arXiv subjects

Frederic A. Rasio

Publications and source records attributed to Frederic A. Rasio.

At least 145 records · Page 8Linked to original sources

arXiv2007

Effects of stellar collisions on star cluster evolution and core collapse

We systematically study the effects of collisions on the overall dynamical evolution of dense star clusters using Monte Carlo simulations over many relaxation times. We derive many observable properties of these clusters, including their core radii and the radial distribution of collision products. We also study different aspects of collisions in a cluster taking into account the shorter lifetimes of more massive stars, which has not been studied in detail before. Depending on the lifetimes of the significantly more massive collision products, observable properties of the cluster can be modified qualitatively; for example, even without binaries, core collapse can sometimes be avoided simply because of stellar collisions.

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arXiv2007

From mean-motion resonances to scattered planets: Producing the Solar System, eccentric exoplanets and Late Heavy Bombardments

We show that interaction with a gas disk may produce young planetary systems with closely-spaced orbits, stabilized by mean-motion resonances between neighbors. On longer timescales, after the gas is gone, interaction with a remnant planetesimal disk tends to pull these configurations apart, eventually inducing dynamical instability. We show that this can lead to a variety of outcomes; some cases resemble the Solar System, while others end up with high-eccentricity orbits reminiscent of the observed exoplanets. A similar mechanism has been previously suggested as the cause of the lunar Late Heavy Bombardment. Thus, it may be that a large-scale dynamical instability, with more or less cataclysmic results, is an evolutionary step common to many planetary systems, including our own.

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arXiv2007

Role of Dynamical Research in the Detection and Characterization of Exoplanets

(Abridged) The discovery of extrasolar planetary systems revealed an unexpected diversity of planetary systems that has revolutionized planet formation theory. A strong program of theoretical research is essential to maximize both the discovery potential and the scientific returns of future observational programs, so as to achieve a deeper understanding of the formation and evolution of planetary systems. We outline three broad categories of theoretical research: detailed studies of specific planetary systems, testing planet formation models by comparing their predictions to the observed exoplanet population, and detailed modeling of specific physical processes. We describe how such theoretical research plays an important role in analyzing observations for a wide range detection methods and contributes to understanding the Earth's place in the universe and the potential for Earth-like life beyond our solar system. In this white paper, we suggest how to maximize the scientific return of future exoplanet observations. Our recommendations include a strong theory program, support for multiple observational programs that will study a diverse set of planets and stars, significant observing time devoted to follow-up observations, and healthy collaboration between observers and theorists.

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arXiv2007

On the rarity of double black hole binaries: consequences for gravitational-wave detection

Double black hole binaries are among the most important sources of gravitational radiation for ground-based detectors such as LIGO or VIRGO. Even if formed with lower efficiency than double neutron star binaries, they could dominate the predicted detection rates, since black holes are more massive than neutron stars and therefore could be detected at greater distances. Here we discuss an evolutionary process that can very significantly limit the formation of close double black hole binaries: the vast majority of their potential progenitors undergo a common envelope (CE) phase while the donor, one of the massive binary components, is evolving through the Hertzsprung gap. Our latest theoretical understanding of the CE process suggests that this will probably lead to a merger, inhibiting double black hole formation. Barring uncertainties in the physics of CE evolution, we use population synthesis calculations, and find that the corresponding reduction in the merger rate of double black holes formed in galactic fields is so great (by ~500) that their contribution to inspiral detection rates for ground-based detectors could become relatively small (1 in 10) compared to double neutron star binaries. A similar process also reduces the merger rates for double neutron stars, by factor of ~5, eliminating most of the previously predicted ultracompact NS-NS systems. Our predicted detection rates for Advanced LIGO are now much lower for double black holes (~2/yr), but are still quite high for double neutron stars (~20/yr). If double black holes were found to be dominant in the detected inspiral signals, this could indicate that they mainly originate from dense star clusters (not included here) or that our theoretical understanding of the CE phase requires significant revision.

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arXiv2006

Monte Carlo Simulations of Globular Cluster Evolution. IV. Direct Integration of Strong Interactions

We study the dynamical evolution of globular clusters containing populations of primordial binaries, using our newly updated Monte Carlo cluster evolution code with the inclusion of direct integration of binary scattering interactions. We describe the modifications we have made to the code, as well as improvements we have made to the core Monte Carlo method. We present several test calculations to verify the validity of the new code, and perform many comparisons with previous analytical and numerical work in the literature. We simulate the evolution of a large grid of models, with a wide range of initial cluster profiles, and with binary fractions ranging from 0 to 1, and compare with observations of Galactic globular clusters. We find that our code yields very good agreement with direct N-body simulations of clusters with primordial binaries, but yields some results that differ significantly from other approximate methods. Notably, the direct integration of binary interactions reduces their energy generation rate relative to the simple recipes used in Paper III, and yields smaller core radii. Our results for the structural parameters of clusters during the binary-burning phase are now in the tail of the range of parameters for observed clusters, implying that either clusters are born significantly more or less centrally concentrated than has been previously considered, or that there are additional physical processes beyond two-body relaxation and binary interactions that affect the structural characteristics of clusters.

