arXiv subjects

N. W. C. Leigh

Publications and source records attributed to N. W. C. Leigh.

12 recordsLinked to original sources

arXiv2024

New candidate hypervelocity red clump stars in the inner Galactic bulge

We search for high-velocity stars in the inner region of the Galactic bulge using a selected sample of red clump stars. Some of those stars might be considered hypervelocity stars (HVSs). Even though the HVSs ejection relies on an interaction with the supermassive black hole (SMBH) at the centre of the Galaxy, there are no confirmed detections of HVSs in the inner region of our Galaxy. With the detection of HVSs, ejection mechanism models can be constrained by exploring the stellar dynamics in the Galactic centre through a recent stellar interaction with the SMBH. Based on a previously developed methodology by our group, we searched with a sample of preliminary data from version 2 of the Vista Variables in the Via Lactea (VVV) Infrared Astrometric Catalogue (VIRAC2) and Gaia DR3 data, including accurate optical and NIR proper motions. This search resulted in a sample of 46 stars with transverse velocities larger than the local escape velocity within the Galactic bulge, of which 4 are prime candidate HVSs with high-proper motions consistent with being ejections from the Galactic centre. Adding to that, we studied a sample of reddened stars without a Gaia DR3 counterpart and found 481 stars with transverse velocities larger than the local escape velocity, from which 65 stars have proper motions pointing out of the Galactic centre and are candidate HVSs. In total, we found 69 candidate HVSs pointing away from the Galactic centre with transverse velocities larger than the local escape velocity.

astro-ph.GA↗

arXiv2022

The thermodynamics of stellar multiplicity: dynamical evolution of binary star populations in dense stellar environments

We recently derived, using the density-of-states approximation, analytic distribution functions for the outcomes of direct single-binary scatterings (Stone & Leigh 2019). Using these outcome distribution functions, we present in this paper a self-consistent statistical mechanics-based analytic model obtained using the Fokker-Planck limit of the Boltzmann equation. Our model quantifies the dominant gravitational physics, combining both strong and weak single-binary interactions, that drives the time evolution of binary orbital parameter distributions in dense stellar environments. We focus in particular the distributions of binary orbital energies and eccentricities. We find a novel steady state distribution of binary eccentricities, featuring strong depletions of both the highest and the lowest eccentricity binaries. In energy space, we compare the predictions of our analytic model to the results of numerical N-body simulations, and find that the agreement is good for the initial conditions considered here. This work is a first step toward the development of a fully self-consistent semi-analytic model for dynamically evolving binary star populations in dense stellar environments due to direct few-body interactions.

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arXiv2022

AGN as Potential Factories for Eccentric Black Hole Mergers

There is some weak evidence that the black hole merger named GW190521 had a non-zero eccentricity. In addition, the component black holes' masses exceeded the limit predicted by stellar evolution. The large masses can be explained by successive mergers, which may be efficient in gas disks surrounding active galactic nuclei (AGN), but it is difficult to maintain an eccentric orbit all the way to the merger, as basic physics would argue for circularization. Here we show that AGN-disk environments can lead to an excess of eccentric mergers, if the interactions between single and binary black holes are frequent, and occur with mutual inclinations of less than a few degrees. We further illustrate that this eccentric population has a different distribution of the inclination between the spin vectors of the black holes and their orbital angular momentum at merger, referred to as the spin-orbit tilt, compared to the remaining circular mergers.

astro-ph.HE↗

arXiv2021

Starfall: A heavy rain of stars in 'turning on' AGN

As active galactic nuclei (AGN) `turn on', some stars end up embedded in accretion disks around supermassive black holes (SMBHs) on retrograde orbits. Such stars experience strong headwinds, aerodynamic drag, ablation and orbital evolution on short timescales. Loss of orbital angular momentum in the first $\sim 0.1$~Myr of an AGN leads to a heavy rain of stars (`starfall') into the inner disk and onto the SMBH. A large AGN loss cone ($θ_{\rm AGN,lc}$) can result from binary scatterings in the inner disk and yield tidal disruption events (TDEs). Signatures of starfall include optical/UV flares that rise in luminosity over time, particularly in the inner disk. If the SMBH mass is $M_{\rm SMBH} \ge 10^{8}M_{\odot}$, flares truncate abruptly and the star is swallowed. If $M_{\rm SMBH}<10^{8}M_{\odot}$, and if the infalling orbit lies within $θ_{\rm AGN,lc}$, the flare is followed by a TDE which can be prograde or retrograde relative to the AGN inner disk. Retrograde AGN TDEs are over-luminous and short-lived as in-plane ejecta collide with the inner disk and a lower AGN state follows. Prograde AGN TDEs add angular momentum to inner disk gas and so start off looking like regular TDEs but are followed by an AGN high state. Searches for such flare signatures test models of AGN `turn on', SMBH mass, as well as disk properties and the embedded population.

