Results for "Simon P. Goodwin"

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The ellipticities of Galactic and LMC globular clustersFeb 03 1997The globular clusters of the LMC are found to be significantly more elliptical than Galactic globular clusters, but very similar in virtually all other respects. The ellipticity of the LMC globular clusters is shown not be correlated with the age or mass ... More
Binaries in star clusters and the origin of the field stellar populationNov 04 2009Many, possibly most, stars form in binary and higher-order multiple systems. Therefore, the properties and frequency of binary systems provide strong clues to the star-formation process, and constraints on star-formation models. However, the majority ... More
The effect of the dynamical state of clusters on gas expulsion and infant mortalityFeb 15 2008Mar 13 2008The star formation efficiency (SFE) of a star cluster is thought to be the critical factor in determining if the cluster can survive for a significant (>50 Myr) time. There is an often quoted critical SFE of ~30 per cent for a cluster to survive gas expulsion. ... More
Residual gas expulsion from young globular clustersJul 17 1996The results of N-body simulations of the expulsion of residual gas from young globular clusters are presented. Globular clusters with a variety of initial masses, Galactocentric radii, concentration and initial mass function slope with star formation ... More
Gas expulsion and the destruction of massive young clustersSep 18 2006We examine the luminosity and dynamical mass estimates for young massive stellar clusters. For many young (<50 Myr) clusters, the luminosity and dynamical mass estimates differ by a significant amount. We explain this as being due to many young clusters ... More
Brown dwarf formation by binary disruptionMar 06 2007Context: The principal mechanism by which brown dwarfs form, and its relation to the formation of higher-mass (i.e. hydrogen-burning) stars, is poorly understood. Aims: We advocate a new model for the formation of brown dwarfs. Methods: In this model, ... More
Limits on the primordial stellar multiplicityMay 23 2005Most stars - especially young stars - are observed to be in multiple systems. Dynamical evolution is unable to pair stars efficiently, which leads to the conclusion that star-forming cores must usually fragment into \geq 2 stars. However, the dynamical ... More
The dynamical evolution of fractal star clusters: the survival of substructureOct 13 2003We simulate the dynamics of fractal star clusters, in order to investigate the evolution of substructure in recently formed clusters. The velocity dispersion is found to be the key parameter determining the survival of substructure. In clusters with a ... More
How do brown dwarves form?Jan 04 2005We review and evaluate four mechanisms for forming brown dwarves: (i) dynamical ejection of a stellar embryo from its placental prestellar core; (ii) opacity-limited fragmentation of a shock-compressed layer; (iii) gravitational instabilities in discs, ... More
The role of cluster evolution in disrupting planetary systems and disks: the Kozai mechanismMay 06 2009May 18 2009We examine the effects of dynamical evolution in clusters on planetary systems or protoplanetary disks orbiting the components of binary stars. In particular, we look for evidence that the companions of host stars of planetary systems or disks could have ... More
SNII enrichment and the star cluster mass functionOct 04 2004Changing the form of the star cluster mass function (CMF) can effectively change the upper end of the stellar initial mass function. The yields of from supernovae are very sensitive to the mass of the progenitor star. We show that by changing the parameters ... More
On mini-halo encounters with starsApr 06 2006Dec 11 2006We study, analytically and numerically, the energy input into dark matter mini-haloes by interactions with stars. We find that the fractional energy input in simulations of Plummer spheres agrees well with the impulse approximation for small and large ... More
Collisions in young triple systemsSep 20 2011We perform N-body simulations of young triple systems consisting of two low-mass companions orbiting around a significantly more massive primary. We find that, when the orbits of the companions are coplanar and not too widely separated, the chance of ... More
Comparisons between different techniques for measuring mass segregationMar 09 2015We examine the performance of four different methods which are used to measure mass segregation in star-forming regions: the radial variation of the mass function $\mathcal{M}_{\rm MF}$; the minimum spanning tree-based $\Lambda_{\rm MSR}$ method; the ... More