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arXiv2006

Structure and Evolution of Nearby Stars with Planets II. Physical Properties of ~1000 Cool Stars from the SPOCS Catalog

We derive detailed theoretical models for 1074 nearby stars from the SPOCS (Spectroscopic Properties of Cool Stars) Catalog. The California and Carnegie Planet Search has obtained high-quality echelle spectra of over 1000 nearby stars taken with the Hamilton spectrograph at Lick Observatory, the HIRES spectrograph at Keck, and UCLES at the Anglo Australian Observatory. A uniform analysis of the high-resolution spectra has yielded precise stellar parameters, enabling systematic error analyses and accurate theoretical stellar modeling. We have created a large database of theoretical stellar evolution tracks using the Yale Stellar Evolution Code (YREC) to match the observed parameters of the SPOCS stars. Our very dense grids of evolutionary tracks eliminate the need for interpolation between stellar evolutionary tracks and allow precise determinations of physical stellar parameters (mass, age, radius, size and mass of the convective zone, etc.). Combining our stellar models with the observed stellar atmospheric parameters and uncertainties, we compute the likelihood for each set of stellar model parameters separated by uniform time steps along the stellar evolutionary tracks. The computed likelihoods are used for a Bayesian analysis to derive posterior probability distribution functions for the physical stellar parameters of interest. We provide a catalog of physical parameters for 1074 stars that are based on a uniform set of high quality spectral observations, a uniform spectral reduction procedure, and a uniform set of stellar evolutionary models. We explore this catalog for various possible correlations between stellar and planetary properties, which may help constrain the formation and dynamical histories of other planetary systems.

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arXiv2006

Observing IMBH-IMBH Binary Coalescences via Gravitational Radiation

Recent numerical simulations have suggested the possibility of forming double intermediate mass black holes (IMBHs) via the collisional runaway scenario in young dense star clusters. The two IMBHs formed would exchange into a common binary shortly after their birth, and quickly inspiral and merge. Since space-borne gravitational wave (GW) observatories such as LISA will be able to see the late phases of their inspiral out to several Gpc, and LIGO will be able to see the merger and ringdown out to similar distances, they represent potentially significant GW sources. In this Letter we estimate the rate at which LISA and LIGO will see their inspiral and merger in young star clusters, and discuss the information that can be extracted from the observations. We find that LISA will likely see tens of IMBH--IMBH inspirals per year, while advanced LIGO could see ~10 merger and ringdown events per year, with both rates strongly dependent on the distribution of cluster masses and densities.

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arXiv2006

Black Hole-Neutron Star Binary Merger Calculations: GRB Progenitors and the Stability of Mass Transfer

We have calculated the first dynamical evolutions of merging black hole-neutron star binaries that treat the combined spacetime in a nonperturbative general relativistic framework. Using the conformal flatness approximation, we have studied how the location of the tidal disruption radius with respect to the the black hole horizon and innermost stable circular orbit (ISCO) affects the qualitative evolution of the system. Based on simple arguments, we show that for a binary mass ratio q>~0.24, tidal disruption occurs outside the ISCO, while the opposite is true for q<~0.24. When tidal disruption occurs sufficiently far outside the ISCO, mass is transferred unstably from the neutron star to the black hole, resulting in the complete disruption of the neutron star. When tidal disruption occurs slightly within the ISCO, we find that some of the mass forms an extremely hot disk around the black hole. The resulting configurations in this case are excellent candidates for the progenitors of short-hard gamma ray bursts.

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arXiv2006

Massive Black Hole Binaries from Collisional Runaways

Recent theoretical work has solidified the viability of the collisional runaway scenario in young dense star clusters for the formation of very massive stars (VMSs), which may be precursors to intermediate-mass black holes (IMBHs). We present first results from a numerical study of the collisional runaway process in dense star clusters containing primordial binaries. Stellar collisions during binary scattering encounters offer an alternate channel for runaway growth, somewhat independent of direct collisions between single stars. We find that clusters with binary fractions >~10% yield two VMSs via collisional runaways, presenting the exotic possibility of forming IMBH--IMBH binaries in star clusters. We discuss the implications for gravitational wave observations, and the impact on cluster structure.

