arXiv subjects

Simon P. Goodwin

Publications and source records attributed to Simon P. Goodwin.

At least 19 recordsLinked to original sources

arXiv2025

Can planet-planet binaries survive in star-forming regions?

Significant numbers of free-floating planetary-mass objects have been discovered in nearby star-forming regions by the James Webb Space Telescope, including a substantial number (42) of Jupiter Mass Binary Objects ('JuMBOs') in the Orion Nebula Cluster. The JuMBOs have much wider separations than other populations of substellar binaries, and their existence challenges conventional theories of substellar and planetary-mass object formation. Whilst several theories have been proposed to explain their formation, there has yet to be a study that determines whether they could survive the dynamical encounters prevalent within a dense star-forming region. We place a population of planet-planet binaries in N-body simulations of dense star-forming regions and calculate their binary fraction over time. We find that between 50-90 per cent of planet-planet binaries are destroyed on timescales of a few Myr, which implies that many more must form if we are to observe them in their current numbers. Furthermore, if the ONC was much more dense at formation, the initial separation distribution of the JuMBOs must have been even wider (and less similar to other substellar binaries) than the observed distribution.

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arXiv2024

Dynamical Evolution of Substructured Star Clusters at 3 kpc from the Galactic Center

We investigate the evolution of initial fractal clusters at 3 kpc from the Galactic Center (GC) of the Milky Way and show how red supergiant clusters (RSGCs)-like objects, which are considered to be the result of active star formation in the Scutum complex, can form by 16 Myr. We find that initial tidal filling and tidal over-filling fractals are shredded by the tidal force, but some substructures can survive as individual subclusters, especially when the initial virial ratio is $\leq$0.5.These surviving subclusters are weakly mass segregated and show a top-heavy mass function. This implies the possibility that a single substructured star cluster can evolve into multiple `star clusters'.

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arXiv2024

Deviations from the universal Initial Mass Function in binary star clusters

The stellar mass distribution in star-forming regions, stellar clusters and associations, the Initial Mass Function (IMF), appears to be invariant across different star-forming environments, and is consistent with the IMF observed in the Galactic field. Deviations from the field, or standard, IMF, if genuine, would be considered strong evidence for a different set of physics at play during the formation of stars in the birth region in question. We analyse N-body simulations of the evolution of spatially and kinematically substructured star-forming regions to identify the formation of binary star clusters, where two (sub)clusters which form from the same Giant Molecular Cloud orbit a common centre of mass. We then compare the mass distributions of stars in each of the subclusters and compare them to the standard IMF, which we use to draw the stellar masses in the star-forming region from which the binary cluster(s) form. In each binary cluster that forms, the mass distributions of stars in one subcluster deviates from the standard IMF, and drastically so when we apply similar mass resolution limits as for the observed binary clusters. Therefore, if a binary subcluster is observed to have an unusual IMF, this may simply be the result of dynamical evolution, rather than different physical conditions for star formation in these systems.

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arXiv2023

The RMS survey: a census of massive YSO multiplicity in the $K$-band

Close to 100 per cent of massive stars are thought to be in binary systems. The multiplicity of massive stars seems to be intrinsically linked to their formation and evolution, and Massive Young Stellar Objects are key in observing this early stage of star formation. We have surveyed three samples totalling hundreds of MYSOs ($>8M_\odot$) across the Galaxy from the RMS catalogue, using UKIDSS and VVV point source data, and UKIRT $K-$band imaging to probe separations between 0.8-9 arcsec (approx 1000-100,000 au). We have used statistical methods to determine the binary statistics of the samples, and we find binary fractions of $64\pm 4$ per cent for the UKIDSS sample, $53\pm 4$ per cent for the VVV sample, and $49\pm 8$ per cent for the RMS imaging sample. Also we use the $J-$ and $K-$band magnitudes as a proxy for the companion mass, and a significant fraction of the detected systems have estimated mass ratios greater than 0.5, suggesting a deviation from the capture formation scenario which would be aligned with random IMF sampling. Finally, we find that YSOs located in the outer Galaxy have a higher binary fraction than those in the inner Galaxy. This is likely due to a lower stellar background density than observed towards the inner Galaxy, resulting in higher probabilities for visual binaries to be physical companions. It does indicate a binary fraction in the probed separation range of close to 100 per cent without the need to consider selection biases.

