arXiv subjects

Sean N. Raymond

Publications and source records attributed to Sean N. Raymond.

At least 127 records · Page 7Linked to original sources

arXiv2014

Very Wide Binary Stars as the Primary Source of Stellar Collisions in the Galaxy

We present numerical simulations modeling the orbital evolution of very wide binaries, pairs of stars separated by over ~1000 AU. Due to perturbations from other passing stars and the Milky Way's tide, the orbits of very wide binary stars occasionally become extremely eccentric, which forces close encounters between the companion stars (Kaib et al. 2013). We show that this process causes a stellar collision between very wide binary companion stars once every 1000-7500 years on average in the Milky Way. One of the main uncertainties in this collision rate is the amount of energy dissipated by dynamic tides during close (but not collisional) periastron passages. This dissipation presents a dynamical barrier to stellar collisions and can instead transform very wide binaries into close or contact binaries. However, for any plausible tidal dissipation model, very wide binary stars are an unrealized, and potentially the dominant, source of stellar collisions in our Galaxy. Such collisions should occur throughout the thin disk of the Milky Way. Stellar collisions within very wide binaries should yield a small population of single, Li-depleted, rapidly rotating massive stars.

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arXiv2013

The Long-Term Dynamical Evolution of Planetary Systems

This chapter concerns the long-term dynamical evolution of planetary systems from both theoretical and observational perspectives. We begin by discussing the planet-planet interactions that take place within our own Solar System. We then describe such interactions in more tightly-packed planetary systems. As planet-planet interactions build up, some systems become dynamically unstable, leading to strong encounters and ultimately either ejections or collisions of planets. After discussing the basic physical processes involved, we consider how these interactions apply to extrasolar planetary systems and explore the constraints provided by observed systems. The presence of a residual planetesimal disc can lead to planetary migration and hence cause instabilities induced by resonance crossing; however, such discs can also stabilise planetary systems. The crowded birth environment of a planetary system can have a significant impact: close encounters and binary companions can act to destabilise systems, or sculpt their properties. In the case of binaries, the Kozai mechanism can place planets on extremely eccentric orbits which may later circularise to produce hot Jupiters.

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arXiv2013

Solar System Moons as Analogs for Compact Exoplanetary Systems

The field of exoplanetary science has experienced a recent surge of new systems that is largely due to the precision photometry provided by the Kepler mission. The latest discoveries have included compact planetary systems in which the orbits of the planets all lie relatively close to the host star, which presents interesting challenges in terms of formation and dynamical evolution. The compact exoplanetary systems are analogous to the moons orbiting the giant planets in our Solar System, in terms of their relative sizes and semi-major axes. We present a study that quantifies the scaled sizes and separations of the Solar System moons with respect to their hosts. We perform a similar study for a large sample of confirmed Kepler planets in multi-planet systems. We show that a comparison between the two samples leads to a similar correlation between their scaled sizes and separation distributions. The different gradients of the correlations may be indicative of differences in the formation and/or long-term dynamics of moon and planetary systems.

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arXiv2013

Secular Behavior of Exoplanets: Self-Consistency and Comparisons with the Planet-Planet Scattering Hypothesis

If mutual gravitational scattering among exoplanets occurs, then it may produce unique orbital properties. For example, two-planet systems that lie near the boundary between circulation and libration of their periapses could result if planet-planet scattering ejected a former third planet quickly, leaving one planet on an eccentric orbit and the other on a circular orbit. We first improve upon previous work that examined the apsidal behavior of known multiplanet systems by doubling the sample size and including observational uncertainties. This analysis recovers previous results that demonstrated that many systems lay on the apsidal boundary between libration and circulation. We then performed over 12,000 three-dimensional N-body simulations of hypothetical three-body systems that are unstable, but stabilize to two-body systems after an ejection. Using these synthetic two-planet systems, we test the planet-planet scattering hypothesis by comparing their apsidal behavior, over a range of viewing angles, to that of the observed systems and find that they are statistically consistent regardless of the multiplicity of the observed systems. Finally, we combine our results with previous studies to show that, from the sampled cases, the most likely planetary mass function prior to planet-planet scattering follows a power law with index -1.1. We find that this pre-scattering mass function predicts a mutual inclination frequency distribution that follows an exponential function with an index between -0.06 and -0.1.