Star cluster evolution in the Magellanic Clouds revisitedAug 19 2008The evolution of star clusters in the Magellanic Clouds has been the subject of significant recent controversy, particularly regarding the importance and length of the earliest, largely mass-independent disruption phase (referred to as "infant mortality"). ... More
Star cluster "infant mortality" in the Small Magellanic Cloud (Redivivus)Sep 24 2007The early evolution of star clusters in the Small Magellanic Cloud (SMC) has been the subject of significant recent controversy, particularly regarding the importance and length of the earliest, largely mass-independent disruption phase (referred to as ... More
What does a universal IMF imply about star formation?Apr 29 2009We show that the same initial mass function (IMF) can result from very different modes of star formation from very similar underlying core and/or system mass functions. In particular, we show that the canonical IMF can be recovered from very similar system ... More
Do O-stars form in isolation?Jul 04 2007Around 4% of O-stars are observed in apparent isolation, with no associated cluster, and no indication of having been ejected from a nearby cluster. We define an isolated O-star as a star > 17.5 M_\odot in a cluster with total mass <100 M_\odot which ... More
The same, but different: Stochasticity in binary destructionApr 26 2012Observations of binaries in clusters tend to be of visual binaries with separations of 10s - 100s au. Such binaries are 'intermediates' and their destruction or survival depends on the exact details of their individual dynamical history. We investigate ... More
The dynamical evolution of very-low mass binaries in open clustersSep 16 2010Very low-mass binaries (VLMBs), with system masses <0.2 Msun appear to have very different properties to stellar binaries. This has led to the suggestion that VLMBs form a distinct and different population. As most stars are born in clusters, dynamical ... More
Why do starless cores appear more flattened than protostellar cores?Aug 07 2003We evaluate the intrinsic three dimensional shapes of molecular cores, by analysing their projected shapes. We use the recent catalogue of molecular line observations of Jijina et al. and model the data by the method originally devised for elliptical ... More
Simulating star formation in molecular cores II. The effects of different levels of turbulenceMay 06 2004(Abridged) We explore, by means of a large ensemble of SPH simulations, how the level of turbulence affects the collapse and fragmentation of a star-forming core. All our simulated cores have the same, except that we vary (a) the initial level of turbulence ... More
Simulating star formation in molecular cloud cores I. The influence of low levels of turbulence on fragmentation and multiplicitySep 30 2003We present the results of an ensemble of simulations of the collapse and fragmentation of dense star-forming cores. We show that even with very low levels of turbulence the outcome is usually a binary, or higher-order multiple, system. We take as the ... More
The Origin of the IMF from Core Mass FunctionsJul 06 2004We examine the initial mass functions (IMFs) of stars produced by different molecular core mass functions. Simulations suggest that more massive cores produce more stars, so we propose a model in which the average number of stars formed in a core is equal ... More
An explanation for the unusual IMF in TaurusMar 01 2004In comparison with other well studied star formation regions, Taurus is unusual in several respects. (i) Its stellar initial mass function (IMF) peaks at relatively high mass ($\sim 0.8 M_\odot$), but contains very few stars much more massive than $1 ... More
No compelling evidence of significant early star cluster disruption in the Large Magellanic CloudAug 15 2013Whether or not the rich star cluster population in the Large Magellanic Cloud (LMC) is affected by significant disruption during the first few x 10^8 yr of its evolution is an open question and the subject of significant current debate. Here, we revisit ... More
Massive, wide binaries as tracers of massive star formationFeb 13 2018Massive stars can be found in wide (hundreds to thousands AU) binaries with other massive stars. We use $N$-body simulations to show that any bound cluster should always have approximately one massive wide binary: one will probably form if none are present ... More
Discs in misaligned binary systemsMay 25 2016We perform SPH simulations to study precession and changes in alignment between the circumprimary disc and the binary orbit in misaligned binary systems. We find that the precession process can be described by the rigid-disc approximation, where the disc ... More
The evolution of binary populations in cool, clumpy star clustersAug 17 2011Observations and theory suggest that star clusters can form in a subvirial (cool) state and are highly substructured. Such initial conditions have been proposed to explain the level of mass segregation in clusters through dynamics, and have also been ... More
