arXiv subjects

Alison Sills

Publications and source records attributed to Alison Sills.

At least 19 recordsLinked to original sources

arXiv2026

Dynamical Cluster Assembly Framework (D-CAF): The Link Between Star Cluster Formation and Expansion Rates

We introduce the Dynamical Cluster Assembly Framework (D-CAF), an AMUSE-based framework designed to connect embedded star formation histories to the dynamical evolution of young stellar systems. We model star formation through the gradual formation of stars inside an evolving background potential, where the global gas evolution is extracted from realistic magneto-hydrodynamical (MHD) simulations. In this first work, we focus on the global evolution of the natal gas and its dynamical imprint on the stellar population. Across all explored MHD setups, we find that the gas continues to collapse while stars are forming, increasing both the central concentration and velocity scale of the embedded stellar population before gas expulsion. Using a controlled grid of direct $N$-body simulations, we show that this embedded evolution strongly regulates both the survival and later expansion of young stellar systems. In particular, gas contraction shortens the stellar crossing time prior to gas expulsion, making the same gas-removal timescale effectively more adiabatic for the stars. We find that the present-day expansion of stellar associations still preserves information about the embedded dynamical state reached during formation. The expansion rate is limited by the velocity scale reached before gas expulsion, while the efficiency with which this velocity field is transformed into expansion depends on the gas-expulsion timescale. Finally, we show that some commonly used expansion diagnostics can directly trace the physical expansion rate of young stellar systems when full kinematic information is available, opening the possibility of using stellar kinematics to constrain the dynamical conditions of embedded star formation.

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arXiv2026

Simulating Star Formation and Star Cluster Assembly in the Aquila Rift Using Archival Observations

We simulate star formation and star cluster assembly inside a molecular cloud with parameters we derive directly from observations of the Aquila Rift. We model the evolution of stars and gas together while resolving close encounters between stars, the formation of new stars, and stellar feedback to follow cluster formation up to the expulsion of the surrounding gas. We find that star formation takes place in clumps spaced unevenly along Serpens South and that these clumps accrete surrounding gas to grow and form new stars. Gas flows along the filament promote the merger of these clumps into a star cluster inside the Serpens South filament. The imprints of these mergers are seen in the dynamics of the Serpens South cluster in the form of velocity space anisotropies, cluster rotation, and cluster expansion. Before gas is removed from the simulation, the Serpens South cluster merges with the nearby cluster W40 non-monolithically resulting in a fractal cluster at the end of the simulation. The dynamics inherited from the mergers throughout the simulation are still seen in the final bound stellar system after the gas has been removed. We compare these results with recent observations of Milky Way clusters to comment on their formation histories. We also study how our results change when lowering the mass resolution of our simulation and removing observations of dense gas tracers from our initial condition setup. Each of the three simulations result in different final cluster configurations pointing towards the importance of gas in cluster assembly.

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arXiv2026

Revisiting the Evidence for Double Sequences of Blue Straggler Stars in Globular Clusters

Blue straggler stars (BSSs) are believed to form through mass transfer in binary systems or stellar collisions. The reported presence of double BSS sequences in some globular clusters (GCs) has been interpreted as evidence that these two formation channels produce distinct sequences in color-magnitude diagram (CMD). We reassess this claim using HST UV Globular Cluster Survey (HUGS) photometry of 56 Galactic GCs. We used the Hartigan Dip Test to test bimodality, and Akaike model comparison to test whether BSS distance distributions are better described by a mixture of two unskewed Gaussians or a skewed unimodal Gaussian model. We find no strong statistical evidence for bimodality; no cluster yields a dip test p-value below 0.15, and Akaike model comparison favors the skewed unimodal model in 94 out of 112 cases. We re-examine NGC 7099 (M30), the prototypical case of a double BSS sequence, using three reductions of HST data. We find bimodality is detected at p = 4 x 10-3 , vs. the originally reported p ~ 10-5 , in the original photometry. The observed uncertainties derived from the subgiant branch widths are comparable to the suggested separation between the proposed BSS sequences, making the detection of statistically significant bimodality challenging. Our results suggest that the dip between two BSS sequences in M30 photometry is a coincidence, and that later bifurcation claims can be explained as skew in the BSS color distribution, rather than two separate distributions.

