Nickolas Moeckel

University of St Andrews, Saint Andrews, SCT, United Kingdom

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Publications (23)113.38 Total impact

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    ABSTRACT: The details of how massive stars form are poorly known. Orion BN/KL is the closest known region with ongoing massive star formation, and hence offers unique chances for a detailed study and an excellent laboratory to test competing theories. In this poster, I illustrates highlights from a long-term study of the region based on a wealth of interferometric data from (E)VLA, VLBA, and ALMA, in particular: 1) a beautiful example of disk-mediated accretion and (magnetic) outflow recollimation in a high-mass protostar; 2) a dynamical model to explain the famous explosive BN/KL flow; 3) a new hypothesis for the excitation of the eponymous Orion Hot Core; 4) the effects of the complex (clustered) environment on an actively accreting massive protostar (and viceversa). This detailed study has enabled us not only to achieve a better understanding of Orion BN/KL but also to significantly advance our understanding of high-mass star formation.
    07/2013;
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    Nickolas Moeckel, Ian A. Bonnell
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    ABSTRACT: Massive stars are known to have a high multiplicity, with examples of higher order multiples among the nearest and best studied objects. In this paper we study hierarchical multiple systems (an inner binary as a component of a wider binary) of massive stars in a clustered environment, in which a system with a size of 100--1000 au will undergo many close encounters during the short lifetime of a massive star. Using two types of N-body experiment we determine the post-formation collision probabilities of these massive hierarchies. We find that, depending on the specifics of the environment, the hierarchy, and the amount of time that is allowed to pass, tens of percent of hierarchies will experience a collision, typically between the two stars of the inner binary. In addition to collisions, clusters hosting a hierarchical massive system produce high velocity runaways at an enhanced rate. The primordial multiplicity specifics of massive stars appear to play a key role in the generation of these relatively small number events in cluster simulations, complicating their use as diagnostics of a cluster's history.
    01/2013;
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    Dimitri Veras, Nickolas Moeckel
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    ABSTRACT: During their main sequence lifetimes, the majority of all Galactic Disc field stars must endure at least one stellar intruder passing within a few hundred AU. Mounting observations of planet-star separations near or beyond this distance suggest that these close encounters may fundamentally shape currently-observed orbital architectures and hence obscure primordial orbital features. We consider the commonly-occurring fast close encounters of two single-planet systems in the Galactic Disc, and investigate the resulting change in the planetary eccentricity and semimajor axis. We derive explicit 4-body analytical limits for these variations and present numerical cross-sections which can be applied to localized regions of the Galaxy. We find that each wide-orbit planet has a few percent chance of escape and an eccentricity that will typically change by at least 0.1 due to these encounters. The orbital properties established at formation of millions of tight-orbit Milky Way exoplanets are likely to be disrupted.
    Monthly Notices of the Royal Astronomical Society 06/2012; 425(1). · 5.52 Impact Factor
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    ABSTRACT: Infant mortality brought about by the expulsion of a star cluster's natal gas is widely invoked to explain cluster statistics at different ages. While a well studied problem, most recent studies of gas expulsion's effect on a cluster have focused on massive clusters, with stellar counts of order $10^4$. Here we argue that the evolutionary timescales associated with the compact low-mass clusters typical of the median cluster in the Solar neighborhood are short enough that significant dynamical evolution can take place over the ages usually associated with gas expulsion. To test this we perform {\it N}-body simulations of the dynamics of a very young star forming region, with initial conditions drawn from a large-scale hydrodynamic simulation of gravitational collapse and fragmentation. The subclusters we analyse, with populations of a few hundred stars, have high local star formation efficiencies and are roughly virialised even after the gas is removed. Over 10 Myr they expand to a similar degree as would be expected from gas expulsion if they were initially gas-rich, but the expansion is purely due to the internal stellar dynamics of the young clusters. The expansion is such that the stellar densities at 2 Myr match those of YSOs in the Solar neighborhood. We argue that at the low-mass end of the cluster mass spectrum, a deficit of clusters at 10s of Myr does not necessarily imply gas expulsion as a disruption mechanism.