On the formation of trapezium-like systemsApr 11 2011We investigate the formation and evolution of high-order massive star multiples similar to the Trapezium in the Orion Nebula Cluster. We perform ensembles of N-body simulations of the evolution of N=1000 Orion-like clusters with initial conditions ranging ... More
What do dynamical cluster masses really tell us about dynamics?Oct 03 2008The diagnostic age versus mass-to-light ratio diagram is often used in attempts to constrain the shape of the stellar initial mass function, and the stability and the potential longevity of extragalactic young to intermediate-age massive star clusters. ... More
Translation for finite W-algebrasAug 19 2009A finite $W$-algebra $U(\g,e)$ is a certain finitely generated algebra that can be viewed as the enveloping algebra of the Slodowy slice to the adjoint orbit of a nilpotent element $e$ of a complex reductive Lie algebra $\g$. It is possible to give the ... More
Intermediate to low-mass stellar content of Westerlund 1Nov 10 2007We have analysed near-infrared NTT/SofI observations of the starburst cluster Westerlund 1, which is among the most massive young clusters in the Milky Way. A comparison of colour-magnitude diagrams with theoretical main-sequence and pre-main sequence ... More
The early dynamical evolution of cool, clumpy star clustersApr 29 2010Observations and theory both suggest that star clusters form sub-virial (cool) with highly sub-structured distributions. We perform a large ensemble of N-body simulations of moderate-sized (N=1000) cool, fractal clusters to investigate their early dynamical ... More
On conjugacy of unipotent elements in finite groups of Lie typeNov 19 2007Jul 11 2008Let $\bfG$ be a connected reductive algebraic group defined over $\F_q$, where $q$ is a power of a prime $p$ that is good for $\bfG$. Let $F$ be the Frobenius morphism associated with the $\FF_q$-structure on $\bfG$ and set $G = \bfG^F$, the fixed point ... More
On commuting varieties of parabolic subalgebrasJun 07 2016Aug 30 2016Let $G$ be a connected reductive algebraic group over an algebraically closed field $k$, and assume that the characteristic of $k$ is zero or a pretty good prime for $G$. Let $P$ be a parabolic subgroup of $G$ and let $\mathfrak p$ be the Lie algebra ... More
Calculating conjugacy classes in Sylow $p$-subgroups of finite Chevalley groupsJan 02 2008Sep 23 2008In earlier work, the first author outlined an algorithm for calculating a parametrization of the conjugacy classes in a Sylow $p$-subgroup $U(q)$ of a finite Chevalley group $G(q)$, valid when $q$ is a power of a good prime for $G(q)$. In this paper we ... More
Representation theory of type B and C standard Levi W-algebrasSep 13 2012We classify the finite dimensional irreducible representations with integral central character of finite $W$-algebras $U(\mathfrak g,e)$ associated to standard Levi nilpotent orbits in classical Lie algebras of types B and C. This classification is given ... More
On the variety of 1-dimensional representations of finite $W$-algebras in low rankAug 29 2017Let $\mathfrak g$ be a simple Lie algebra over $\mathbb C$ and let $e \in \mathfrak g$ be nilpotent. We consider the finite $W$-algebra $U(\mathfrak g,e)$ associated to $e$ and the problem of determining the variety $\mathcal E(\mathfrak g,e)$ of 1-dimensional ... More
Parabolic conjugacy in general linear groupsNov 25 2006Mar 21 2007Let q be a power of a prime and n a positive integer. Let P(q) be a parabolic subgroup of the finite general linear group GL(n,q). We show that the number of P(q)-conjugacy classes in GL(n,q) is, as a function of q, a polynomial in q with integer coefficients. ... More
Counting conjugacy classes in the unipotent radical of parabolic subgroups of $\GL_n(q)$Jan 06 2009Jun 16 2009Let $q$ be a power of a prime $p$. Let $P$ be a parabolic subgroup of the general linear group $\GL_n(q)$ that is the stabilizer of a flag in $\FF_q^n$ of length at most 5, and let $U = O_p(P)$. In this note we prove that, as a function of $q$, the number ... More
Richardson elements for parabolic subgroups of classical groups in positive characteristicAug 31 2006Let $G$ be a simple algebraic group of classical type over an algebraically closed field $k$. Let $P$ be a parabolic subgroup of $G$ and let $\p = \Lie P$ be the Lie algebra of $P$ with Levi decomposition $\p = {\l}\oplus \u$, where $\u$ is the Lie algebra ... More
Good grading polytopesOct 10 2005Let g be a finite dimensional semisimple Lie algebra over C and e be a nilpotent element. Elashvili and Kac have recently classified all good Z-gradings for e. We instead consider good R-gradings, which are naturally parameterized by an open convex polytope ... More