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arXiv2026

Evolution of fractality in centrally concentrated young clusters

We investigate the structural evolution of young star clusters forming within centrally concentrated molecular clouds. Our simulations use the Torch framework, which integrates the FLASH magnetohydrodynamics code with the AMUSE environment, enabling a self-consistent treatment of gas dynamics, star formation, stellar evolution, radiative transfer, and gravitational interactions. We quantify cluster structure using the $Q$ parameter for fractality and compute fractal dimensions via two methods: box-counting and correlation dimension. Our results show that clusters generally inherit fractal substructure from their parental clouds, which is typically erased within $\sim 2.5\,t_\mathrm{ff}$ through dynamical relaxation. Massive stars can induce the formation of secondary subclusters via feedback, with outcomes strongly dependent on stellar mass and formation timing. Interactions among subclusters, including mergers and dispersal, can extend fractal structure beyond $4\,t_\mathrm{ff}$. We also find systematic correlations between the fractality parameter $Q$ and the fractal dimension: fractality is positively correlated with both the correlation and box-counting dimensions, with the correlation dimension exhibiting a stronger correlation. These results demonstrate how stellar feedback and internal dynamics jointly shape the measurable fractal properties of embedded star clusters.

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arXiv2025

The collision and merger products of stars do not look alike: A magnetohydrodynamics comparison

A significant fraction of stars experience close interactions, including collisions resulting from gravitational encounters and mergers within close binary systems. These processes can produce more massive stars that may give rise to relatively rare objects such as blue stragglers. Distinguishing the outcomes of collisions and mergers is challenging yet essential for interpreting observations. This study utilizes the magnetohydrodynamics code AREPO to simulate collisions and mergers of $5$ to $10 \,\mathrm{M}_{\odot}$ main-sequence stars, systematically comparing the properties of the resulting products. Both collisions and mergers yield more massive, strongly magnetized, rapidly and differentially rotating stars with cores enriched in hydrogen, but notable quantitative differences emerge. Merger products exhibit core hydrogen fractions up to $10\%$ higher than those of collision products. In both scenarios, turbulent mixing amplifies magnetic field energies by $9$ to $12$ orders of magnitude. However, magnetic fields in small-impact-parameter collision products display small-scale reversals that may dissipate over time, whereas merger products and large-impact-parameter collision products develop large-scale ordered, potentially long-lived magnetic fields. Additionally, only merger products display magnetically driven, bipolar outflows with radial velocities exceeding $300$ to $400 \,\mathrm{km}\,\mathrm{s}^{-1}$. These distinctions may result in different long-term evolutionary outcomes, which warrant further investigation in future studies.

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arXiv2025

The Blue Lurker WOCS 14020: A Long-Period Post-Common-Envelope Binary in M67 Originating from a Merger in a Triple System

We present Hubble Space Telescope far-ultraviolet (FUV) spectra of a blue-lurker$-$white-dwarf (BL-WD) binary system in the 4 Gyr open cluster M67. We fit the FUV spectrum of the WD, determining it is a C/O WD with a mass of $0.72^{+0.05}_{-0.04}$ M$_\odot$ and a cooling age of $\sim400$ Myr. This requires a WD progenitor of $\sim3$ M$_\odot$, significantly larger than the current cluster turnoff mass of 1.3 M$_\odot$. We suggest the WD progenitor star formed several hundred Myr ago via the merger of two stars near the turnoff of the cluster. In this scenario, the original progenitor system was a hierarchical triple consisting of a close, near-equal-mass inner binary, with a tertiary companion with an orbit of a few thousand days. The WD is descended from the merged inner binary, and the original tertiary is now the observed BL. The likely formation scenario involves a common envelope while the WD progenitor is on the AGB, and thus the observed orbital period of 359 days requires an efficient common envelope ejection. The rapid rotation of the BL indicates it accreted some material during its evolution, perhaps via a wind prior to the common envelope. This system will likely undergo a second common envelope in the future, and thus could result in a short-period double WD binary or merger of a 0.72 M$_\odot$ C/O WD and a 0.38 $M_\odot$ Helium WD, making this a potential progenitor of an interesting transient such as a sub-Chandrasekhar Type Ia supernova.