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arXiv2006

Runaway collisions in young star clusters. I. Methods and tests

We present the methods and preparatory work for our study of the collisional runaway scenario to form a very massive star (VMS, M > 400 Msun) at the centre of a young, compact stellar cluster. In the first phase of the process, a very dense central core of massive stars (M =~ 30-120 Msun) forms through mass segregation and gravothermal collapse. This leads to a collisional stage, likely to result in the formation of a VMS (itself a possible progenitor for an intermediate-mass black hole) through a runaway sequence of mergers between the massive stars. In this paper we present the runaway scenario in a general astrophysical context. We then explain the numerical method used to investigate it. Our approach is based on a Monte Carlo code to simulate the stellar dynamics of spherical star clusters using a very large number of particles (a few 1e5 to several 1e6). Finally, we report on test computations carried out to ensure that our implementation of the important physics is sound. In a second paper, we present results from more than 100 cluster simulations realized to determine the conditions leading to the collisional formation of a VMS and the characteristics of the runaway sequences.

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arXiv2006

Runaway collisions in young star clusters. II. Numerical results

We present a new study of the collisional runaway scenario to form an intermediate-mass black hole (IMBH, MBH > 100 Msun) at the centre of a young, compact stellar cluster. The first phase is the formation of a very dense central core of massive stars (Mstar =~ 30-120 Msun) through mass segregation and gravothermal collapse. Previous work established the conditions for this to happen before the massive stars evolve off the main sequence (MS). In this and a companion paper, we investigate the next stage by implementing direct collisions between stars. Using a Monte Carlo stellar dynamics code, we follow the core collapse and subsequent collisional phase in more than 100 models with varying cluster mass, size, and initial concentration. Collisions are treated either as ideal, ``sticky-sphere'' mergers or using realistic prescriptions derived from 3-D hydrodynamics computations. In all cases for which the core collapse happens in less than the MS lifetime of massive stars (~3 Myr), we obtain the growth of a single very massive star (VMS, Mstar =~ 400-4000 Msun) through a runaway sequence of mergers. Mass loss from collisions, even for velocity dispersions as high as sigma1D ~ 1000 km/s, does not prevent the runaway. The region of cluster parameter space leading to runaway is even more extended than predicted in previous work because, in clusters with sigma1D > 300 km/s, collisions accelerate (and, in extreme cases, drive) core collapse. Although the VMS grows rapidly to > 1000 Msun in models exhibiting runaway, we cannot predict accurately its final mass. This is because the termination of the runaway process must eventually be determined by a complex interplay between stellar dynamics, hydrodynamics, and the stellar evolution of the VMS. [abridged]

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arXiv2005

On the Relation Between Hot Jupiters & the Roche Limit

Many of the known extrasolar planets are ``hot Jupiters,'' giant planets with orbital periods of just a few days. We use the observed distribution of hot Jupiters to constrain the location of its inner edge in the mass--period diagram. If we assume a slope corresponding to the classical Roche limit, then we find that the edge corresponds to a separation close to_twice_ the Roche limit, as expected if the planets started on highly eccentric orbits that were later circularized. In contrast, any migration scenario would predict an inner edge right at the Roche limit, which applies to planets approaching on nearly circular orbits. However, the current sample of hot Jupiters is not sufficient to provide a precise constraint simultaneously on both the location and slope of the inner edge.

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arXiv2005

Star Cluster Evolution with Primordial Binaries

Observations and theoretical work suggest that globular clusters may be born with initially very large binary fractions. We present first results from our newly modified Monte-Carlo cluster evolution code, which treats binary interactions exactly via direct N-body integration. It is shown that binary scattering interactions generate significantly less energy than predicted by the recipes that have been used in the past to model them in approximate cluster evolution methods. The new result that the cores of globular clusters in the long-lived binary-burning phase are smaller than previously predicted weakens the agreement with observations, thus implying that more than simply stellar dynamics is at work in shaping the globular clusters we observe today.

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arXiv2005

Dynamical Interactions of Planetary Systems in Dense Stellar Environments

We study dynamical interactions of star--planet binaries with other single stars. We derive analytical cross sections for all possible outcomes, and confirm them with numerical scattering experiments. We find that a wide mass ratio in the binary introduces a region in parameter space that is inaccessible to comparable-mass systems, in which the nature of the dynamical interaction is fundamentally different from what has traditionally been considered in the literature on binary scattering. We study the properties of the planetary systems that result from the scattering interactions for all regions of parameter space, paying particular attention to the location of the "hard--soft" boundary. The structure of the parameter space turns out to be significantly richer than a simple statement of the location of the "hard--soft" boundary would imply. We consider the implications of our findings, calculating characteristic lifetimes for planetary systems in dense stellar environments, and applying the results to previous analytical studies, as well as past and future observations. Recognizing that the system PSR B1620-26 in the globular cluster M4 lies in the "new" region of parameter space, we perform a detailed analysis quantifying the likelihood of different scenarios in forming the system we see today.

