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Ross P. Church

Publications and source records attributed to Ross P. Church.

At least 19 recordsLinked to original sources
arXiv2026

$N$-body simulations of the open cluster population in the Milky Way and the impact of GMC encounters

We investigate the population of open clusters in the Solar Neighbourhood and model the effect of encounters with giant molecular clouds (GMCs) over the last 1 Gyr. We combine a Galactic model with $N$-body simulations of 20692 unique clusters in the mass range $[50-24000]$ $\mathrm{M}_{\odot}$. Each cluster is simulated twice: with and without tidal forces from the GMCs. We find that an initial cluster mass function truncated at $\sim 9000$ $\mathrm{M}_{\odot}$ best reproduces the observed mass function evolution. For the age function, the observations show a decline in clusters for ages older than $\sim 1$ Myr, whereas our simulated clusters show a decline after $\sim 50$ Myr. The observed early disruption suggests that some clusters form supervirial, whereas our simulated clusters are created in virial equilibrium. Low-mass ($<600 \, \mathrm{M}_{\odot}$) clusters are most sensitive to GMC encounters, which accelerate their disruption in the first $\sim 200$ Myr. After $\sim 500$ Myr, the impact of GMCs becomes irrelevant since the clusters will have been destroyed regardless of whether they experience GMC encounters or not. The survival of intermediate-mass ($600-6000\, \mathrm{M}_{\odot}$) clusters is significantly reduced by GMCs at ages up to 1 Gyr. High-mass ($>6000 \, \mathrm{M}_{\odot}$) clusters survive to 1 Gyr with minimal disruption with or without GMCs. We find that clusters that have had strong GMC encounters within the last $20$ Myr should have tidal tails that are randomly orientated with respect to the Galactic centre.

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arXiv2026

Observational constraints on the origin of the elements. X. Combining NLTE and machine learning for chemical diagnostics of 4 million stars in the 4MIDABLE-HR survey

We present the 4MOST-HR resolution Non-Local Thermal Equilibrium (NLTE) Payne artificial neural network (ANN), trained on $404\,793$ new FGK spectra with 16 elements computed in NLTE. This network will be part of the Stellar Abundances and atmospheric Parameters Pipeline (SAPP), which will analyse 4 million stars during the five year long 4MOST consortium 4: 4MOST MIlky way Disc And BuLgE High-Resolution (4MIDABLE-HR) survey. A fitting algorithm using this ANN is also presented that is able to fully-automatically and self-consistently derive both stellar parameters and elemental abundances. The ANN is validated by fitting 121 observed spectra of low-mass FGKM type stars, including main-sequence dwarf, subgiant and giant stars down to [Fe/H] $\approx -3.3$ degraded to 4MOST-HR resolution of $R\approx20\,000$, and comparing the derived abundances with the output of the classical radiative transfer code TSFitPy. We are able to recover all 18 elemental abundances with a bias~$<0.13$ and spread~$<0.16$\,dex, although the typical values are $<0.09$ dex for most elements. These abundances are compared to the OMEGA+ Galactic Chemical Evolution model, showcasing for the first time, the expected performance and results obtained from high-resolution spectra of the quality expected to be obtained with 4MOST. The expected Galactic trends are recovered, and we highlight the potential of using many chemical elements to constrain the formation history of the Galaxy.