A natural formation scenario for misaligned and short-period eccentric extrasolar planetsJul 11 2011Jan 31 2012Recent discoveries of strongly misaligned transiting exoplanets pose a challenge to the established planet formation theory which assumes planetary systems to form and evolve in isolation. However, the fact that the majority of stars actually do form ... More
The relationship between the prestellar core mass function and the stellar initial mass functionNov 12 2007Stars form from dense molecular cores, and the mass function of these cores (the CMF) is often found to be similar to the form of the stellar initial mass function (IMF). This suggests that the form of the IMF is the result of the form of the CMF. However, ... More
Tidally induced brown dwarf and planet formation in circumstellar discsMay 17 2010Jun 17 2010Most stars are born in clusters and the resulting gravitational interactions between cluster members may significantly affect the evolution of circumstellar discs and therefore the formation of planets and brown dwarfs. Recent findings suggest that tidal ... More
Mass segregation in star clusters is not energy equipartitionApr 01 2016Mass segregation in star clusters is often thought to indicate the onset of energy equipartition, where the most massive stars impart kinetic energy to the lower-mass stars and brown dwarfs/free floating planets. The predicted net result of this is that ... More
Constraints on Massive Star Formation: Cygnus OB2 was always an AssociationNov 18 2013We examine substructure and mass segregation in the massive OB association Cygnus OB2 to better understand its initial conditions. Using a well understood Chandra X-ray selected sample of young stars we find that Cyg OB2 exhibits considerable physical ... More
Dynamical evolution of star forming regionsNov 14 2013We model the dynamical evolution of star forming regions with a wide range of initial properties. We follow the evolution of the regions' substructure using the Q-parameter, we search for dynamical mass segregation using the Lambda_MSR technique, and ... More
Testing the universality of star formation - II. Comparing separation distributions of nearby star-forming regions and the fieldSep 11 2012We have measured the multiplicity fractions and separation distributions of seven young star-forming regions using a uniform sample of young binaries. Both the multiplicity fractions and separation distributions are similar in the different regions. A ... More
Local structure correlations in plastic cyclohexane - a Reverse Monte Carlo studyJan 15 2013Two solid phases of cyclohexane have been investigated over a temperature range spanning 13 to 266 K on a powdered, perdeuterated sample using neutron total scattering. Phase II has an ordered structure (C2/c) that forms below 186 K. Between 186 and 280 ... More
The binary fraction of the young cluster NGC 1818 in the Large Magellanic CloudJan 18 2008Sep 27 2010We use high-resolution {\sl Hubble Space Telescope} imaging observations of the young ($\sim 15-25$ Myr-old) star cluster NGC 1818 in the Large Magellanic Cloud to derive an estimate for the binary fraction of F stars ($1.3 < M_\star/M_\odot < 1.6$). ... More
An Idealized Pulsar Magnetosphere: the Relativistic Force-Free ApproximationJul 12 2004The non-dissipative relativistic force-free condition should be a good approximation to describe the electromagnetic field in much of the pulsar magnetosphere, but we may plausibly expect it to break down in singular domains. Self-consistent magnetospheric ... More
The local to global $H_0$ ratio and the SNe Ia resultsJun 10 1999The effects of differences between the local and global values of the Hubble parameter on the cosmologies consistent with studies of high-redshift Type Ia supernov\ae are discussed. It is found that with a local Hubble parameter around 10 per cent higher ... More
Testing the universality of star formation - I. Multiplicity in nearby star-forming regionsJan 05 2012We have collated multiplicity data for five clusters (Taurus, Chamaeleon I, Ophiuchus, IC348, and the Orion Nebula Cluster). We have applied the same mass ratio (flux ratios of delta K <= 2.5) and primary mass cuts (~0.1-3.0 Msun) to each cluster and ... More
Do binaries in clusters form in the same way as in the field?May 13 2009We examine the dynamical destruction of binary systems in star clusters of different densities. We find that at high densities (10^4 - 10^5 Msun pc^-3) almost all binaries with separations > 10^3 AU are destroyed after a few crossing times. At low densities ... More
Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?Aug 14 2017We examine the level of substructure and mass segregation in the massive, young cluster Westerlund 1. We find that it is relatively smooth, with little or no mass segregation, but with the massive stars in regions of significantly higher than average ... More