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arXiv2023

Modelling uncertainties in wide binary constraints on primordial black holes

Dark matter in the form of compact objects with mass $M_{\rm co} \gtrsim 10 M_{\odot}$ can be constrained by its dynamical effects on wide binary stars. Motivated by the recent interest in Primordial Black Hole dark matter, we revisit the theoretical modelling involved in these constraints. We improve on previous studies in several ways. Specifically, we i) implement a physically motivated model for the initial wide-binary semi-major axis distribution, ii) include unbound binaries, and iii) take into account the uncertainty in the relationship between semi-major axis and observed angular separation. These effects all tend to increase the predicted number of wide binaries (for a given compact object population). Therefore the constraints on the halo fraction in compact objects, $f_{\rm co}$, are significantly weakened. For the wide binary sample used in the most recent calculation of the constraints, we find the fraction of halo dark matter in compact objects is $f_{\rm co} < 1$ for $M_{\rm co} \approx 300 \, M_{\odot}$, tightening with increasing $M_{\rm co}$ to $f_{\rm co} < 0.26$ for $M_{\rm co} \gtrsim 1000 \, M_{\odot}$.

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arXiv2023

On encounter rates in star clusters

Close encounters between stars in star forming regions are important as they can perturb or destroy protoplanetary discs, young planetary systems, and stellar multiple systems. We simulate simple, viralised, equal-mass $N$-body star clusters and find that both the rate and total number of encounters between stars varies by factors of several in statistically identical clusters due to the stochastic/chaotic details of orbits and stellar dynamics. Encounters tend to rapidly `saturate' in the core of a cluster, with stars there each having many encounters, while more distant stars have none. However, we find that the fraction of stars that have had at least one encounter within a particular distance grows in the same way (scaling with crossing time and half-mass radius) in all clusters, and we present a new (empirical) way of estimating the fraction of stars that have had at least one encounter at a particular distance.

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arXiv2022

Few Observation Binary Orbit Solver (FOBOS) from two (or more) astrometric observations

We have developed a new, fast method of estimating the orbital properties of a binary or triple system using as few as two epochs of astrometric data. FOBOS (Few Observation Binary Orbit Solver) uses a flat prior brute force Monte Carlo method to produce probability density functions of the likely orbital parameters. We test the code on fake observations and show that it can (fairly often) constrain the semi-major axis to within a factor of 2-3, and the inclination to within $\sim$20$^{\circ}$ from only two astrometric observations. We also show that the 68 and 95 per cent confidence intervals are statistically reliable. Applying this method to triple systems allows the relative inclination of the secondary and tertiary star orbits to be constrained. FOBOS can usually find a statistically significant number of possible matches in CPU minutes for binary systems, and CPU hours for triple systems.

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arXiv2021

External photoevaporation of protoplanetary discs: does location matter?

Many theoretical studies have shown that external photoevaporation from massive stars can severely truncate, or destroy altogether, the gaseous protoplanetary discs around young stars. In tandem, several observational studies report a correlation between the mass of a protoplanetary disc and its distance to massive ionising stars in star-forming regions, and cite external photoevaporation by the massive stars as the origin of this correlation. We present N-body simulations of the dynamical evolution of star-forming regions and determine the mass-loss in protoplanetary discs from external photoevaporation due to far ultraviolet (FUV) and extreme ultraviolet (EUV) radiation from massive stars. We find that projection effects can be significant, in that low-mass disc-hosting stars that appear close to the ionising sources may be fore- or background stars in the star-forming region. We find very little evidence in our simulations for a trend in increasing disc mass with increasing distance from the massive star(s), even when projection effects are ignored. Furthermore, the dynamical evolution of these young star-forming regions moves stars whose discs have been photoevaporated to far-flung locations, away from the ionising stars, and we suggest that any correlation between disc mass and distance the ionising star is either coincidental, or due to some process other than external photoevaporation.