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arXiv2025

The young stellar clusters in M51 and the impact of GMC encounters

We investigate the young stellar cluster population of M51 and how it is affected by encounters with giant molecular clouds (GMCs). We combine a galactic model with $N$-body simulations of 5000 unique clusters in the mass range [600 - 24000] $M_{\odot}$. We simulate each cluster twice: with ($C_R$) and without ($C_N$) tidal perturbations from the GMCs. We are able to reproduce the majority of the observed mass- and age functions. However, for the age function we see a large discrepancy for clusters with masses $\sim$ 5000 $M_{\odot}$, which is likely related to incompleteness in the observations. We find that old low-mass clusters, located close to the galactic centre, are most likely to be disrupted. The effect of GMC encounters causes a decrease in survivability by 8 per cent points for the oldest clusters with initial masses below 6000 $M_{\odot}$. For 15 clusters we find that the presence of GMCs can protect the $C_R$ clusters from the rest of the galactic tidal field and thereby cause them to retain a significantly higher fraction of stars compared to the $C N$ clusters. For clusters that are super-virialized we find that the $C_R$ clusters have a higher virial ratio compared to the $C_N$ clusters, which is a result of interactions with GMCs. We see no significant difference between the $C_R$ and $C_N$ populations, indicating that over a time period of 200 Myr the effect of the GMCs cannot be detected.

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arXiv2025

Binary stars in the Milky Way nuclear stellar cluster

Intermediate-mass galaxies, including the Milky Way, typically host both a supermassive black hole (SMBH) and a nuclear stellar cluster (NSC). Binaries in an NSC evolve via close encounters with surrounding stars and secular processes related to the SMBH. We study moderately soft and hard binaries ($0.03$-$2.5\,\mathrm{au}$, $M \lesssim 2\,M_\odot$) initially at galactocentric radii 0.1 and 0.3 pc using three-body simulations including von Zeipel-Lidov-Kozai oscillations and tidal dissipation over $\sim 10$ Gyr. Binaries migrate both inward and outward as a consequence of kicks received in the three-body encounters. Inward migration leads to destruction via mergers and evaporation, while outward migration is a pathway to retaining intact binaries for $\gtrsim 10$ Gyr. All surviving binaries are hard and circular, but outcomes for binaries initially at the hard-soft boundary are stochastic. We find that: (i) about $0.3$ percent of evaporated binaries fall into the SMBH's loss cone, (ii) at least $1$ percent of mergers occur late enough to appear as blue straggler stars (BSSs) on the main sequence or as recently evolved red giants, (iii) about $1$ percent of binaries initially at 0.1 pc merge within the inner arcsec of the NSC, and (iv) less than about $80$ percent of field-star collisions with a binary star lead to a subsequent merger; a three-body pile up, which are relatively common in the first 1-2 Gyr and could serve as a way to form more massive BSSs in the NSC. We predict that a small fraction of binaries originate closer to the SMBH than their present-day orbits, and vice versa for evaporated binaries and BSSs. The mergers confined to the inner arcsec occur after $\gtrsim 300$ Myr, too long to be directly related to the formation of the S-stars or G-objects, but suggest that the inner arcsec is contaminated with BSSs from earlier star formation events.

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arXiv2024

A Wide Metallicity Range for Gyr-old Stars in the Nuclear Star Cluster

We report metallicities for three $\sim$Gyr-old stars in the Milky Way nuclear star cluster (NSC) using high-resolution near-infrared spectroscopy. We derive effective temperatures from a calibration with Sc line strength, which yields results in good agreement with other methods, and metallicities from spectral fits to Fe I lines. Our derived metallicities range from -1.2 < [Fe/H] < +0.5, a span of 1.7 dex. In addition we use isochrone projection to obtain masses of 1.6 to 4.3 M$_\odot$, and ages assuming single-star evolution. The oldest of these stars is 1.5 Gyr while the youngest and most metal-rich is only 100 Myr. The wide range in metallicity poses interesting questions concerning the chemical evolution and enrichment of the NSC and adds to the evidence for the presence of a young, metal-rich population in the NSC. We suggest that the candidate intermediate-age, metal-poor ([Fe/H] = -1.2) star may be best explained as a blue straggler from an underlying old population.