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arXiv2025

Massive Interacting Binaries Enhance Feedback in Star-Forming Regions

We present a new framework to incorporate feedback from massive interacting binaries in simulations of star cluster formation. Our new feedback model adds binary stellar evolution to the cluster formation code Torch, and couples it in AMUSE to the pre-existing modules for collisional stellar dynamics, magnetohydrodynamics, and mechanical and radiative feedback. Our model accounts for the effects of mass transfer on the stars' mass loss rates, their radiation spectra, and the timing of core-collapse supernovae. It also injects mass lost through non-conservative mass transfer and common envelope ejection into the interstellar medium. We demonstrate the use of our feedback model through simulations of isolated binaries in a gaseous medium, and of embedded clusters of massive binaries. Feedback from interacting binaries efficiently couples with the surrounding interstellar medium. It increases the size of HII regions, increases the kinetic and thermal energy of the gas, and increases the pressure within HII regions compared to models that use single star stellar evolution. Those differences arise from the ionizing radiation, which increases by three orders of magnitude, resulting in HII regions that expand due to thermal pressure rather than radiation pressure. The effects of stellar dynamics and the gravitational potential of the background gas cause the evolution of individual binaries to deviate from the predictions made by secular evolution, impacting the subsequent feedback from the binary. We conclude that massive interacting binaries are an important source of feedback in cluster-forming regions, and must be considered when studying the emerging timescales of young star clusters.

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arXiv2025

The Evolution of Substructure during Star Cluster Assembly

Star cluster formation and assembly occurs inside filamentary and turbulent molecular clouds, which imprints both spatial and kinematic substructure on the young cluster. In this paper, we quantify the amount and evolution of this substructure in simulations of star cluster formation that include radiation magnetohydrodynamical evolution of the gas, coupled with detailed stellar dynamics, binary formation and evolution, and stellar feedback. We find that both spatial and kinematic substructure are present at early times. Both are erased as the cluster assembles through the formation of new stars as well as the merger of sub-clusters. Spatial substructure is erased over a timescale of approximately 2.5 times the initial free-fall time of the cloud. Kinematic substructure persists for longer, and is still present to the end of our simulations. We also explored our simulations for evidence of early dynamical mass segregation, and conclude that the presence of a population of binary stars can accelerate and enhance the mass segregation process.

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arXiv2025

Dynamics of Star Cluster Formation: The Effects of Ongoing Star Formation and Stellar Feedback

We perform a high resolution zoom-in simulation of star cluster assembly including the merger of two sub-clusters with initial conditions taken from previous large scale giant molecular cloud (GMC) simulations. We couple hydrodynamics to N-body dynamics to simulate the individual stars themselves, and the gas-rich environment in which they evolve. We include prescriptions for star formation and stellar feedback and compare directly to previous simulations of the same region without these prescriptions to determine their role in shaping the dynamics inherited from the cluster assembly process. The stellar mass of the cluster grows through star formation within the cluster and accretion of new stars and star forming gas from a nearby filament. This growth results in an enhancement in the cluster's rotation and anisotropic expansion compared to simulations without star formation. We also analyze the internal kinematics of the cluster once it has lost most of its gas and find that the rotational velocity and the velocity anisotropy profiles are qualitatively similar to those expected of clusters that have undergone violent relaxation. As well, rotation and anisotropic expansion are still present by the time of gas removal. This implies that evolution within the GMC was unable to completely erase the kinematics inherited by the merger.

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arXiv2024

Massive Star Cluster Formation with Binaries. I. Evolution of Binary Populations

We study the evolution of populations of binary stars within massive cluster-forming regions. We simulate the formation of young massive star clusters within giant molecular clouds with masses ranging from 2 x 10$^{4}$ to 3.2 x 10$^{5}$ M$_{\odot}$. We use Torch, which couples stellar dynamics, magnetohydrodynamics, star and binary formation, stellar evolution, and stellar feedback through the AMUSE framework. We find that the binary fraction decreases during cluster formation at all molecular cloud masses. The binaries' orbital properties also change, with stronger and quicker changes in denser, more massive clouds. Most of the changes we see can be attributed to the disruption of binaries wider than 100 au, although the close binary fraction also decreases in the densest cluster-forming region. The binary fraction for O stars remains above 90%, but exchanges and dynamical hardening are ubiquitous, indicating that O stars undergo frequent few-body interactions early during the cluster formation process. Changes to the populations of binaries are a by-product of hierarchical cluster assembly: most changes to the binary population take place when the star formation rate is high and there are frequent mergers between sub-clusters in the cluster-forming region. A universal primordial binary distribution based on observed inner companions in the Galactic field is consistent with the binary populations of young clusters with resolved stellar populations, and the scatter between clusters of similar masses could be explained by differences in their formation history.