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arXiv2021

Stellar collisions in flattened and rotating Pop. III star clusters

Fragmentation often occurs in disk-like structures, both in the early Universe and in the context of present-day star formation. Supermassive black holes (SMBHs) are astrophysical objects whose origin is not well understood; they weigh millions of solar masses and reside in the centers of galaxies. An important formation scenario for SMBHs is based on collisions and mergers of stars in a massive cluster, in which the most massive star moves to the center of the cluster due to dynamical friction. This increases the rate of collisions and mergers since massive stars have larger collisional cross sections. This can lead to runaway growth of a very massive star which may collapse to become an intermediate-mass black hole. Here we investigate the dynamical evolution of Miyamoto-Nagai models that allow us to describe dense stellar clusters, including flattening and different degrees of rotation. We find that the collisions in these clusters depend mostly on the number of stars and the initial stellar radii for a given radial size of the cluster. By comparison, rotation seems to affect the collision rate by at most $20\%$. For flatness, we compared spherical models with systems that have a scale height of about $10\%$ of their radial extent, in this case finding a change in the collision rate of less than $25\%$. Overall, we conclude that the parameters only have a minor effect on the number of collisions. Our results also suggest that rotation helps to retain more stars in the system, reducing the number of escapers by a factor of $2-3$ depending on the model and the specific realization. After two million years, a typical lifetime of a very massive star, we find that about $630$ collisions occur in typical models with $N=10^4$, $R=100$ $\rm~R_\odot$ and a half-mass radius of $0.1$ $\rm~pc$, leading to a mass of about $6.3\times10^3$ $\rm~M_\odot$ for the most massive object.

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arXiv2020

A phylogenetic analysis of galaxies in the Coma Cluster and the field: a new approach to galaxy evolution

We propose a phylogenetic approach (PA) as a novel and robust tool to detect galaxy populations (GPs) based on their chemical composition. The branches of the tree are interpreted as different GPs and the length between nodes as the internal chemical variation along a branch. We apply the PA using 30 abundance indices from the Sloan Digital Sky Survey to 475 galaxies in the Coma Cluster and 438 galaxies in the field. We find that a dense environment, such as Coma, shows several GPs, which indicates that the environment is promoting galaxy evolution. Each population shares common properties that can be identified in colour magnitude space, in addition to minor structures inside the red sequence. The field is more homogeneous, presenting one main GP. We also apply a principal component analysis (PCA) to both samples, and find that the PCA does not have the same power in identifying GPs.

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arXiv2020

Mergers of Equal-Mass Binaries with Compact Object Companions from Mass Transfer in Triple Star Systems

In this paper, we consider triple systems composed of main-sequence (MS) stars, and their internal evolution due to stellar and binary evolution. Our focus is on triples that produce white dwarfs (WDs), where Roche lobe overflow of an evolving tertiary triggers accretion onto the inner binary via a circumbinary disk (CBD) driving it toward a mass ratio of unity. We present a combination of analytic- and population synthesis-based calculations performed using the \texttt{SeBa} code to constrain the expected frequency of such systems, given a realistic initial population of MS triples, and provide the predicted distributions of orbital periods. We identify the parameter space for triples that can accommodate a CBD, to inform future numerical simulations of suitable initial conditions. We find that $\lesssim$ 10\% of all MS triples should be able to accommodate a CBD around the inner binary, and compute lower limits for the production rates. This scenario broadly predicts mergers of near equal-mass binaries, producing blue stragglers (BSs), Type Ia supernovae, gamma ray bursts and gravitational wave-induced mergers, along with the presence of an outer WD tertiary companion. We compare our predicted distributions to a sample of field BS binaries, and argue that our proposed mechanism explains the observed range of orbital periods. Finally, the mechanism considered here could produce hypervelocity MS stars, WDs and even millisecond pulsars with masses close to the Chandrasekhar mass limit, and be used to constrain the maximum remnant masses at the time of any supernova explosion.

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arXiv2020

The effects of a background potential in star cluster evolution: a delay in the relaxation time-scale and runaway collision processes

Runaway stellar collisions in dense star clusters are invoked to explain the presence of very massive stars or blue stragglers in the center of those systems. This process has also been explored for the first star clusters in the Universe and shown to yield stars that may collapse at some points into an intermediate mass black hole. Although the early evolution of star clusters requires the explicit modeling of the gas out of which the stars form, these calculations would be extremely time-consuming and often the effects of the gas can be accurately treated by including a background potential to account for the extra gravitational force. We apply this approximation to model the early evolution of the first dense star clusters formed in the Universe by performing $N$-body simulations, our goal is to understand how the additional gravitational force affects the growth of a very massive star through stellar mergers in the central parts of the star cluster. Our results show that the background potential increases the velocities of the stars, causing an overall delay in the evolution of the clusters and in the runaway growth of a massive star at the center. The population of binary stars is lower due to the increased kinetic energy of the stars, initially reducing the number of stellar collisions, and we show that relaxation processes are also affected. Despite these effects, the external potential enhances the mass of the merger product by a factor $\sim$2 if the collisions are maintained for long times.