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arXiv2022

Influence of tidal dissipation on outcomes of binary-single encounters between stars and black holes in stellar clusters

In the cores of dense stellar clusters, close gravitational encounters between binary and single stars can frequently occur. Using the Tsunami code, we computed the outcome of a large number of binary-single interactions involving two black holes (BHs) and a star to check how the inclusion of orbital energy losses due to tidal dissipation can change the outcome of these chaotic interactions. Each interaction was first simulated without any dissipative processes and then we systematically added orbital energy losses due to gravitational wave emission (using post-Newtonian (PN) corrections) and dynamical tides and recomputed the interactions. We find that the inclusion of tides increases the number of BH-star mergers by up to 75 per cent but it does not affect the number of BH-BH mergers. These results highlight the importance of including orbital energy dissipation due to dynamical tides during few-body encounters and evolution of close binary systems within stellar cluster simulations. Consistent with previous studies, we find that the inclusion of PN terms increases the number of BH-BH mergers during binary-single encounters. However, BH-star mergers are largely unaffected by the inclusion of these terms.

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arXiv2021

Formation of supermassive black holes in galactic nuclei II: retention and growth of seed intermediate-mass black holes

In many galactic nuclei, a nuclear stellar cluster (NSC) co-exists with a supermassive black hole (SMBH). In this work, we explore the idea that the NSC forms before the SMBH through the merger of several stellar clusters that may contain intermediate-mass black holes (IMBHs). These IMBHs can subsequently grow by mergers and accretion to form an SMBH. To check the observable consequences of this proposed SMBH seeding mechanism, we created an observationally motivated mock population of galaxies, in which NSCs are constructed by aggregating stellar clusters that may or may not contain IMBHs. We model the growth of IMBHs in the NSCs through gravitational wave (GW) mergers with other IMBHs and gas accretion. In the case of GW mergers, the merged BH can either be retained or ejected depending on the GW recoil kick it receives. The likelihood of retaining the merged BH increases if we consider growth of IMBHs in the NSC through gas accretion. We find that nucleated lower-mass galaxies ($\rm M_{\star} \lesssim 10^{9} \ M_{\odot}$; e.g. M33) have an SMBH seed occupation fraction of about 0.3 to 0.5. This occupation fraction increases with galaxy stellar mass and for more massive galaxies ($\rm 10^{9} \ M_{\odot} \lesssim \rm M_{\star} \lesssim 10^{11} \ M_{\odot}$), it is between 0.5 and 0.8, depending on how BH growth is modelled. These occupation fractions are consistent with observational constraints. Furthermore, allowing for BH growth also allows us to reproduce the observed diversity in the mass range of SMBHs in the $\rm M_{\rm NSC} - M_{\rm BH}$ plane.

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arXiv2021

Close stellar encounters at the Galactic Centre I: The effect on the observed stellar populations

We model the effects of collisions and close encounters on the stellar populations observed in the Milky Way nuclear stellar cluster (NSC). Our analysis is based on $N$-body simulations in which the NSC forms by accretion of massive stellar clusters around a supermassive black hole. We attach stellar populations to our $N$-body particles and follow the evolution of their stars, and the rate of collisions and close encounters. The most common encounters are collisions between pairs of main-sequence stars, which lead to mergers: destructive collisions between main-sequence stars and compact objects are rare. We find that the effects of collisions on the stellar populations are small for three reasons. First, our models possess a core which limits the maximum stellar density. Secondly, the velocity dispersion in the NSC is similar to the surface escape velocities of the stars, which minimises the collision rate. Finally, whilst collisions between main-sequence stars destroy bright giants by accelerating their evolution, they also create them by accelerating the evolution of lower-mass stars. These two effects approximately cancel out. We also investigate whether the G2 cloud could be a fuzzball: a compact stellar core which has accreted a tenuous envelope in a close encounter with a red giant. We conclude that fuzzballs with cores below $2\,M_\odot$ have thermal times-scales too short to reproduce G2. A fuzzball with a black-hole core could reproduce the surface properties of G2 but the production rate of such objects in our model is low.

