arXiv subjects

Benjamin C. Bromley

Publications and source records attributed to Benjamin C. Bromley.

At least 55 records · Page 3Linked to original sources

arXiv2015

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.

astro-ph.EP↗

arXiv2014

The Fate of Scattered Planets

As gas giant planets evolve, they may scatter other planets far from their original orbits to produce hot Jupiters or rogue planets that are not gravitationally bound to any star. Here, we consider planets cast out to large orbital distances on eccentric, bound orbits through a gaseous disk. With simple numerical models, we show that super-Earths can interact with the gas through dynamical friction to settle in the remote outer regions of a planetary system. Outcomes depend on planet mass, the initial scattered orbit, and the evolution of the time-dependent disk. Efficient orbital damping by dynamical friction requires planets at least as massive as the Earth. More massive, longer-lived disks damp eccentricities more efficiently than less massive, short-lived ones. Transition disks with an expanding inner cavity can circularize orbits at larger distances than disks that experience a global (homologous) decay in surface density. Thus, orbits of remote planets may reveal the evolutionary history of their primordial gas disks. A remote planet with an orbital distance ~100 AU from the Sun is plausible and might explain correlations in the orbital parameters of several distant trans-Neptunian objects.

astro-ph.EP↗

arXiv2014

Predicted Space Motions for Hypervelocity and Runaway Stars: Proper Motions and Radial Velocities for the GAIA Era

We predict the distinctive three dimensional space motions of hypervelocity stars (HVSs) and runaway stars moving in a realistic Galactic potential. For nearby stars with distances less than 10~kpc, unbound stars are rare; proper motions alone rarely isolate bound HVSs and runaways from indigenous halo stars. At large distances of 20-100 kpc, unbound HVSs are much more common than runaways; radial velocities easily distinguish both from indigenous halo stars. Comparisons of the predictions with existing observations are encouraging. Although the models fail to match observations of solar-type HVS candidates from SEGUE, they agree well with data for B-type HVS and runaways from other surveys. Complete samples of g <= 20 stars with GAIA should provide clear tests of formation models for HVSs and runaways and will enable accurate probes of the shape of the Galactic potential.

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arXiv2014

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.

astro-ph.CO↗

arXiv2014

Fomalhaut b as a Cloud of Dust: Testing Aspects of Planet Formation Theory

We consider the ability of three models - impacts, captures, and collisional cascades - to account for a bright cloud of dust in Fomalhaut b. Our analysis is based on a novel approach to the power-law size distribution of solid particles central to each model. When impacts produce debris with (i) little material in the largest remnant and (ii) a steep size distribution, the debris has enough cross-sectional area to match observations of Fomalhaut b. However, published numerical experiments of impacts between 100 km objects suggest this outcome is unlikely. If collisional processes maintain a steep size distribution over a broad range of particle sizes (300 microns to 10 km), Earth-mass planets can capture enough material over 1-100 Myr to produce a detectable cloud of dust. Otherwise, capture fails. When young planets are surrounded by massive clouds or disks of satellites, a collisional cascade is the simplest mechanism for dust production in Fomalhaut b. Several tests using HST or JWST data - including measuring the expansion/elongation of Fomalhaut b, looking for trails of small particles along Fomalhaut b's orbit, and obtaining low resolution spectroscopy - can discriminate among these models.

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arXiv2013

The Formation of Pluto's Low Mass Satellites

Motivated by the New Horizons mission, we consider how Pluto's small satellites -- currently Styx, Nix, Kerberos, and Hydra -- grow in debris from the giant impact that forms the Pluto-Charon binary. After the impact, Pluto and Charon accrete some of the debris and eject the rest from the binary orbit. During the ejection, high velocity collisions among debris particles produce a collisional cascade, leading to the ejection of some debris from the system and enabling the remaining debris particles to find stable orbits around the binary. Our numerical simulations of coagulation and migration show that collisional evolution within a ring or a disk of debris leads to a few small satellites orbiting Pluto-Charon. These simulations are the first to demonstrate migration-induced mergers within a particle disk. The final satellite masses correlate with the initial disk mass. More massive disks tend to produce fewer satellites. For the current properties of the satellites, our results strongly favor initial debris masses of 3-10 x 10^{19} g and current satellite albedos of roughly 0.4-1. We also predict an ensemble of smaller satellites with radii less than roughly 1-3 km, and very small particles, with radii of roughly 1-100 cm and optical depth smaller than 10^-10. These objects should have semimajor axes outside the current orbit of Hydra.

astro-ph.EP↗

arXiv2013

Coagulation Calculations of Icy Planet Formation Around 0.1--0.5~\msun\ Stars: Super-Earths From Large Planetestimals