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arXiv2013

Making systems of Super Earths by inward migration of planetary embryos

Using N-body simulations with planet-disk interactions, we present a mechanism capable of forming compact systems of hot super Earths such as Kepler 11. Recent studies show that outward migration is common in the inner parts of radiative disks. However we show that two processes naturally tip the balance in favor of inward migration. First the corotation torque is too weak to generate outward migration for planetary embryos less massive than $4\mearth$. Second, system of multiple embryos generate sustained non-zero eccentricities that damp the corotation torque and again favor inward migration. Migration and accretion of planetary embryos in realistic disks naturally produce super Earths in resonant chains near the disk inner edge. Their compact configuration is similar to the observed systems.

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arXiv2013

Dynamical and collisional constraints on a stochastic late veneer on the terrestrial planets

Given their tendency to be incorporated into the core during differentiation, the highly-siderophile elements (HSEs) in Earth's mantle are thought to have been accreted as a `late veneer' after the end of the giant impact phase. Bottke et al (2010) proposed that the large Earth-to-Moon HSE abundance ratio can be explained if the late veneer was characterized by large (D = 1000-4000km) impactors. Here we simulate the evolution of the terrestrial planets during a stochastic late veneer phase from the end of accretion until the start of the late heavy bombardment ~500 Myr later. We show that a late veneer population of 0.05 Earth masses dominated by large (D > 1000km) bodies naturally delivers a ~0.01 Earth mass veneer to Earth, consistent with constraints. The eccentricities and inclinations of the terrestrial planets are excited by close encounters with the largest late veneer bodies. We find the best agreement with their post-veneer orbits if either a) the terrestrial planets' pre-veneer angular momentum deficit AMD_0 was less than half of the current one AMD_now, or b) AMD_0 <= AMD_now and the veneer was limited to smaller (D_max <= 2000km) bodies. Veneer impacts on Venus, Earth and Mars were mostly accretionary but on Mercury and the Moon they were mostly erosive. In ~20% of simulations an energetic impact occurred that could have removed >25% of Mercury's mass, thereby increasing its iron mass fraction. We show that, due to the erosive nature of larger impacts, the Moon cannot accrete any material from objects larger than 500-1000km. The large Earth-to-Moon HSE abundance ratio is explained if the late veneer included large impactors (D >= 500-1000km) regardless of their size distribution, as long as most of Earth's veneer came from large bodies. The spin angular momenta imparted by stochastic late veneer impacts was far in excess of the current ones for Mercury and Venus.

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arXiv2013

Habitable Planets Eclipsing Brown Dwarfs: Strategies for Detection and Characterization

Given the very close proximity of their habitable zones, brown dwarfs represent high-value targets in the search for nearby transiting habitable planets that may be suitable for follow-up occultation spectroscopy. In this paper we develop search strategies to find habitable planets transiting brown dwarfs depending on their maximum habitable orbital period (PHZ out). Habitable planets with PHZ out shorter than the useful duration of a night (e.g. 8-10 hrs) can be screened with 100 percent completeness from a single location and in a single night (near-IR). More luminous brown dwarfs require continuous monitoring for longer duration, e.g. from space or from a longitude-distributed network (one test scheduling achieved - 3 telescopes, 13.5 contiguous hours). Using a simulated survey of the 21 closest known brown dwarfs (within 7 pc) we find that the probability of detecting at least one transiting habitable planet is between 4.5 +5.6-1.4 and 56 +31-13 percent, depending on our assumptions. We calculate that brown dwarfs within 5-10 pc are characterizable for potential biosignatures with a 6.5 m space telescope using approx. 1 percent of a 5-year mission's life-time spread over a contiguous segment only one fifth to one tenth of this duration.