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arXiv2021

Formation of super-massive black holes in galactic nuclei I: delivering seed intermediate-mass black holes in massive stellar clusters

Supermassive black holes (SMBHs) are found in most galactic nuclei. A significant fraction of these nuclei also contain a nuclear stellar cluster (NSC) surrounding the SMBH. In this paper, we consider the idea that the NSC forms first, from the merger of several stellar clusters that may contain intermediate-mass black holes (IMBHs). These IMBHs can subsequently grow in the NSC and form an SMBH. We carry out $N$-body simulations of the simultaneous merger of three stellar clusters to form an NSC, and investigate the outcome of simulated runs containing zero, one, two and three IMBHs. We find that IMBHs can efficiently sink to the centre of the merged cluster. If multiple merging clusters contain an IMBH, we find that an IMBH binary is likely to form and subsequently merge by gravitational wave emission. We show that these mergers are catalyzed by dynamical interactions with surrounding stars, which systematically harden the binary and increase its orbital eccentricity. The seed SMBH will be ejected from the NSC by the recoil kick produced when two IMBHs merge, if their mass ratio $q\gtrsim 0.15$. If the seed is ejected then no SMBH will form in the NSC. This is a natural pathway to explain those galactic nuclei that contain an NSC but apparently lack an SMBH, such as M33. However, if an IMBH is retained then it can seed the growth of an SMBH through gas accretion and tidal disruption of stars.

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arXiv2020

Merger rates in primordial black hole clusters without initial binaries

Primordial black holes formed through the collapse of cosmological density fluctuations have been hypothesised as contributors to the dark matter content of the Universe. At the same time, their mergers could contribute to the recently observed population of gravitational-wave sources. We investigate the scenario in which primordial black holes form binaries at late times in the Universe. Specifically, we re-examine the mergers of primordial black holes in small clusters of ~30 objects in the absence of initial binaries. Binaries form dynamically through Newtonian gravitational interactions. These binaries act as heat sources for the cluster, increasing the cluster's velocity dispersion, which inhibits direct mergers through gravitational-wave two-body captures. Meanwhile, three-body encounters of tight binaries are too rare to tighten binaries sufficiently to allow them to merge through gravitational-wave emission. We conclude that in the absence of initial binaries, merger rates of primordial black holes in the Bird et al. (2016) initial cluster configuration are at least an order of magnitude lower than previously suggested, which makes gravitational-wave detections of such sources improbable.

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arXiv2019

Super-Earth ingestion can explain the anomalously high metal abundances of M67 Y2235

We investigate the hypothesis that ingestion of a terrestrial or super-Earth planet could cause the anomalously high metal abundances seen in a turn-off star in the open cluster M67, when compared to other turn-off stars in the same cluster. We show that the mass in convective envelope of the star is likely only $3.45\,\times 10^{-3}\,{\rm M}_\odot$, and hence $5.2\,{\rm M}_\oplus$ of rock is required to obtain the observed 0.128 dex metal enhancement. Rocky planets dissolve entirely in the convective envelope if they enter it with sufficiently tangential orbits: we find that the critical condition for dissolution is that the planet's radial speed must be less than 40% of its total velocity at the stellar surface; or, equivalently, the impact parameter must be greater than about 0.9. We model the delivery of rocky planets to the stellar surface both by planet-planet scattering in a realistic multi-planet system, and by Lidov-Kozai cycles driven by a more massive planetary or stellar companion. In both cases almost all planets that are ingested arrive at the star on grazing orbits and hence will dissolve in the surface convection zone. We conclude that super-Earth ingestion is a good explanation for the metal enhancement in M67 Y2235, and that a high-resolution spectroscopic survey of stellar abundances around the turn-off and main sequence of M67 has the potential to constrain the frequency of late-time dynamical instability in planetary systems.