How do binary clusters form?Jul 10 2017Approximately 10 per cent of star clusters are found in pairs, known as binary clusters. We propose a mechanism for binary cluster formation; we use N-body simulations to show that velocity substructure in a single (even fairly smooth) region can cause ... More
Using the minimum spanning tree to trace mass segregationJan 14 2009We present a new method to detect and quantify mass segregation in star clusters. It compares the minimum spanning tree (MST) of massive stars with that of random stars. If mass segregation is present, the MST length of the most massive stars will be ... More
On the coadjoint orbits of maximal unipotent subgroups of reductive groupsSep 16 2014Jan 26 2015Let G be a simple algebraic group defined over an algebraically closed field of characteristic 0 or a good prime for G. Let U be a maximal unipotent subgroup of G and \u its Lie algebra. We prove the separability of orbit maps and the connectedness of ... More
The orbit structure of Dynkin curvesAug 23 2006Jan 17 2007Let G be a simple algebraic group over an algebraically closed field k; assume that Char k is zero or good for G. Let \cB be the variety of Borel subgroups of G and let e in Lie G be nilpotent. There is a natural action of the centralizer C_G(e) of e ... More
Calculating conjugacy classes in Sylow p-subgroups of finite Chevalley groups of rank six and sevenFeb 27 2013Let G(q) be a finite Chevalley group, where q is a power of a good prime p, and let U(q) be a Sylow p-subgroup of G(q). Then a generalized version of a conjecture of Higman asserts that the number k(U(q)) of conjugacy classes in U(q) is given by a polynomial ... More
Non-integral representation theory of even multiplicity finite W-algebrasDec 29 2011We complete the classification of the finite dimensional irreducible representations of finite W-algebras associated to even multiplicity nilpotent elements in classical Lie algebras. This extends earlier work where this classification is determined for ... More
Highest weight theory for finite W-algebrasJan 09 2008We define analogues of Verma modules for finite W-algebras. By the usual ideas of highest weight theory, this is a first step towards the classification of finite dimensional irreducible modules. Motivated by known results in type A, we then formulate ... More
The generic character table of a Sylow $p$-subgroup of a finite Chevalley group of type $D_4$Aug 27 2015Let $U$ be a Sylow $p$-subgroup of the finite Chevalley group of type $D_4$ over the field of $q$ elements, where $q$ is a power of a prime $p$. We describe a construction of the generic character table of $U$.
On 1-dimensional representations of finite W-algebras associated to simple Lie algebras of exceptional typeMay 22 2009Mar 19 2010We consider the finite $W$-algebra $U(\g,e)$ associated to a nilpotent element $e \in \g$ in a simple complex Lie algebra $\g$ of exceptional type. Using presentations obtained through an algorithm based on the PBW-theorem, we verify a conjecture of Premet, ... More
Finite dimensional irreducible representations of finite W-algebras associated to even multiplicity nilpotent orbits in classical Lie algebrasSep 20 2010Oct 11 2010We consider finite W-algebras U(g,e) associated to even multiplicity nilpotent elements in classical Lie algebras. We give a classification of finite dimensional irreducible U(g,e)-modules with integral central character in terms of the highest weight ... More
Principal W-algebras for GL(m|n)May 04 2012Dec 05 2012We consider the (finite) $W$-algebra $W_{m|n}$ attached to the principal nilpotent orbit in the general linear Lie superalgebra $\mathfrak{gl}_{m|n}(\mathbb C)$. Our main result gives an explicit description of $W_{m|n}$ as a certain truncation of a shifted ... More
Rational points on generalized flag varieties and unipotent conjugacy in finite groups of Lie typeFeb 01 2006Feb 29 2008Let $G$ be a connected reductive algebraic group defined over the finite field $\FF_q$, where $q$ is a power of a good prime for $G$. We write $F$ for the Frobenius morphism of $G$ corresponding to the $\FF_q$-structure, so that $G^F$ is a finite group ... More
Conjugacy classes in Sylow p-subgroups of finite Chevalley groups in bad characteristicJan 06 2012Let $U = \mathbf U(q)$ be a Sylow $p$-subgroup of a finite Chevalley group $G = \mathbf G(q)$. In [GR}] R\"ohrle and the second author determined a parameterization of the conjugacy classes of $U$, for $\mathbf G$ of small rank when $q$ is a power of ... More
On changing highest weight theories for finite W-algebrasMay 17 2011A highest weight theory for a finite W-algebra U(g,e) was developed in [BGK]. This leads to a strategy for classifying the irreducible finite dimensional U(g,e)-modules. The highest weight theory depends on the choice of a parabolic subalgebra of g leading ... More