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arXiv2013

Tidal dissipation and eccentricity pumping: Implications for the depth of the secondary eclipse of 55 Cnc e

We use the super Earth 55 Cnc e as a case study to address an observable effect of tidal heating. We investigate whether planet-planet interactions can force the eccentricity of this planet to a level affecting the eclipse depth observed with Spitzer. Using the constant time lag tidal model, we first calculate the observed planet flux as a function of albedo and eccentricity, for different tidal dissipation constants and for two extreme cases: a planet with no heat redistribution and a planet with full heat redistribution. We derive the values of albedo and eccentricity that match the observed transit depth. We then perform N-body simulations of the planetary system including tides and General Relativity to follow the evolution of the eccentricity of planet e. We compare the range of eccentricities given by the simulations with the eccentricities required to alter the eclipse depth. We find that the eccentricity of planet e can be large enough to contribute at a measurable level to the thermal emission measured with Spitzer. This affects the constraints on the albedo of the planet, which can be as high as 0.9 (instead of 0.55 when ignoring tidal heating). We also derive a maximum value for the eccentricity of planet e directly from the eclipse depth: e<0.015 assuming Earth's dissipation constant. Transiting exoplanets in multiple planet systems are exceptional targets for testing tidal models because their tidal luminosity may be observable. Future multi-wavelengths observations of eclipse depth and phase curves (for instance with EChO and JWST) should allow us to better resolve the temperature map of these planets and break the degeneracy between albedo and tidal heating that remains for single band observations. In addition, an accurate determination of the eccentricity will make it possible to constrain the dissipation rate of the planet and to probe its internal structure.

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arXiv2013

Planetary System Disruption by Galactic Perturbations to Wide Binary Stars

Nearly half of the exoplanets found within binary star systems reside in very wide binaries with average stellar separations beyond 1,000 AU (1 AU being the Earth-Sun distance), yet the influence of such distant binary companions on planetary evolution remains largely unstudied. Unlike their tighter counterparts, the stellar orbits of wide binaries continually change under the influence of the Galactic tide and impulses from other passing stars. Here we report numerical simulations demonstrating that the variable nature of wide binary star orbits dramatically reshapes the planetary systems they host, typically Gyrs after formation. Contrary to previous understanding, wide binary companions may often strongly perturb planetary systems, triggering planetary ejections and exciting orbital eccentricities of surviving planets. Indeed, observed exoplanet eccentricities offer evidence of this; giant exoplanet orbits within wide binaries are statistically more eccentric than those around isolated stars. Both eccentricity distributions are well-reproduced when we assume isolated stars and wide binaries host similar planetary systems whose outermost giant planets are scattered beyond ~10 AU from their parent stars via early internal instabilities. Consequently, our results suggest that although wide binaries eventually truncate their planetary systems, most isolated giant exoplanet systems harbor additional distant, still undetected planets.

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arXiv2012

Primordial origins of Earth's carbon

In this chapter we review the astrophysical origins of Earth's carbon, starting from the products of the Big Bang and culminating with the Earth's formation. We review the measured compositions of different primitive objects including comets, various classes of meteorites and interstellar dust particles. We discuss the composition of the Solar Nebula, especially with regards to the distribution of volatiles such as carbon. We discuss dynamical models of planetary formation from planetesimals and planetary embryos, and the timescale for volatile delivery to the growing Earth from different sources. Finally, we review Earth's carbon reservoirs. Throughout the chapter we highlight open questions related to planet formation, meteoritics, and geochemistry.

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arXiv2012

Mini-Oort clouds: Compact isotropic planetesimal clouds from planet-planet scattering

Starting from planetary systems with three giant planets and an outer disk of planetesimals, we use dynamical simulations to show how dynamical instabilities can transform planetesimal disks into 100-1000 AU-scale isotropic clouds. The instabilities involve a phase of planet-planet scattering that concludes with the ejection of one or more planets and the inward-scattering of the surviving gas giant(s) to remove them from direct dynamical contact with the planetesimals. "Mini-Oort clouds" are thus formed from scattered planetesimals whose orbits are frozen by the abrupt disappearance of the perturbing giant planet. Although the planetesimal orbits are virtually isotropic, the surviving giant planets tend to have modest inclinations (typically ~10 degrees) with respect to the initial orbital plane. The collisional lifetimes of mini-Oort clouds are long (10 Myr to >10 Gyr) and there is a window of ~100 Myr or longer during which they produce spherical clouds of potentially observable dust at 70 microns. If the formation channel for hot Jupiters commonly involves planetary close encounters, we predict a correlation between this subset of extrasolar planetary systems and mini-Oort clouds.