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arXiv2020

The velocity structure of Cygnus OB2

The kinematic structure of the Cygnus OB2 association is investigated. No evidence of expansion or contraction is found at any scale within the region. Stars that are within $\sim$ 0.5 parsecs of one another are found to have more similar velocities than would be expected by random chance, and so it is concluded that velocity substructure exists on these scales. At larger scales velocity substructure is not found. We suggest that bound substructures exist on scales of $\sim$ 0.5 parsecs, despite the region as a whole being unbound. We further suggest that any velocity substructure that existed on scales > 0.5 parsecs has been erased. The results of this study are then compared to those of other kinematic studies of Cygnus OB2.

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arXiv2020

Making top-heavy IMFs from canonical IMFs near the Galactic Centre

We show that dynamical evolution in a strong (Galactic Centre-like) tidal field can create clusters that would appear to have very top-heavy IMFs. The tidal disruption of single star forming events can leave several bound `clusters' spread along 20 pc of the orbit within 1-2 Myr. These surviving (sub)clusters tend to contain an over-abundance of massive stars, with low-mass stars tending to be spread along the whole `tidal arm'. Therefore observing a cluster in a strong tidal field with a top-heavy IMF might well not mean the stars formed with a top-heavy IMF.

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arXiv2019

A pilot survey of the binarity of Massive Young Stellar Objects with $K$ band adaptive optics

We present the first search for binary companions of Massive Young Stellar Objects (MYSOs) using AO-assisted $K$ band observations, with NaCo at the VLT. We have surveyed 32 MYSOs from the RMS catalogue, probing the widest companions, with a physical separation range of 400 - 46,000 au, within the predictions of models and observations for multiplicity of MYSOs. Statistical methods are employed to discern whether these companions are physical rather than visual binaries. We find 18 physical companions around 10 target objects, amounting to a multiplicity fraction of 31$\pm$8\% and a companion fraction of 53$\pm$9\%. For similar separation and mass ratio ranges, MYSOs seem to have more companions than T Tauri or O stars, respectively. This suggests that multiplicity increases with mass and decreases with evolutionary stage. We compute very rough estimates for the mass ratios from the $K$ band magnitudes, and these appear to be generally larger than 0.5. This is inconsistent with randomly sampling the IMF, as predicted by the binary capture formation theory. Finally, we find that MYSOs with binaries do not show any different characteristics to the average MYSO in terms of luminosity, distance, outflow or disc presence.

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arXiv2018

The early dynamical evolution of star clusters near the Galactic Centre

We examine the dynamical evolution of both Plummer sphere and substructured (fractal) star forming regions in Galactic Centre (GC) strong tidal fields to see what initial conditions could give rise to an Arches-like massive star cluster by $\sim 2$ Myr. We find that any initial distribution has to be contained within its initial tidal radius to survive, which sets a lower limit of the initial density of the Arches of $\sim$ 600 M$_\odot$ pc$^{-3}$ if the Arches is at 30 pc from the GC, or $\sim$ 200 M$_\odot$ pc$^{-3}$ if the Arches is at 100 pc from the GC. Plummer spheres that survive change little other than to dynamically mass segregate, but initially fractal distributions rapidly erase substructure, dynamically mass segregate and by 2 Myr look extremely similar to initial Plummer spheres, therefore it is almost impossible to determine the initial conditions of clusters in strong tidal fields.