    Monthly Notices of the Royal Astronomical Society 05/2012; 425(1). · 5.52 Impact Factor
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    Nickolas Moeckel, Dimitri Veras
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    ABSTRACT: Exoplanetary systems are found not only among single stars, but also binaries of widely varying parameters. Binaries with separations of 100--1000 au are prevalent in the Solar neighborhood; at these separations planet formation around a binary member may largely proceed as if around a single star. During the early dynamical evolution of a planetary system, planet--planet scattering can eject planets from a star's grasp. In a binary, the motion of a planet ejected from one star has effectively entered a restricted three-body system consisting of itself and the two stars, and the equations of motion of the three body problem will apply as long as the ejected planet remains far from the remaining planets. Depending on its energy, escape from the binary as a whole may be impossible or delayed until the three-body approximation breaks down, and further close interactions with its planetary siblings boost its energy when it passes close to its parent star. Until then this planet may be able to transition from the space around one star to the other, and chaotically `bounce' back and forth. In this paper we directly simulate scattering planetary systems that are around one member of a circular binary, and quantify the frequency of bouncing in scattered planets. We find that a great majority (70 to 85 per cent) of ejected planets will pass at least once through the space of it's host's binary companion, and depending on the binary parameters about 45 to 75 per cent will begin bouncing. The time spent bouncing is roughly log-normally distributed with a peak at about $10^4$ years, with only a small percentage bouncing for more than a Myr. This process may perturb and possibly incite instability among existing planets around the companion star. In rare cases, the presence of multiple planets orbiting both stars may cause post-bouncing capture or planetary swapping.
    Monthly Notices of the Royal Astronomical Society 01/2012; 422(1). · 5.52 Impact Factor
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    ABSTRACT: The fraction of star formation that results in bound star clusters is influenced by the density spectrum in which stars are formed and by the response of the stellar structure to gas expulsion. We analyse hydrodynamical simulations of turbulent fragmentation in star-forming regions to assess the dynamical properties of the resulting population of stars and (sub)clusters. Stellar subclusters are identified using a minimum spanning tree algorithm. When considering only the gravitational potential of the stars and ignoring the gas, we find that the identified subclusters are close to virial equilibrium (the typical virial ratio Q_vir~0.59, where virial equilibrium would be Q_vir~0.5). This virial state is a consequence of the low gas fractions within the subclusters, caused by the accretion of gas onto the stars and the accretion-induced shrinkage of the subclusters. Because the subclusters are gas-poor, up to a length scale of 0.1-0.2 pc at the end of the simulation, they are only weakly affected by gas expulsion. The fraction of subclusters that reaches the high density required to evolve to a gas-poor state increases with the density of the star-forming region. We extend this argument to star cluster scales, and suggest that the absence of gas indicates that the early disruption of star clusters due to gas expulsion (infant mortality) plays a smaller role than anticipated, and is potentially restricted to star-forming regions with low ambient gas densities. We propose that in dense star-forming regions, the tidal shocking of young star clusters by the surrounding gas clouds could be responsible for the early disruption. This `cruel cradle effect' would work in addition to disruption by gas expulsion. We suggest possible methods to quantify the relative contributions of both mechanisms.
    Monthly Notices of the Royal Astronomical Society 01/2012; 419(1):841. · 5.52 Impact Factor
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    Nickolas Moeckel, Ciriaco Goddi
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    ABSTRACT: The Orion BN/KL complex is the nearest site of ongoing high-mass star formation. Recent proper motion observations provide convincing evidence of a recent (about 500 years ago) dynamical interaction between two massive young stellar objects in the region resulting in high velocities: the BN object and radio Source I. At the same time, Source I is surrounded by a nearly edge-on disc with radius ~50 au. These two observations taken together are puzzling: a dynamical encounter between multiple stars naturally yields the proper motions, but the survival of a disc is challenging to explain. In this paper we take the first steps to numerically explore the preferred dynamical scenario of Goddi et al., in which Source I is a binary that underwent a scattering encounter with BN, in order to determine if a pre-existing disc can survive this encounter in some form. Treating only gravitational forces, we are able to thoroughly and efficiently cover a large range of encounter parameters. We find that disc material can indeed survive a three-body scattering event if 1) the encounter is close, i.e. BN's closest approach to Source I is comparable to Source I's semi-major axis; and 2) the interplay of the three stars is of a short duration. Furthermore, we are able to constrain the initial conditions that can broadly produce the orientation of the present-day system's disc relative to its velocity vector. To first order we can thus confirm the plausibility of the scattering scenario of Goddi et al., and we have significantly constrained the parameters and narrowed the focus of future, more complex and expensive attempts to computationally model the complicated BN/KL region.