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arXiv2005

The Dynamical Evolution of Black Hole-Neutron Star Binaries in General Relativity: Simulations of Tidal Disruption

We calculate the first dynamical evolutions of merging black hole-neutron star binaries that construct the combined black hole-neutron star spacetime in a general relativistic framework. We treat the metric in the conformal flatness approximation, and assume that the black hole mass is sufficiently large compared to that of the neutron star so that the black hole remains fixed in space. Using a spheroidal spectral methods solver, we solve the resulting field equations for a neutron star orbiting a Schwarzschild black hole. The matter is evolved using a relativistic, Lagrangian, smoothed particle hydrodynamics (SPH) treatment. We take as our initial data recent quasiequilibrium models for synchronized neutron star polytropes generated as solutions of the conformal thin-sandwich (CTS) decomposition of the Einstein field equations. We are able to construct from these models relaxed SPH configurations whose profiles show good agreement with CTS solutions. Our adiabatic evolution calculations for neutron stars with low compactness show that mass transfer, when it begins while the neutron star orbit is still outside the innermost stable circular orbit, is more unstable than is typically predicted by analytical formalisms. This dynamical mass loss is found to be the driving force in determining the subsequent evolution of the binary orbit and the neutron star, which typically disrupts completely within a few orbital periods. The majority of the mass transferred onto the black hole is accreted promptly; a significant fraction (~30%) of the mass is shed outward as well, some of which will become gravitationally unbound and ejected completely from the system. The remaining portion forms an accretion disk around the black hole, and could provide the energy source for short-duration gamma ray bursts.

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arXiv2005

Eccentricities of Planets in Binary Systems

The most puzzling property of the extrasolar planets discovered by recent radial velocity surveys is their high orbital eccentricities, which are very difficult to explain within our current theoretical paradigm for planet formation. Current data reveal that at least 25% of these planets, including some with particularly high eccentricities, are orbiting a component of a binary star system. The presence of a distant companion can cause significant secular perturbations in the orbit of a planet. At high relative inclinations, large-amplitude, periodic eccentricity perturbations can occur. These are known as "Kozai cycles" and their amplitude is purely dependent on the relative orbital inclination. Assuming that every planet host star also has a (possibly unseen, e.g., substellar) distant companion, with reasonable distributions of orbital parameters and masses, we determine the resulting eccentricity distribution of planets and compare it to observations? We find that perturbations from a binary companion always appear to produce an excess of planets with both very high (e>0.6) and very low (e<0.1) eccentricities. The paucity of near-circular orbits in the observed sample implies that at least one additional mechanism must be increasing eccentricities. On the other hand, the overproduction of very high eccentricities observed in our models could be combined with plausible circularization mechanisms (e.g., friction from residual gas) to create more planets with intermediate eccentricities (e=0.1-0.6).

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arXiv2005

On the relation between Hot-Jupiters and the Roche Limit

Many of the known extrasolar planets are ``hot Jupiters,'' giant planets with orbital periods of just a few days. We use the observed distribution of hot Jupiters to constrain the location of the ``inner edge'' and planet migration theory. If we assume the location of the inner edge is proportional to the Roche limit, then we find that this edge is located near twice the Roche limit, as expected if the planets were circularized from a highly eccentric orbit. If confirmed, this result would place significant limits on migration via slow inspiral. However, if we relax our assumption for the slope of the inner edge, then the current sample of hot Jupiters is not sufficient to provide a precise constraint on both the location and power law index of the inner edge.

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arXiv2005

Binary Mergers and Growth of Black Holes in Dense Star Clusters

We model the dynamical evolution of primordial black holes (BHs) in dense star clusters using a simplified treatment of stellar dynamics in which the BHs are assumed to remain concentrated in an inner core, completely decoupled from the background stars. Dynamical interactions involving BH binaries are computed exactly and are generated according to a Monte Carlo prescription. Recoil and ejections lead to complete evaporation of the BH core on a timescale ~10^9 yr for typical globular cluster parameters. Orbital decay driven by gravitational radiation can make binaries merge and, in some cases, successive mergers can lead to significant BH growth. Our highly simplified treatment of the cluster dynamics allows us to study a large number of models and to compute statistical distributions of outcomes, such as the probability of massive BH growth and retention in a cluster. We find that, in most models, there is a significant probability (~20-80%) of BH growth with final masses > 100 M_{\sun}. In one case, a final BH formed with mass ~ 620 M_{\sun}. However, if the typical merger recoil speed (due to asymmetric emission of gravitational radiation) significantly exceeds the cluster escape speed, no growth ever occurs. Independent of the recoil speed, we find that BH-BH mergers enhanced by dynamical interactions in cluster cores present an important source of gravitational waves for ground--based laser interferometers. Under optimistic conditions, the total rate of detections by Advanced LIGO, could be as high as a few tens of events per year from inspiraling BHs from clusters.

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