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arXiv2018

Massive, wide binaries as tracers of massive star formation

Massive 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 initially; and probably only one will survive if more than one are present initially. Therefore any region that contains many massive wide binaries must have been composed of many individual subregions. Observations of Cyg OB2 show that the massive wide binary fraction is at least a half (38/74) which suggests that Cyg OB2 had at least 30 distinct massive star formation sites. This is further evidence that Cyg OB2 has always been a large, low-density association. That Cyg OB2 has a normal high-mass IMF for its total mass suggests that however massive stars form they 'randomly sample' the IMF (as the massive stars did not 'know' about each other).

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arXiv2017

Hierarchical formation of Westerlund 1: a collapsing cluster with no primordial mass segregation?

We 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 surface density. While an expanding or bouncing-back scenario for the evolution of Westerlund 1 cannot be ruled out, we argue that the most natural model to explain these observations is one in which Westerlund 1 formed with no primordial mass segregation and at a similar or larger size than we now observe.

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arXiv2017

How do binary clusters form?

Approximately 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 binary clusters to form. This process is highly stochastic and it is not obvious from a region's initial conditions whether a binary will form and, if it does, which stars will end up in which cluster. We find the probability that a region will divide is mainly determined by its virial ratio, and a virial ratio above 'equilibrium' is generally necessary for binary formation. We also find that the mass ratio of the two clusters is strongly influenced by the initial degree of spatial substructure in the region.

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arXiv2016

Discs in misaligned binary systems

We 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 is considered as a rigid body interacting with the binary companion only gravitationally. Precession also causes change in alignment between the rotational axis of the disc and the spin axis of the primary star. This type of alignment is of great important for explaining the origin of spin-orbit misaligned planetary systems. However, we find that the rigid-disc approximation fails to describe changes in alignment between the disc and the binary orbit. This is because the alignment process is a consequence of interactions that involve the fluidity of the disc, such as the tidal interaction and the encounter interaction. Furthermore, simulation results show that there are not only alignment processes, which bring the components towards alignment, but also anti-alignment processes, which tend to misalign the components. The alignment process dominates in systems with misalignment angle near 90 degrees, while the anti-alignment process dominates in systems with the misalignment angle near 0 or 180 degrees. This means that highly misaligned systems will become more aligned but slightly misaligned systems will become more misaligned.

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arXiv2016

Mass segregation in star clusters is not energy equipartition

Mass 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 the centrally concentrated massive stars should have significantly lower velocities than fast-moving low-mass objects on the periphery of the cluster. We search for energy equipartition in initially spatially and kinematically substructured N-body simulations of star clusters with N = 1500 stars, evolved for 100 Myr. In clusters that show significant mass segregation we find no differences in the proper motions or radial velocities as a function of mass. The kinetic energies of all stars decrease as the clusters relax, but the kinetic energies of the most massive stars do not decrease faster than those of lower-mass stars. These results suggest that dynamical mass segregation -- which is observed in many star clusters -- is not a signature of energy equipartition from two-body relaxation.

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arXiv2015

Comparisons between different techniques for measuring mass segregation

We 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 $Λ_{\rm MSR}$ method; the local surface density $Σ_{\rm LDR}$ method; and the $Ω_{\rm GSR}$ technique, which isolates groups of stars and determines whether the most massive star in each group is more centrally concentrated than the average star. All four methods have been proposed in the literature as techniques for quantifying mass segregation, yet they routinely produce contradictory results as they do not all measure the same thing. We apply each method to synthetic star-forming regions to determine when and why they have shortcomings. When a star-forming region is smooth and centrally concentrated, all four methods correctly identify mass segregation when it is present. However, if the region is spatially substructured, the $Ω_{\rm GSR}$ method fails because it arbitrarily defines groups in the hierarchical distribution, and usually discards positional information for many of the most massive stars in the region. We also show that the $Λ_{\rm MSR}$ and $Σ_{\rm LDR}$ methods can sometimes produce apparently contradictory results, because they use different definitions of mass segregation. We conclude that only $Λ_{\rm MSR}$ measures mass segregation in the classical sense (without the need for defining the centre of the region), although $Σ_{\rm LDR}$ does place limits on the amount of previous dynamical evolution in a star-forming region.

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