    Monthly Notices of the Royal Astronomical Society 09/2011; 419. · 5.52 Impact Factor
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    Nickolas Moeckel, Philip J. Armitage
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    ABSTRACT: A significant fraction of unstable multiple planet systems likely scatter during the transitional disc phase as gas damping becomes ineffectual. Using an ensemble of FARGO hydrodynamic simulations and MERCURY n-body integrations, we directly follow planet-disc and planet-planet interactions through the clearing phase and on through 50 Myr of dynamical evolution. Disc clearing occurs via X-ray driven photoevaporation. The hydrodynamic evolution of individual scattering systems is complex, and involves phases in which massive planets orbit within eccentric gaps, or accrete directly from the disc without a gap. Comparing the results to a gas-free model, we find that the n-body dynamics and hydrodynamics of scattering into one- and two-planet final states are almost identical. The eccentricity distributions in these channels are almost unaltered by the presence of gas. The hydrodynamic simulations, however, also form low eccentricity three-planet systems in long-term stable configurations, and the admixture of these systems results in modestly lower eccentricities in hydrodynamic as opposed to gas-free simulations. The incidence of these three-planet systems is likely a function of the initial conditions; different planet setups (number or spacing) may change the character of this result. We analyze the properties of surviving multiple planet systems, and show that only a small fraction (a few percent) enter mean-motion resonances after scattering, while a larger fraction form stable resonant chains and avoid scattering entirely. Our results remain consistent with the hypothesis that exoplanet eccentricity results from scattering, though the detailed agreement between observations and gas-free simulation results is likely coincidental. We discuss the prospects for testing scattering models by observing planets or non-axisymmetric gas structure in transitional discs.
    Monthly Notices of the Royal Astronomical Society 08/2011; 419. · 5.52 Impact Factor
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    Nickolas Moeckel, Cathie J. Clarke
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    ABSTRACT: Wide, fragile binary stellar systems are found in the galactic field, and have recently been noted in the outskirts of expanding star clusters in numerical simulations. Energetically soft, with semi-major axes exceeding the initial size of their birth cluster, it is puzzling how these binaries are created and preserved. We provide an interpretation of the formation of these binaries that explains the total number formed and their distribution of energies. A population of weakly bound binaries can always be found in the cluster, in accordance with statistical detailed balance, limited at the soft end only by the current size of the cluster and whatever observational criteria are imposed. At any given time, the observed soft binary distribution is predominantly a snapshot of a transient population. However, there is a constantly growing population of long-lived soft binaries that are removed from the detailed balance cycle due to the changing density and velocity dispersion of an expanding cluster. The total number of wide binaries that form, and their energy distribution, are insensitive to the cluster population; the number is approximately one per cluster. This suggests that a population composed of many dissolved small-N clusters will more efficiently populate the field with wide binaries than that composed of dissolved large-N clusters. Locally such binaries are present at approximately the 2% level; thus the production rate is consistent with the field being populated by clusters with a median of a few hundred stars rather than a few thousand.
    Monthly Notices of the Royal Astronomical Society 03/2011; 415. · 5.52 Impact Factor
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    C. Weidner, I. A. Bonnell, N. Moeckel
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    ABSTRACT: With the use of N-body calculations, the numbers and properties of escaping stars from low-N (N= 100 and 1000) young embedded star clusters prior to gas expulsion are studied over the first 5 Myr of their existence. Besides the numbers of stars, different initial radii and binary populations are also examined as well as virialized and collapsing clusters. It is found that these clusters can lose substantial amounts (up to 20 per cent) of stars within 5 Myr, with considerable velocities of up to more than 100 km s−1. Even with their mean velocities between 2 and 8 km s−1, these stars will still travel between 2 and 30 pc during the 5 Myr. Therefore large numbers of distributed stars in star-forming regions cannot necessarily be counted as evidence for the isolated formation of stars.