The modality of a Borel subgroup in a simple algebraic group of type $E_8$Feb 09 2018Let $G$ be a simple algebraic group over an algebraically closed field $k$, where $\mathrm{char}\, k$ is either 0 or a good prime for $G$. We consider the modality $\mathrm{mod}(B : \mathfrak u)$ of the action of a Borel subgroup $B$ of $G$ on the Lie ... More
Modular finite $W$-algebrasMay 17 2017Nov 03 2017Let $k$ be an algebraically closed field of characteristic $p > 0$ and let $G$ be a connected reductive algebraic group over $k$. Under some standard hypothesis on $G$, we give a direct approach to the finite $W$-algebra $U(\mathfrak g,e)$ associated ... More
Life and Death of Very Massive StarsDec 16 2010We recently determined the mass of the most massive star known to the date, R136a1 with a mass at birth 320 times the mass of our sun, as well as the mass of several other stars that are more massive than 150 M. Such massive stars (~150-300 M) may end ... More
On the mass segregation of stars and brown dwarfs in TaurusNov 24 2010We use the new minimum spanning tree (MST) method to look for mass segregation in the Taurus association. The method computes the ratio of MST lengths of any chosen subset of objects, including the most massive stars and brown dwarfs, to the MST lengths ... More
The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limitJul 19 2010Spectroscopic analyses of H-rich WN5-6 stars within the young star clusters NGC 3603 and R136 are presented, using archival HST & VLT spectroscopy, & high spatial resolution near-IR photometry. We derive high T* for the WN stars in NGC 3603 (T*~42+/-2 ... More
Dynamical Masses of Young Star Clusters: Constraints on the Stellar IMF and Star-Formation EfficiencyApr 21 2006Many recent works have attempted to constrain the stellar initial mass function (IMF) inside massive clusters by comparing their dynamical mass estimates to the measured light. These studies have come to different conclusions, with some claiming standard ... More
Evidence for the Strong Effect of Gas Removal on the Internal Dynamics of Young Stellar ClustersFeb 21 2006Mar 22 2006We present detailed luminosity profiles of the young massive clusters M82-F, NGC 1569-A, and NGC 1705-1 which show significant departures from equilibrium (King and EFF) profiles. We compare these profiles with those from N-body simulations of clusters ... More
Surviving infant mortality in the hierarchical merging scenarioFeb 25 2011We examine the effects of gas expulsion on initially sub-structured and out-of-equilibrium star clusters. We perform $N$-body simulations of the evolution of star clusters in a static background potential before removing that potential to model gas expulsion. ... More
Constructing characters of Sylow $p$-subgroups of finite Chevalley groupsDec 08 2015Aug 17 2016Let $q$ be a power of a prime $p$, let $G$ be a finite Chevalley group over $\mathbb{F}_q$ and let $U$ be a Sylow $p$-subgroup of $G$; we assume that $p$ is not a very bad prime for $G$. We explain a procedure of reduction of irreducible complex characters ... More
Multiplicity in Early Stellar EvolutionMar 08 2014Observations from optical to centimeter wavelengths have demonstrated that multiple systems of two or more bodies is the norm at all stellar evolutionary stages. Multiple systems are widely agreed to result from the collapse and fragmentation of cloud ... More
The formation of brown dwarfsDec 16 2005We review four mechanisms for forming brown dwarfs: (i) turbulent fragmentation (producing very low-mass prestellar cores); (ii) gravitational instabilities in discs; (iii) dynamical ejection of stellar embryos from their placental cores; and (iv) photo-erosion ... More
The distribution of ejected brown dwarfs in clustersJan 12 2006We examine the spatial distribution of brown dwarfs produced by the decay of small-N stellar systems as expected from the embryo ejection scenario. We model a cluster of several hundred stars grouped into 'cores' of a few stars/brown dwarfs. These cores ... More
Evolution of Prolate Molecular Clouds at HII Boundaries: I. Formation of fragment-core structuresJul 25 2014The evolution of a prolate cloud at an Hii boundary is investigated using Smoothed Particle Hydrodynamics (SPH). The prolate molecular clouds in our investigation are set with their semi-major axis perpendicular to the radiative direction of a plane parallel ... More
Infant mortality in the hierarchical merging scenario: Dependency on gas expulsion timescalesOct 02 2012We examine the effects of gas expulsion on initially sub-structured and out-of-equilibrium star clusters. We perform N-body simulations of the evolution of star clusters in a static background potential before adjusting that potential to model gas expulsion. ... More
On the Origin of Mass Segregation in NGC 3603Dec 19 2012We present deep Hubble Space Telescope/Wide Field and Planetary Camera 2 photometry of the young HD 97950 star cluster in the giant H {\sc ii} region NGC 3603. The data were obtained in 1997 and 2007 permitting us to derive membership based on proper ... More