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arXiv2019

The Ecology of the Galactic Centre: Nuclear Stellar Clusters and Supermassive Black Holes

Supermassive black holes are found in most galactic nuclei. A large fraction of these nuclei also contain a nuclear stellar cluster surrounding the black hole. Here we consider the idea that the nuclear stellar cluster formed first and that the supermassive black hole grew later. In particular we consider the merger of three stellar clusters to form a nuclear stellar cluster, where some of these clusters contain a single intermediate-mass black hole (IMBH). In the cases where multiple clusters contain IMBHs, we discuss whether the black holes are likely to merge and whether such mergers are likely to result in the ejection of the merged black hole from the nuclear stellar cluster. In some cases, no supermassive black hole will form as any merger product is not retained. This is a natural pathway to explain those galactic nuclei that contain a nuclear stellar cluster but apparently lack a supermassive black hole; M33 being a nearby example. Alternatively, if an IMBH merger product is retained within the nuclear stellar cluster, it may subsequently grow, e.g. via the tidal disruption of stars, to form a supermassive black hole.

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arXiv2019

Rapid destruction of protoplanetary discs due to externalphotoevaporation in star-forming regions

We analyse N-body simulations of star-forming regions to investigate the effects of external far and extreme ultra-violet photoevaporation from massive stars on protoplanetary discs. By varying the initial conditions of simulated star-forming regions, such as the spatial distribution, net bulk motion (virial ratio), and density, we investigate which parameters most affect the rate at which discs are dispersed due to external photoevaporation. We find that disc dispersal due to external photoevaporation is faster in highly substructured star-forming regions than in smooth and centrally concentrated regions. Sub-virial star-forming regions undergoing collapse also show higher rates of disc dispersal than regions that are in virial equilibrium or are expanding. In moderately dense ($\sim$100 M$_{\odot}$ pc$^{-3}$) regions, half of all protoplanetary discs with radii $\geq$ 100 AU are photoevaporated within 1 Myr, three times faster than is currently suggested by observational studies. Discs in lower-density star-forming regions ($\sim$10 M$_{\odot}$ pc$^{-3}$) survive for longer, but half are still dispersed on short timescales ($\sim$2 Myr). This demonstrates that the initial conditions of the star forming regions will greatly impact the evolution and lifetime of protoplanetary discs. These results also imply that either gas giant planet formation is extremely rapid and occurs before the gas component of discs is evaporated, or gas giants only form in low-density star-forming regions where no massive stars are present to photoevaporate gas from protoplanetary discs.

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arXiv2018

Estimating stellar ages and metallicities from parallaxes and broadband photometry - successes and shortcomings

A deep understanding of the Milky Way galaxy, its formation and evolution requires observations of huge numbers of stars. Stellar photometry, therefore, provides an economical method to obtain intrinsic stellar parameters. With the addition of distance information - deriving reliable ages from photometry is a possibility. We have developed a Bayesian method that generates 2D probability maps of a star's age and metallicity from photometry and parallax using isochrones. Our synthetic tests show that including a near-UV passband enables us to break the degeneracy between a star's age and metallicity for certain evolutionary stages. It is possible to find well-constrained ages and metallicities for turn-off and sub-giant stars with colours including a U band and a parallax with uncertainty less than ~20%. Metallicities alone are possible for the main sequence and giant branch. We find good agreement with the literature when we apply our method to the Gaia benchmark stars, particularly for turn-off and young stars. Further tests on the old open cluster NGC 188, however, reveal significant limitations in the stellar isochrones. The ages derived for the cluster stars vary with evolutionary stage, such that turn-off ages disagree with those on the sub-giant branch, and metallicities vary significantly throughout. Furthermore, the parameters vary appreciably depending on which colour combinations are used in the derivation. We identify the causes of these mismatches and show that improvements are needed in the modelling of giant branch stars and in the creation and calibration of synthetic near-UV photometry. Our results warn against applying isochrone fitting indiscriminately. In particular, the uncertainty on the stellar models should be quantitatively taken into account. Further efforts to improve the models will result in significant advancements in our ability to study the Galaxy.