    Monthly Notices of the Royal Astronomical Society 01/2011; 410(3):1861 - 1869. · 5.52 Impact Factor
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    ABSTRACT: The explosive BN/KL outflow emerging from OMC1 behind the Orion Nebula may have been powered by the dynamical decay of a non-hierarchical multiple system $\sim$500 years ago that ejected the massive stars I, BN, and source n, with velocities of about 10 to 30 km s$^{-1}$. New proper motion measurements of H$_2$ features show that within the errors of measurement, the outflow originated from the site of stellar ejection. Combined with published data, these measurements indicate an outflow age of $\sim$500 years, similar to the time since stellar ejection. The total kinetic energy of the ejected stars and the outflow is about 2 to $6 \times 10^{47}$ ergs. It is proposed that the gravitational potential energy released by the formation of a short-period binary, most likely source I, resulted in stellar ejection and powered the outflow. A scenario is presented for the formation of a compact, non-hierarchical multiple star system, its decay into an ejected binary and two high-velocity stars, and launch of the outflow. Three mechanisms may have contributed to the explosion in the gas: (i) Unbinding of the circum-cluster envelope following stellar ejection, (ii) disruption of circumstellar disks and high-speed expulsion of the resulting debris during the final stellar encounter, and (iii) the release of stored magnetic energy. Plausible proto-stellar disk end envelope properties can produce the observed outflow mass, velocity, and kinetic energy distributions. The ejected stars may have acquired new disks by fall-back or Bondi-Hoyle accretion with axes roughly orthogonal to their velocities. The expulsion of gas and stars from OMC1 may have been driven by stellar interactions. Comment: 36 pages, 7 figures, 2 tables
    The Astrophysical Journal 11/2010; · 6.73 Impact Factor
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    Nickolas Moeckel, Cathie J. Clarke
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    ABSTRACT: We investigate the contraction of accreting protoclusters using an extension of n-body techniques that incorporates the accretional growth of stars from the gaseous reservoir in which they are embedded. Following on from Monte Carlo studies by Davis et al., we target our experiments toward populous clusters likely to experience collisions as a result of accretion-driven contraction. We verify that in less extreme star forming environments, similar to Orion, the stellar density is low enough that collisions are unimportant, but that conditions suitable for stellar collisions are much more easily satisfied in large-n clusters, i.e. n ~ 30,000 (we argue, however, that the density of the Arches cluster is insufficient for us to expect stellar collisions to have occurred in the cluster's prior evolution). We find that the character of the collision process is not such that it is a route toward smoothly filling the top end of the mass spectrum. Instead, runaway growth of one or two extreme objects can occur within less than 1 Myr after accretion is shut off, resulting in a few objects with masses several times the maximum reached by accretion. The rapid formation of these objects is due to not just the post-formation dynamical evolution of the clusters, but an interplay of dynamics and the accretional growth of the stars. We find that accretion-driven cluster shrinkage results in a distribution of gas and stars that offsets the disruptive effect of gas expulsion, and we propose that the process can lead to massive binaries and early mass segregation in star clusters. Comment: 8 pages, accepted to MNRAS
    Monthly Notices of the Royal Astronomical Society 09/2010; · 5.52 Impact Factor
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    Nickolas Moeckel, Matthew R. Bate
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    ABSTRACT: We investigate the evolution, following gas dispersal, of a star cluster produced from a hydrodynamical calculation. We find that when the gas, initially comprising 60% of the mass, is removed, the system settles into a bound cluster containing 30-40% of the stellar mass surrounding by an expanding halo of ejected stars. The bound cluster expands from an initial radius of <0.05 pc to 1-2 pc over 4-10 Myr, depending on how quickly the gas is removed, implying that stellar clusters may begin with far higher stellar densities than usually assumed. With rapid gas dispersal the most massive stars are found to be mass segregated for the first ~1 Myr of evolution, but classical mass segregation only develops for cases with long gas removal timescales. Eventually, many of the most massive stars are expelled from the bound cluster. Despite the high initial stellar density and the extensive dynamical evolution of the system, we find that the stellar multiplicity is almost constant during the 10 Myr of evolution. This is because the primordial multiple systems are formed in a clustered environment and, thus, by their nature are already resistant to further evolution. The majority of multiple system evolution is confined to the decay of high-order systems and the formation of a significant population of very wide (10^4-10^5 AU) multiple systems in the expanding halo. This formation mechanism for wide binaries potentially solves the problem of how most stars apparently form in clusters and yet a substantial population of wide binaries exist in the field. Many of these wide binaries and the binaries produced by the decay of high-order multiple systems have unequal mass components, potentially solving the problem that hydrodynamical simulations of star formation are found to under-produce unequal-mass solar-type binaries. Comment: Accepted by MNRAS, 18 pages, 13 figures