We investigate formation mechanisms for icy super-Earth mass planets orbiting at 2-20 AU around 0.1-0.5 solar mass stars. A large ensemble of coagulation calculations demonstrates a new formation channel: disks composed of large planetesimals with radii of 30-300 km form super-Earths on time scales of roughly 1 Gyr. In other gas-poor disks, a collisional cascade grinds planetesimals to dust before the largest planets reach super-Earth masses. Once icy Earth-mass planets form, they migrate through the leftover swarm of planetesimals at rates of 0.01-1 AU per Myr. On time scales of 10 Myr to 1 Gyr, many of these planets migrate through the disk of leftover planetesimals from semimajor axes of 5-10 AU to 1-2 AU. A few per cent of super-Earths might migrate to semimajor axes of 0.1-0.2 AU. When the disk has an initial mass comparable with the minimum mass solar nebula scaled to the mass of the central star, the predicted frequency of super-Earths matches the observed frequency.

astro-ph.EP↗

arXiv2013

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.

astro-ph.EP↗

arXiv2012

Binary Disruption by Massive Black Holes: Hypervelocity Stars, S Stars, and Tidal Disruption Events

We examine whether disrupted binary stars can fuel black hole growth. In this mechanism, tidal disruption produces a single hypervelocity star (HVS) ejected at high velocity and a former companion star bound to the black hole. After a cluster of bound stars forms, orbital diffusion allows the black hole to accrete stars by tidal disruption at a rate comparable to the capture rate. In the Milky Way, HVSs and the S star cluster imply similar rates of 10^{-5}--10^{-3} yr^{-1} for binary disruption. These rates are consistent with estimates for the tidal disruption rate in nearby galaxies and imply significant black hole growth from disrupted binaries on 10 Gyr time scales.

astro-ph.GA↗

arXiv2012

Coagulation Calculations of Icy Planet Formation at 15--150 AU: A Correlation Between the Maximum Radius and the Slope of the Size Distribution for Transneptunian Objects

We investigate whether coagulation models of planet formation can explain the observed size distributions of transneptunian objects (TNOs). Analyzing published and new calculations, we demonstrate robust relations between the size of the largest object and the slope of the size distribution for sizes 0.1 km and larger. These relations yield clear, testable predictions for TNOs and other icy objects throughout the solar system. Applying our results to existing observations, we show that a broad range of initial disk masses, planetesimal sizes, and fragmentation parameters can explain the data. Adding dynamical constraints on the initial semimajor axis of `hot' KBOs along with probable TNO formation times of 10-700 Myr restricts the viable models to those with a massive disk composed of relatively small (1-10 km) planetesimals.

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arXiv2011

Gravitationally Focused Dark Matter Around Compact Stars

If dark matter self-annihilates then it may produce an observable signal when its density is high. The details depend on the intrinsic properties of dark matter and how it clusters in space. For example, the density profile of some dark matter candidates may rise steeply enough toward the Galactic Center that self-annihilation produces detectable gamma-ray emission. Here, we discuss the possibility that an annihilation signal may arise near a compact object (e.g., neutron star or black hole) even when the density of dark matter in the neighborhood of the object is uniform. Gravitational focusing produces a local enhancement of density, with a profile that falls off approximately as the inverse square-root of distance from the compact star. While geometric dilution may overwhelm the annihilation signal from this local enhancement, magnetic fields tied to the compact object can increase the signal's contrast relative to the background.

astro-ph.HE↗

arXiv2011

Migration of planets embedded in a circumstellar disk

Planetary migration poses a serious challenge to theories of planet formation. In gaseous and planetesimal disks, migration can remove planets as quickly as they form. To explore migration in a planetesimal disk, we combine analytic and numerical approaches. After deriving general analytic migration rates for isolated planets, we use N-body simulations to confirm these results for fast and slow migration modes. Migration rates scale as 1/m (for massive planets) and 1/(1+(e_H/3)^3), where m is the mass of a planet and e_H is the eccentricity of the background planetesimals in Hill units. When multiple planets stir the disk, our simulations yield the new result that large-scale migration ceases. Thus, growing planets do not migrate through planetesimal disks. To extend these results to migration in gaseous disks, we compare physical interactions and rates. Although migration through a gaseous disk is an important issue for the formation of gas giants, we conclude that migration has little impact on the formation of terrestrial planets.

astro-ph.EP↗

arXiv2011

A New Hybrid N-Body-Coagulation Code for the Formation of Gas Giant Planets

We describe an updated version of our hybrid N-body-coagulation code for planet formation. In addition to the features of our 2006-2008 code, our treatment now includes algorithms for the 1D evolution of the viscous disk, the accretion of small particles in planetary atmospheres, gas accretion onto massive cores, and the response of N-bodies to the gravitational potential of the gaseous disk and the swarm of planetesimals. To validate the N-body portion of the algorithm, we use a battery of tests in planetary dynamics. As a first application of the complete code, we consider the evolution of Pluto-mass planetesimals in a swarm of 0.1-1 cm pebbles. In a typical evolution time of 1-3 Myr, our calculations transform 0.01-0.1 solar mass disks of gas and dust into planetary systems containing super-Earths, Saturns, and Jupiters. Low mass planets form more often than massive planets; disks with smaller alpha form more massive planets than disks with larger alpha. For Jupiter-mass planets, masses of solid cores are 10-100 Earth masses.