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arXiv2012

The Compositional Diversity of Extrasolar Terrestrial Planets: II. Migration Simulations

Prior work has found that a variety of terrestrial planetary compositions are expected to occur within known extrasolar planetary systems. However, such studies ignored the effects of giant planet migration, which is thought to be very common in extra-solar systems. Here we present calculations of the compositions of terrestrial planets that formed in dynamical simulations incorporating varying degrees of giant planet migration. We used chemical equilibrium models of the solid material present in the disks of five known planetary host stars: the Sun, GJ 777, HD4203, HD19994 and HD213240. Giant planet migration has a strong effect on the compositions of simulated terrestrial planets as the migration results large-scale mixing between terrestrial planet building blocks that condensed at a range of temperatures. This mixing acts to 1) increase the typical abundance of Mg-rich silicates in the terrestrial planets feeding zones and thus increase the frequency of planets with Earth-like compositions compared with simulations with static giant planet orbits; and 2) drastically increase the efficiency of the delivery of hydrous phases (water and serpentine) to terrestrial planets and thus produce water worlds and/or wet Earths. Our results demonstrate that although a wide variety of terrestrial planet compositions can still be produced, planets with Earth-like compositions should be common within extrasolar planetary systems.

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arXiv2012

Building Terrestrial Planets

This paper reviews our current understanding of terrestrial planets formation. The focus is on computer simulations of the dynamical aspects of the accretion process. Throughout the chapter, we combine the results of these theoretical models with geochemical, cosmochemical and chronological constraints, in order to outline a comprehensive scenario of the early evolution of our Solar System. Given that the giant planets formed first in the protoplanetary disk, we stress the sensitive dependence of the terrestrial planet accretion process on the orbital architecture of the giant planets and on their evolution. This suggests a great diversity among the terrestrial planets populations in extrasolar systems. Issues such as the cause for the different masses and accretion timescales between Mars and the Earth and the origin of water (and other volatiles) on our planet are discussed at depth.

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arXiv2012

Effect of the stellar spin history on the tidal evolution of close-in planets

We investigate how the evolution of the stellar spin rate affects, and is affected by, planets in close orbits, via star-planet tidal interactions. To do this, we used a standard equilibrium tidal model to compute the orbital evolution of single planets orbiting both Sun-like stars and 0.1 M\odot M-dwarfs. We tested two stellar spin evolution profiles, one with fast initial rotation (P=1.2 day) and one with slow initial rotation (P=8 day). We tested the effect of varying the stellar and planetary dissipation and the planet's mass and initial orbital radius. Conclusions: Tidal evolution allows to differentiate the early behaviors of extremely close-in planets orbiting either a rapidly rotating star or a slowly rotating star. The early spin-up of the star allows the close-in planets around fast rotators to survive the early evolution. For planets around M-dwarfs, surviving the early evolution means surviving on Gyr timescales whereas for Sun-like stars the spin-down brings about late mergers of Jupiter planets. In light of this study, we can say that differentiating between one spin evolution from another given the present position of planets can be very tricky. Unless we can observe some markers of former evolution it is nearly impossible to distinguish the two very different spin profiles, let alone intermediate spin profiles. Though some conclusions can still be drawn from statistical distributions of planets around fully convective M-dwarfs. However, if the tidal evolution brings about a merger late in its history it can also entail a noticeable acceleration of the star in late ages, so that it is possible to have old stars that spin rapidly. This raises the question of better constraining the age of stars.