    Monthly Notices of the Royal Astronomical Society 01/2010; · 5.52 Impact Factor
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    Nickolas Moeckel, Henry B. Throop
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    ABSTRACT: Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this `tail-end' accretion for planet formation was recently highlighted by Throop & Bally (2008). While accretion onto a point mass is successfully modeled by the classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of accretion onto a star-disk system defies analytic solution. In this paper we investigate via direct hydrodynamic simulations the accretion of dense interstellar material onto a star with an associated gaseous protoplanetary disk. We discuss the changes to the structure of the accretion flow caused by the disk, and vice versa. We find that immersion in a dense accretion flow can redistribute disk material such that outer disk migrates inwards, increasing the inner disk surface density and reducing the outer radius. The accretion flow also triggers the development of spiral density features, and changes to the disk inclination. The mean accretion rate onto the star remains roughly the same with and without the presence of a disk. We discuss the potential impact of this process on planet formation, including the possibility of triggered gravitational instability; inclination differences between the disk and the star; and the appearance of spiral structure in a gravitationally stable system. Comment: Accepted to ApJ. Version 2 replaces a mislabeled figure. Animations of the simulations and a version of the paper with slightly less-compressed images can be found at http://origins.colorado.edu/~moeckel/BHLpaper
    The Astrophysical Journal 10/2009; · 6.73 Impact Factor
  • Nickolas Moeckel, Ian A. Bonnell
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    ABSTRACT: The nearest site of massive star formation in Orion is dominated by the Trapezium subsystem, with its four OB stars and numerous companions. The question of how these stars came to be in such close proximity has implications for our understanding of massive star formation and early cluster evolution. A promising route towards rapid mass segregation was proposed by McMillan et al., who showed that the merger product of faster evolving subclusters can inherit their apparent dynamical age from their progenitors. In this paper, we briefly consider this process at a size and time-scale more suited for local and perhaps more typical star formation, with stellar numbers from hundreds to thousands. We find that for reasonable ages and cluster sizes, the merger of subclusters can indeed lead to compact configurations of the most massive stars, a signal seen both in nature and in large-scale hydrodynamic simulations of star formation from collapsing molecular clouds, and that subvirial initial conditions can make an unmerged cluster display a similar type of mass segregation. Additionally, we discuss a variation of the minimum spanning tree mass-segregation technique introduced by Allison et al.
    Monthly Notices of the Royal Astronomical Society 09/2009; 400(2):657 - 664. · 5.52 Impact Factor
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    Nickolas Moeckel, Ian A. Bonnell
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    ABSTRACT: The nearest site of massive star formation in Orion is dominated by the Trapezium subsystem, with its four OB stars and numerous companions. The question of how these stars came to be in such close proximity has implications for our understanding of massive star formation and early cluster evolution. A promising route toward rapid mass segregation was proposed by McMillan et al. (2007), who showed that the merger product of faster-evolving sub clusters can inherit their apparent dynamical age from their progenitors. In this paper we briefly consider this process at a size and time scale more suited for local and perhaps more typical star formation, with stellar numbers from the hundreds to thousands. We find that for reasonable ages and cluster sizes, the merger of sub-clusters can indeed lead to compact configurations of the most massive stars, a signal seen both in Nature and in large-scale hydrodynamic simulations of star formation from collapsing molecular clouds, and that sub-virial initial conditions can make an un-merged cluster display a similar type of mass segregation. Additionally, we discuss a variation of the minimum spanning tree mass-segregation technique introduced by Allison et al. (2009). Comment: 9 pages, submitted to MNRAS
    08/2009;
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    Nickolas Moeckel, Ian A. Bonnell
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    ABSTRACT: Mass segregation is observed in many star clusters, including several that are less than a few Myr old. Timescale arguments are frequently used to argue that these clusters must be displaying primordial segregation, because they are too young to be dynamically relaxed. Looking at this argument from the other side, the youth of these clusters and the limited time available to mix spatially distinct populations of stars can provide constraints on the amount of initial segregation that is consistent with current observations. We present n-body experiments testing this idea, and discuss the implications of our results for theories of star formation. For system ages less than a few crossing times, we show that star formation scenarios predicting general primordial mass segregation are inconsistent with observed segregation levels.