astro-ph.EP↗

arXiv2010

Variations on Debris Disks II. Icy Planet Formation as a Function of the Bulk Properties and Initial Sizes of Planetesimals

We describe comprehensive calculations of the formation of icy planets and debris disks at 30-150 AU around 1-3 solar mass stars. Disks composed of large, strong planetesimals produce more massive planets than disks composed of small, weak planetesimals. The maximum radius of icy planets ranges from roughly 1500 km to 11,500 km. The formation rate of 1000 km objects - `Plutos' - is a useful proxy for the efficiency of icy planet formation. Plutos form more efficiently in massive disks, in disks with small planetesimals, and in disks with a range of planetesimal sizes. Although Plutos form throughout massive disks, Pluto production is usually concentrated in the inner disk. Despite the large number of Plutos produced in many calculations, icy planet formation is inefficient. At the end of the main sequence lifetime of the central star, Plutos contain less than 10% of the initial mass in solid material. This conclusion is independent of the initial mass in the disk or the properties of planetesimals. Debris disk formation coincides with the formation of planetary systems containing Plutos. As Plutos form, they stir leftover planetesimals to large velocities. A cascade of collisions then grinds the leftovers to dust, forming an observable debris disk. In disks with small (< 1-10 km) planetesimals, collisional cascades produce luminous debris disks with maximum luminosity roughly 0.01 times the stellar luminosity. Disks with larger planetesimals produce much less luminous debris disks. Observations of debris disks around A-type and G-type stars strongly favor models with small planetesimals. In these models, our predictions for the time evolution and detection frequency of debris disks agree with published observations. We suggest several critical observations that can test key features of our calculations.

astro-ph.SR↗

arXiv2009

Runaway Stars, Hypervelocity Stars, and Radial Velocity Surveys

Runaway stars ejected from the Galactic disk populate the halo of the Milky Way. To predict the spatial and kinematic properties of runaways, we inject stars into a Galactic potential, compute their trajectories through the Galaxy, and derive simulated catalogs for comparison with observations. Runaways have a flattened spatial distribution, with higher velocity stars at Galactic latitudes less than 30 degrees. Due to their shorter stellar lifetimes, massive runaway stars are more concentrated towards the disk than low mass runaways. Bound (unbound) runaways that reach the halo probably originate from distances of 6--12 kpc (10--15 kpc) from the Galactic center, close to the estimated origin of the unbound runaway star HD 271791. Because runaways are brighter and have smaller velocities than hypervelocity stars (HVSs), radial velocity surveys are unlikely to confuse runaway stars with HVSs. We estimate that at most 1 runaway star contaminates the current sample. We place an upper limit of 2% on the fraction of A-type main sequence stars ejected as runaways.

astro-ph.GA↗

arXiv2008

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.

astro-ph↗

arXiv2008

The Anisotropic Spatial Distribution of Hypervelocity Stars

We study the distribution of angular positions and angular separations of unbound hypervelocity stars (HVSs). HVSs are spatially anisotropic at the 3-sigma level. The spatial anisotropy is significant in Galactic longitude, not in latitude, and the inclusion of lower velocity, possibly bound HVSs reduces the significance of the anisotropy. We discuss how the observed distribution of HVSs may be linked to their origin. In the future, measuring the distribution of HVSs in the southern sky will provide additional constraints on the spatial anisotropy and the origin of HVSs.

astro-ph↗

arXiv2008

Variations on Debris Disks: Icy Planet Formation at 30-150 AU for 1-3 Solar Mass Main Sequence Stars

We describe calculations for the formation of icy planets and debris disks at 30-150 AU around 1-3 solar mass stars. Debris disk formation coincides with the formation of planetary systems. As protoplanets grow, they stir leftover planetesimals to large velocities. A cascade of collisions then grinds the leftovers to dust, forming an observable debris disk. Stellar lifetimes and the collisional cascade limit the growth of protoplanets. The maximum radius of icy planets, roughly 1750 km, is remarkably independent of initial disk mass, stellar mass, and stellar age. These objects contain no more than 3% to 4% of the initial mass in solid material. Collisional cascades produce debris disks with maximum luminosity of roughly 0.002 times the stellar luminosity. The peak 24 micron excess varies from roughly 1% of the stellar photospheric flux for 1 solar mass stars to roughly 50 times the stellar photospheric flux for 3 solar mass stars. The peak 70-850 micron excesses are roughly 30-100 times the stellar photospheric flux. For all stars, the 24-160 micron excesses rise at stellar ages of 5-20 Myr, peak at 10-50 Myr, and then decline. The decline is roughly a power law, with f propto t^{-n} and n = 0.6-1.0. This predicted evolution agrees with published observations of A-type and solar-type stars. The observed far-IR color evolution of A-type stars also matches model predictions.

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