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arXiv2024

Magnetic Field Amplification during Stellar Collisions between Low-Mass Stars

Blue straggler stars in stellar clusters are a subset of stars that are bluer and appear younger than other cluster members, seemingly straggling behind in their evolution. They offer a unique opportunity to understand the stellar dynamics and populations within their hosts. In the collisional formation scenario, a persistent challenge is the excessive angular momentum in the collision product. The consequent significant mass loss during transition to a stable state leads to a star with too low a mass to be a blue straggler, unless it spins down efficiently. While many proposed spin-down mechanisms involve boosted angular momentum loss via magnetic braking within the collision product, the existence or strength of these magnetic fields has not been confirmed. Here, we report three-dimensional magnetohydrodynamical simulations of collisions between two low-mass main-sequence stars and investigate magnetic field amplification. Magnetic field energy is amplified by a factor of $10^{8}-10^{10}$, resulting in the magnetic field strength of $10^{7}-10^{8}$G at the core of the collision product, independent of collision parameters. The surface magnetic field strengths have increased up to $10-10^{4}$ G. In addition, a distinctly flattened, rotating gas structure appears around the collision products in off-axis collisions, which may be a hint of possible disk formation. Such significant magnetic amplification and potential disk formation suggest the possibility of efficient spin-down of collision products via magnetic braking and magnetic disk locking, which can result in their appearance as blue stragglers.

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arXiv2024

Binary Disruption and Ejected Stars from Hierarchical Star Cluster Assembly

We simulate mergers between star clusters embedded within their natal giant molecular cloud. We extract initial conditions from cloud-scale simulations of cluster formation and introduce different prescriptions for primordial binaries. We find that simulations that do not include primordial binaries result in a larger fraction of unbound stars than simulations which include a prescription for binaries based on observations. We also find a preferred direction of motion for stars that become unbound during the merger. Sub-cluster mergers within realistic gas environments promote binary disruption while mergers between idealized, gas-rich spherical clusters do not produce the same disruption. Binary systems with smaller semi-major axes are disrupted in simulations of sub-cluster mergers within their natal environment compared to simulations that do not include the realistic gas environment. We conclude that binary disruption and the production of an anisotropic distribution of unbound stars are the natural consequences of sub-cluster mergers during star cluster assembly.

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arXiv2024

The Star Clusters As Links between galaxy Evolution and Star formation (SCALES) project I: Numerical method

Stellar clusters are critical constituents within galaxies: they are the result of highest-density star formation, and through their spatially and temporally correlated feedback they regulate their host galaxy evolution. We present a novel numerical method to model star clusters as individual units of star formation using sink particles. In our method, star clusters grow via gas accretion and via merging with less massive clusters. We describe the implementation in the radiation hydrodynamics code GIZMO and run a large grid of marginally bound, turbulent clouds of $10^7~{\rm M}_{\odot}$ to explore the effect of modeling ingredients on the evolution of the clouds and the star clusters. We find both gas accretion and mergers to be critical processes to form star clusters of masses up to $\sim10^5$-$10^6~{\rm M}_{\odot}$, while ionising radiation is the main feedback mechanism regulating the growth of star clusters. The majority of our star clusters assemble their mass in $0.3$-$2.6~{\rm Myr}$, and the most massive ones take $\sim10~{\rm Myr}$. By removing high density gas by accretion, our sink-based cluster formation prescription allows the newly-formed star clusters to inject their stellar feedback in less dense environments. This makes feedback more efficient at ionising and disrupting the cloud than if we were to use a standard star formation approach, indicating that our numerical method is the missing critical step to model the interplay between star clusters and their host galaxies.

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arXiv2024

Massive interacting binaries as an enrichment source for multiple populations in star clusters

We present a suite of binary evolution models with massive primaries (10 $\leq$ M$_1$ $\leq$ 40 M$_\odot$) and periods and mass ratios chosen such that the systems undergo non-conservative mass transfer while the primaries have helium cores. We track the total mass and chemical composition of the ejecta from these systems. This material shows the abundance signatures of hot hydrogen burning which are needed to explain the abundance patterns seen in multiple populations in massive star clusters. We then calculate the total yield of a population of binary stars with masses, mass ratios, and periods consistent with their distribution in a field population. We show that the overall abundance of this material is enriched in helium, nitrogen, sodium, and aluminum, and depleted in carbon, oxygen, and magnesium, by amounts that are consistent with observations. We also show that such a population of binaries will return approximately 25% of its mass in this ejecta (compared to 4% if all the stars were single), over a characteristic timescale of about 12 Myr. We argue that massive binaries must be seriously considered as a contributor to the source of enriched material needed to explain the multiple populations in massive clusters, since essentially all massive stars are formed in binaries or higher order multiples, massive binaries are primarily formed in clusters, and massive binaries naturally produce material of the right composition.