    Monthly Notices of the Royal Astronomical Society 04/2009; · 5.52 Impact Factor
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    Nathaniel J. Cunningham, Nickolas Moeckel, John Bally
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    ABSTRACT: We present near-infrared H2, radio CO, and thermal infrared observations of the nearby massive star-forming region Cepheus A (Cep A). From H2 bow shocks arranged along four distinct jet axes, we infer that the massive protostellar source HW2 drives a pulsed, precessing jet that has changed its orientation by about 45 degrees in roughly 104 years. The current HW2 radio jet represents the most recent event in this time series of eruptions. This scenario is consistent with the recent discovery of a disk around HW2, perpendicular to the current jet orientation, and with the presence of companions at projected distances comparable to the disk radius. We propose that the Cep A system formed by the disk-assisted capture of a sibling star by HW2. We present a numerical model of a 15 M_sun star with a circumstellar disk, orbited by a companion in an inclined, eccentric orbit. Close passages of the companion through or near the disk result in periods of enhanced accretion and mass loss, as well as forced precession of the disk and associated orientation changes in the jet. The observations reveal a second powerful outflow that emerges from radio source HW3c or HW3d. This flow is associated with blueshifted CO emission and a faint H2 bow shock to the east, and with HH 168 to the west. A collision between the flows from HW2 and HW3c/d may be responsible for X-ray and radio continuum emission in Cep A West.
    The Astrophysical Journal 02/2009; 692(2). · 6.73 Impact Factor
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    Nickolas Moeckel, Sean N. Raymond, Philip J. Armitage
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    ABSTRACT: Gravitational scattering between massive planets has been invoked to explain the eccentricity distribution of extrasolar planets. For scattering to occur, the planets must either form in -- or migrate into -- an unstable configuration. In either case, it is likely that a residual gas disk, with a mass comparable to that of the planets, will be present when scattering occurs. Using explicit hydrodynamic simulations, we study the impact of gas disks on the outcome of two-planet scattering. We assume a specific model in which the planets are driven toward instability by gravitational torques from an outer low mass disk. We find that the accretion of mass and angular momentum that occurs when a scattered planet impacts the disk can strongly influence the subsequent dynamics by reducing the number of close encounters. The eccentricity of the innermost surviving planet at the epoch when the system becomes Hill stable is not substantially altered from the gas-free case, but the outer planet is circularized by its interaction with the disk. The signature of scattering initiated by gas disk migration is thus a high fraction of low eccentricity planets at larger radii accompanying known eccentric planets. Subsequent secular evolution of the two planets in the presence of damping can further damp both eccentricities, and tends to push systems away from apsidal alignment and toward anti-alignment. We note that the late burst of accretion when the outer planet impacts the disk is in principle observable, probably via detection of a strong near-IR excess in systems with otherwise weak disk and stellar accretion signatures. Comment: 7 pages, 7 figures. Accepted to ApJ
    The Astrophysical Journal 07/2008; · 6.73 Impact Factor
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    Nickolas Moeckel, John Bally
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    ABSTRACT: The high multiplicity of massive stars in dense, young clusters is established early in their evolution. The mechanism behind this remains unresolved. Recent results suggest that massive protostars may capture companions through disk interactions with much higher efficiency than their solar mass counterparts. However, this conclusion is based on analytic determinations of capture rates and estimates of the robustness of the resulting binaries. We present the results of coupled n-body and SPH simulations of star-disk encounters to further test the idea that disk-captured binaries contribute to the observed multiplicity of massive stars. Comment: 4 pages, 3 figures, accepted to ApJL
    The Astrophysical Journal 04/2007; · 6.73 Impact Factor

Publication Stats

76 Citations
113.38 Total Impact Points

Institutions

  • 2009–2011
    • University of St Andrews
      • School of Physics and Astronomy
      Saint Andrews, SCT, United Kingdom
    • University of Exeter
      Exeter, England, United Kingdom
  • 2010
    • University of Cambridge
      • Institute of Astronomy
      Cambridge, England, United Kingdom
  • 2005–2008
    • University of Colorado at Boulder
      • Department of Astrophysical and Planetary Sciences
      Boulder, Colorado, United States