What does the IMF really tell us about star formation?Oct 23 2009Obtaining accurate measurements of the initial mass function (IMF) is often considered to be the key to understanding star formation, and a universal IMF is often assumed to imply a universal star formation process. Here, we illustrate that different ... More
Binary mass ratios: system mass not primary massNov 26 2012Binary properties are usually expressed (for good observational reasons) as a function of primary mass. It has been found that the distribution of companion masses -- the mass ratio distribution -- is different for different primary masses. We argue that ... More
The initial conditions of young globular clusters in the LMCNov 12 1996N-body simulations are used to model the early evolution of globular clusters. These simulations include residual gas which was not turned into stars which is expelled from the globular cluster by the actions of massive stars. The results of these simulations ... More
A dynamical model for the dusty ring in the CoalsackFeb 03 2006Lada et al. recently presented a detailed near-infrared extinction map of Globule G2 in the Coalsack molecular cloud complex, showing that this starless core has a well-defined central extinction minimum. We propose a model for G2 in which a rapid increase ... More
Adaptive smoothing lengths in SPHJan 31 2007Context: There is a need to improve the fidelity of SPH simulations of self-gravitating gas dynamics. Aims: We remind users of SPH that, if smoothing lengths are adjusted so as to keep the number of neighbours, ${\cal N}$, in the range ${\cal N}_{_{\rm ... More
Star formation in molecular cores III. The effect of the turbulent power spectrumFeb 28 2006We investigate the effect of the turbulent power spectrum (P(k) \propto k^{-n}, with n=3, 4 or 5) on the fragmentation of low-mass cores, by means of SPH simulations. We adopt initial density profiles and low levels of turbulence based on observation, ... More
Resolution requirements for simulating gravitational fragmentation using SPHDec 09 2005Jeans showed analytically that, in an infinite uniform-density isothermal gas, plane-wave perturbations collapse to dense sheets if their wavelength, $\lambda$, satisfies $\lambda > \lambda_{_{\rm JEANS}} = (\pi a^2 / G \rho_{_0})^{1/2}$ (where $a$ is ... More
The Fragmentation of Cores and the Initial Binary PopulationMar 09 2006Almost all young stars are found in multiple systems. This suggests that protostellar cores almost always fragment into multiple objects. The observed properties of multiple systems such as their separation distribution and mass ratios provide strong ... More
The shape of the initial cluster mass function: what it tells us about the local star formation efficiencyFeb 11 2008We explore how the expulsion of gas from star-cluster forming cloud-cores due to supernova explosions affects the shape of the initial cluster mass function, that is, the mass function of star clusters when effects of gas expulsion are over. We demonstrate ... More
Observational characteristics of dense cores with deeply embedded young protostarsNov 03 2005Class 0 objects, which are thought to be the youngest protostars, are identified in terms of NIR or radio emission and/or the presence of molecular outflows. We present combined hydrodynamic and radiative transfer simulations of the collapse of a star-forming ... More
Formation rates of star clusters in the hierarchical merging scenarioMay 10 2011Stars form with a complex and highly structured distribution. For a smooth star cluster to form from these initial conditions, the star cluster must erase this substructure. We study how substructure is removed using N-body simulations that realistically ... More
The Milky Way is just an average spiralApr 22 1997It has been thought for many years that the Milky Way is an overly large spiral galaxy. Using Cephied distances to 17 spiral galaxies we calculate the true linear diameters of those galaxies. These diameters are then compared to that of the Milky Way ... More
The mass ratio and formation mechanisms of Herbig Ae/Be star binary systemsOct 09 2009Nov 18 2009We present B and R band spectroastrometry of a sample of 45 Herbig Ae/Be stars in order to study their binary properties. All but one of the targets known to be binary systems with a separation of ~0.1-2.0 arcsec are detected by a distinctive spectroastrometric ... More
Radiative transfer and the energy equation in SPH simulations of star formationMay 01 2007Aug 02 2007We introduce and test a new and highly efficient method for treating the thermal and radiative effects influencing the energy equation in SPH simulations of star formation. The method uses the density, temperature and gravitational potential of each particle ... More