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arXiv2024

Dynamics of Star Cluster Formation: Mergers in Gas Rich Environments

We perform high resolution simulations of forming star clusters as they merge inside giant molecular clouds (GMCs) using hydrodynamics coupled to N-body dynamics to simultaneously model both the gas and stars. We zoom in to previously run GMC simulations and resolve clusters into their stellar and gas components while including the surrounding GMC environment. We find that GMC gas is important in facilitating the growth of clusters in their embedded phase by promoting cluster mergers. Mergers induce asymmetric expansion of the stellar component of the clusters in our simulations. As well, mergers induce angular momentum in the clusters' stellar and gas components. We find that mergers can lead to an increase in the amount of dense gas present in clusters if a background gas distribution is present. We predict that this can lead to new star formation that can change the overall distribution of cluster stars in velocity space. Our results suggest that subcluster mergers in the presence of background gas can imprint dynamical signatures that can be used to constrain cluster formation histories.

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arXiv2023

Galactic properties that favour star cluster formation: a statistical view

The presence or absence of star clusters in galaxies, and the properties of star cluster populations compared to their host galaxy properties, are important observables for validating models of cluster formation, galaxy formation, and galaxy assembly. In this work, we apply a Bayesian approach to fit two models to data from surveys of young clusters in star forming galaxies. The first model is a logistic regression, which allows us to include galaxies which do not have any young clusters. The second model is a hurdle model, which includes galaxies with zero clusters and also incorporates information about the total mass in the cluster system. We investigate two predictors (star formation rate and total stellar mass in the galaxy) and look at clusters younger than 10 or 100 Myr. We find that in all cases, star formation rate is the better predictor for both the probability of hosting clusters and the total mass in the cluster system. We compare our results to similar models for old globular clusters, and conclude that star cluster formation was more abundant and more efficient at higher redshifts, likely because of the high gas content of galaxies at that time.

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arXiv2023

Initial sizes of star clusters: implications for cluster dissolution during galaxy evolution

Massive star clusters are often used as tracers of galaxy formation and assembly. In order to do so, we must understand their properties at formation, and how those properties change with time, galactic environment, and galaxy assembly history. The two most important intrinsic properties that govern star cluster evolution are mass and radius. In this paper, we investigate 10 theoretically and observationally motivated initial size-mass relations for star clusters, and evolve populations of clusters through galaxy formation models. We compare our results to each other and to observations of cluster populations in M83, M31, and the Milky Way. We find that none of our size-mass relations agree with the observations after 6-10 Gyr of evolution. We can successfully reproduce the cluster mass functions with models that have a small range of initial radii, and which do not allow cluster radii to change with time. However, these models do not agree with our understanding of cluster evolution, which does involve radius evolution, and do not match the observed distributions of radii. We note that there is a region of parameter space where clusters are optimally protected from both tidal shocks and evaporation due to two-body relaxation. Clusters which are allowed to evolve into this parameter space will likely survive. An improved understanding of both mass and radius evolution of star clusters in realistic, time-varying galactic potentials is necessary to appropriately make the connection between present-day cluster properties and their use as tracers of galaxy formation and assembly.

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arXiv2023

Fast rotating Blue Stragglers prefer loose clusters

Blue stragglers are anomalously luminous core hydrogen-burning stars formed through mass-transfer in binary/triple systems and stellar collisions. Their physical and evolutionary properties are largely unknown and unconstrained. Here we analyze 320 high-resolution spectra of blue stragglers collected in eight galactic globular clusters with different structural characteristics and show evidence that the fraction of fast rotating blue stragglers (with rotational velocities larger than 40 km/s) increases for decreasing central density of the host system. This trend suggests that fast spinning blue stragglers prefer low-density environments and promises to open an unexplored route towards understanding the evolutionary processes of these stars. Since large rotation rates are expected in the early stages of both formation channels, our results provide direct evidence for recent blue straggler formation activity in low-density environments and put strong constraints on the timescale of the collisional blue straggler slow-down processes.

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