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arXiv2012

Debris disks as signposts of terrestrial planet formation. II Dependence of exoplanet architectures on giant planet and disk properties

We present models for the formation of terrestrial planets, and the collisional evolution of debris disks, in planetary systems that contain multiple unstable gas giants. We previously showed that the dynamics of the giant planets introduces a correlation between the presence of terrestrial planets and debris disks. Here we present new simulations that show that this connection is qualitatively robust to changes in: the mass distribution of the giant planets, the width and mass distribution of the outer planetesimal disk, and the presence of gas in the disk. We discuss how variations in these parameters affect the evolution. Systems with equal-mass giant planets undergo the most violent instabilities, and these destroy both terrestrial planets and the outer planetesimal disks that produce debris disks. In contrast, systems with low-mass giant planets efficiently produce both terrestrial planets and debris disks. A large fraction of systems with low-mass outermost giant planets have stable gaps between these planets that are frequently populated by planetesimals. Planetesimal belts between outer giant planets may affect debris disk SEDs. If Earth-mass seeds are present in outer planetesimal disks, the disks radially spread to colder temperatures. We argue that this may explain the very low frequency of > 1 Gyr-old solar-type stars with observed 24 micron excesses. Among the (limited) set of configurations explored, the best candidates for hosting terrestrial planets at ~1 AU are stars older than 0.1-1 Gyr with bright debris disks at 70 micron but with no currently-known giant planets. These systems combine evidence for rocky building blocks, with giant planet properties least likely to undergo destructive dynamical evolution. We predict an anti-correlation between debris disks and eccentric giant planets, and a positive correlation between debris disks and terrestrial planets.

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arXiv2012

A low mass for Mars from Jupiter's early gas-driven migration

Jupiter and Saturn formed in a few million years (Haisch et al. 2001) from a gas-dominated protoplanetary disk, and were susceptible to gas-driven migration of their orbits on timescales of only ~100,000 years (Armitage 2007). Hydrodynamic simulations show that these giant planets can undergo a two-stage, inward-then-outward, migration (Masset & Snellgrove 2001, Morbidelli & Crida 2007, Pierens & Nelson 2008). The terrestrial planets finished accreting much later (Klein et al. 2009), and their characteristics, including Mars' small mass, are best reproduced by starting from a planetesimal disk with an outer edge at about one astronomical unit from the Sun (Wetherill 1978, Hansen 2009) (1 AU is the Earth-Sun distance). Here we report simulations of the early Solar System that show how the inward migration of Jupiter to 1.5 AU, and its subsequent outward migration, lead to a planetesimal disk truncated at 1 AU; the terrestrial planets then form from this disk over the next 30-50 million years, with an Earth/Mars mass ratio consistent with observations. Scattering by Jupiter initially empties but then repopulates the asteroid belt, with inner-belt bodies originating between 1 and 3 AU and outer-belt bodies originating between and beyond the giant planets. This explains the significant compositional differences across the asteroid belt. The key aspect missing from previous models of terrestrial planet formation is the substantial radial migration of the giant planets, which suggests that their behaviour is more similar to that inferred for extrasolar planets than previously thought.

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arXiv2012

Planet-Planet Scattering Alone Cannot Explain the Free-Floating Planet Population

Recent gravitational microlensing observations predict a vast population of free-floating giant planets that outnumbers main sequence stars almost twofold. A frequently-invoked mechanism for generating this population is a dynamical instability that incites planet-planet scattering and the ejection of one or more planets in isolated main sequence planetary systems. Here, we demonstrate that this process alone probably cannot represent the sole source of these galactic wanderers. By using straightforward quantitative arguments and N-body simulations, we argue that the observed number of exoplanets exceeds the plausible number of ejected planets per system from scattering. Thus, other potential sources of free-floaters, such as planetary stripping in stellar clusters and post-main-sequence ejection, must be considered.

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arXiv2011

55 Cancri: A Coplanar Planetary System that is Likely Misaligned with its Star

Although the 55 Cnc system contains multiple, closely packed planets that are presumably in a coplanar configuration, we use numerical simulations to demonstrate that they are likely to be highly inclined to their parent star's spin axis. Due to perturbations from its distant binary companion, this planetary system precesses like a rigid body about its parent star. Consequently, the parent star's spin axis and the planetary orbit normal likely diverged long ago. Because only the projected separation of the binary is known, we study this effect statistically, assuming an isotropic distribution for wide binary orbits. We find that the most likely projected spin-orbit angle is ~50 degrees, with a ~30% chance of a retrograde configuration. Transit observations of the innermost planet - 55 Cnc e - may be used to verify these findings via the Rossiter-McLaughlin effect. 55 Cancri may thus represent a new class of planetary systems with well-ordered, coplanar orbits that are inclined with respect to the stellar equator.

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