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arXiv2020

The X-ray Emissivity of Low-Density Stellar Populations

The dynamical production of low-mass X-ray binaries and brighter cataclysmic variables (CVs) in dense globular clusters is well-established. We investigate how the X-ray emissivity of fainter X-ray binaries (principally CVs and coronally active binaries) varies between different environments. We compile calculations (largely from the literature) of the X-ray emissivity of old stellar populations, including open and globular clusters and several galaxies. We investigate three literature claims of unusual X-ray sources in low-density stellar populations. We show that a suggested quiescent neutron star in the open cluster NGC 6819 is a foreground M dwarf. We show that the suggested diffuse X-ray emission from an old nova shell in the globular cluster NGC 6366 is actually a background galaxy cluster. And we show that a suggested population of quiescent X-ray binaries in the Sculptor Dwarf Galaxy is mostly (perhaps entirely) background galaxies. We find that above densities of $10^4$ M$_{\odot}$/pc$^3$, the X-ray emissivity of globular clusters increases, due to dynamical production of X-ray emitting systems. Below this density, globular clusters have lower X-ray emissivity than the other populations, and we do not see a strong dependence of X-ray emissivity due to density effects. We find significant correlations between X-ray emissivity and binary fraction, metallicity, and density. Sampling these fits via bootstrap techniques gives less significant correlations, but confirms the effect of metallicity on low-density populations, and that of density on the full globular cluster sample.

astro-ph.HE↗

arXiv2018

Constraints on blue straggler formation mechanisms in Galactic globular clusters from proper motion velocity distributions

For a sample of 38 Galactic globular clusters (GCs), we confront the observed distributions of blue straggler (BS) proper motions and masses (derived from isochrone fitting) from the BS catalog of Simunovic & Puzia with theoretical predictions for each of the two main competing BS formation mechanisms. These are mass transfer from an evolved donor on to a main-sequence (MS) star in a close binary system, and direct collisions involving MS stars during binary encounters. We use the \texttt{FEWBODY} code to perform simulations of single-binary and binary-binary interactions. This provides collisional velocity and mass distributions for comparison to the observed distributions. Most clusters are consistent with BSs derived from a dynamically relaxed population, supportive of the binary mass-transfer scenario. In a few clusters, including all the post-core collapse clusters in our sample, the collisional velocities provide the best fit.

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arXiv2018

On stellar-mass black hole mergers in AGN disks detectable with LIGO

Black hole mergers detectable with LIGO can occur in active galactic nucleus (AGN) disks. Here we parameterize the merger rates, the mass spectrum and the spin spectrum of black holes (BH) in AGN disks. The predicted merger rate spans $\sim 10^{-4}-10^{4} \rm{Gpc}^{-1} \rm{yr}^{-1}$, so upper limits from LIGO ($<212\rm{Gpc}^{-1}\rm{yr}^{-1}$) already constrain it. The predicted mass spectrum has the form of a broken power-law consisting of a pre-existing BH powerlaw mass spectrum and a harder powerlaw mass spectrum resulting from mergers. The predicted spin spectrum is multi-peaked with the evolution of retrograde spin BH in the gas disk playing a key role. We outline the large uncertainties in each of these LIGO observables for this channel and we discuss ways in which they can be constrained in the future.

astro-ph.HE↗

arXiv2015

Nuclear star cluster formation in energy-space

In a virialized stellar system, the mean-square velocity is a direct tracer of the energy per unit mass of the system. Here, we exploit this to estimate and compare root-mean-square velocities for a large sample of nuclear star clusters and their host (late- or early-type) galaxies. Traditional observables, such as the radial surface brightness and second-order velocity moment profiles, are subject to short-term variations due to individual episodes of matter infall and/or star formation. The total mass, energy and angular momentum, on the other hand, are approximately conserved. Thus, the total energy and angular momentum more directly probe the formation of galaxies and their nuclear star clusters, by offering access to more fundamental properties of the nuclear cluster-galaxy system than traditional observables. We find that there is a strong correlation, in fact a near equality, between the root-mean-square velocity of a nuclear star cluster and that of its host. Thus, the energy per unit mass of a nuclear star cluster is always comparable to that of its host galaxy. We interpret this as evidence that nuclear star clusters do not form independently of their host galaxies, but rather that their formation and subsequent evolution are coupled. We discuss how our results can potentially be used to offer a clear and observationally testable prediction to distinguish between the different nuclear star cluster formation scenarios, and/or quantify their relative contributions.

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