and ground-based occultation
campaigns with 2-3-m class telescopes can detect 1-2 km satellites outside the
orbit of Hydra. Searches for these moons enable new constraints on the masses
of the known satellites and on theories for circumbinary satellite formation.Comment: 34 pages of text, 2 tables, 12 figures, submitted to AAS journals,
comments welcome. Animations associated with the paper are available at
https://www.cfa.harvard.edu/~kenyon/Media/PCSonata.htm
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
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
Variations on Debris Disks III. Collisional Cascades and Giant Impacts in the Terrestrial Zones of Solar-type Stars
We analyze two new sets of coagulation calculations for solid particles
orbiting within the terrestrial zone of a solar-type star. In models of
collisional cascades, numerical simulations demonstrate that the total mass,
the mass in 1 mm and smaller particles, and the dust luminosity decline with
time more rapidly than predicted by analytic models, with 1.1-1.2 instead of 1. Size distributions derived from the numerical
calculations follow analytic predictions at radii less than 0.1 km but are
shallower than predicted at larger sizes. In simulations of planet formation,
the dust luminosity declines more slowly than in pure collisional cascades,
with 0.5-0.8 instead of 1.1-1.2. Throughout this decline, giant
impacts produce large, observable spikes in dust luminosity which last roughly
0.01-0.1 Myr and recur every 1-10 Myr. If most solar-type stars have Earth mass
planets with 1-2 AU, observations of debris around 1-100 Myr stars
allow interesting tests of theory. Current data preclude theories where
terrestrial planets form out of 1000 km or larger planetesimals. Although the
observed frequency of debris disks among 30 Myr old stars agrees with
our calculations, the observed frequency of warm debris among 5-20 Myr old
stars is smaller than predicted.Comment: 43 pages of text, 1 table, 30 figures, ApJ, in pres
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