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arXiv2018

Benchmark ages for the Gaia benchmark stars

In the era of large-scale surveys of stars in the Milky Way, stellar ages are crucial for studying the evolution of the Galaxy. But determining ages of field stars is notoriously difficult; therefore, we attempt to determine benchmark ages for the extensively studied Gaia benchmark stars which can be used for validation purposes. By searching the literature for age estimates from different methods and deriving new ages based on Bayesian isochrone fitting, we are able to put reliable limits on the ages of 16 out of the 33 benchmark stars. The giants with well-defined ages are all young, and an expansion of the sample to include older giants with asteroseismic ages would be beneficial. Some of the stars have surface parameters inconsistent with isochrones younger than 16 Gyr. Including $α$-enhancement in the models when relevant resolves some of these cases, but others clearly highlight discrepancies between the models and observations. We test the impact of atomic diffusion on the age estimates by fitting to the actual surface metallicity of the models instead of the initial value and find that the effect is negligible except for a single turn-off star. Finally, we show that our ability to determine isochrone-based ages for large spectroscopic surveys largely mirrors our ability to determine ages for these benchmark stars, except for stars with $\log g \gtrsim 4.4$ dex since their location in the HR diagram is almost age insensitive. Hence, isochrone fitting does not constrain their ages given the typical uncertainties of spectroscopic stellar parameters.

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arXiv2018

Thermohaline mixing in extremely metal-poor stars

Extremely metal-poor (EMP) stars are an integral piece in the puzzle that is the early Universe, and although anomolous subclasses of EMP stars such as carbon-enhanced metal-poor (CEMP) stars are well-studied, they make up less than half of all EMP stars with [Fe/H] $\sim -3.0$. The amount of carbon depletion occurring on the red giant branch (carbon offset) is used to determine the evolutionary status of EMP stars, and this offset will differ between CEMP and normal EMP stars. The depletion mechanism employed in stellar models (from which carbon offfsets are derived) is very important, however the only widely available carbon offsets in the literature are derived from stellar models using a thermohaline mixing mechanism that cannot simultaneously match carbon and lithium abundances to observations for a single diffusion coeffcient. Our stellar evolution models utilise a modified thermohaline mixing model that can match carbon and lithium in the metal-poor globular cluster NGC 6397. We compare our models to the bulk of the EMP star sample at [Fe/H] $= -3$ and show that our modified models follow the trend of the observations and deplete less carbon compared to the standard thermohaline mixing theory. We conclude that stellar models that employ the standard thermohaline mixing formalism overestimate carbon offsets and hence CEMP star frequencies, particularly at metallicities where carbon-normal stars dominate the EMP star population.

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arXiv2018

Stellar multiplicity in high-resolution spectroscopic surveys. I. Application to APOGEE subgiants and giants

Many field stars reside in binaries, and the analysis and interpretation of photometric and spectroscopic surveys must take this into account. We have developed a model to predict how binaries influence the scientific results inferred from large spectroscopic surveys. Based on the rapid binary evolution code BSE, it allows us to model a representative population of binaries and generate synthetic survey observations. We describe this model in detail, and apply it to the radial velocity variation of subgiant and giant stars in the Galactic disc, as observed by the Apache Point Observatory Galactic Evolution Experiment (APOGEE), part of the Sloan Digital Sky Survey III. APOGEE provides an excellent data set for testing our binary models since a large fraction of the stars have been observed multiple times. By comparing our model to the APOGEE observations we constrain the initial binary fraction of solar-metallicity stars in the sample to be $f_{\rm b,0}=0.35\pm0.01$, in line with the solar neighbourhood. We find that the binary fraction is higher at lower metallicities, consistent with other observational studies. Our model matches the shape of the high-velocity scatter in APOGEE, which suggests that most velocity variability above 0.5 km/s comes from binaries. Our exploration of binary initial properties shows that APOGEE is mostly sensitive to binaries with periods between 3 and 3000 years, and is largely insensitive to the detailed properties of the population. We can, however, rule out a population where the mass of the lower-mass star is drawn from the IMF independently of its companion.

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