arXiv subjects

Eoin Farrell

Publications and source records attributed to Eoin Farrell.

At least 19 recordsLinked to original sources

arXiv2025

The impact of rotational mixing in intermediate-age star clusters with extended main-sequence turn-offs and extended red clumps

The extended main-sequence turn-offs (eMSTOs) and extended red clumps (eRCs) observed in intermediate-age star clusters challenge the traditional understanding of clusters as simple stellar populations. Recently, eMSTOs have been interpreted as signatures of stellar rotation. In this work, we test the effectiveness of rotational mixing in shaping the color-magnitude diagram (CMD) of star clusters. We computed a set of separate single-age synthetic stellar populations, referred to as "Base Stellar Populations" (BSPs), including stellar rotation. These BSPs were generated from two grids of stellar models that share the same input physics but differ in the efficiency of rotational mixing. We used an optimization algorithm to determine the best combination of BSPs to fit the CMDs of two star clusters: the Small Magellanic Cloud cluster NGC 419 and the Milky Way cluster NGC 1817. The synthetic clusters with weak rotational mixing provide the best fit to both the eMSTO and eRC features for both clusters, and are consistent with the luminosities and asteroseismic masses we derived for eRC stars in NGC 1817. In contrast, synthetic clusters with strong rotational mixing result in overly bright post-main-sequence stars, inconsistent with observations. This suggests that, for intermediate-mass stars, the influence of rotational mixing of chemical elements on stellar evolution cannot be so strong as to significantly increase the post-main-sequence luminosity. A simple test suggests that accounting for self-extinction by decretion discs in equator-on fast rotators could influence inferred rotation distributions and help reconcile the projected rotational velocity discrepancy across the eMSTO between models and observations.

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arXiv2025

Detailed Simulations of Massive Hierarchical Triple Star Systems - Exploring the impact of the stellar physics on the evolutionary pathways of massive hierarchical triple systems

Recent observations estimate that 30% of early B and O-type stars are found in triple systems. So far, the evolution of triple star systems has mainly been modeled using fast stellar codes. Their accuracy decreases with increasing mass, limiting their reliability for predicting the evolutionary pathways of massive triple systems. We coupled Tres, which by default uses Seba (fast stellar code) to Mesa to perform the first simulations of triple systems that combine a triple secular evolutionary code with a detailed, on-the-fly stellar code. After examining the differences between the stellar evolution predicted by the two codes, we simulate the evolution of a set of triple systems and compare their predicted evolutionary pathways. The predicted stellar tracks become increasingly divergent with increasing mass and wind mass loss efficiency. The maximal radial extent, crucial for determining whether the components of the triple systems interact, differ by up to two orders of magnitude between the two stellar codes in the considered mass range. This leads to divergences in the triples evolutionary pathways predicted by mesa and seba. Using mesa instead of seba, the minimum period for avoiding inner mass transfer is reduced by three orders of magnitude. This has important consequences for the formation of GW sources through the triple compact object channel. Our simulations offer new insights into the physics of triple systems, as key processes (mass loss, radial expansion, precession) are treated self-consistently. They indicate that the results of triple systems population synthesis studies must be interpreted cautiously, in particular when the considered masses are outside the range of the grid the fast codes are based on and when significant stellar winds are considered.

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arXiv2025

Fluorine production in He-burning regions of massive stars during cosmic history

The origin of fluorine is still a debated question. AGB stars synthesise this element and likely contribute significantly to its synthesis in the present-day Universe. However, it is not clear whether other sources contribute, especially in the early Universe. We discuss variations of the surface abundances of fluorine coming from our massive star models and compare them with available present-day observations. We compute the contribution of massive stars in producing 19F over metallicities covering the whole cosmic history. We used models in the mass range of 9Msol < Mini < 300Msol at metallicities from Pop III up to super-solar while accounting for the required nuclear network to follow the evolution of 19F during the core H- and He-burning phases. Results from models with and without rotational mixing are presented. We find that rotating models predict a slight depletion of fluorine at their surface at the end of the MS phase. In more advanced evolutionary phases, only models with an initial mass larger than 25Msol at metallicities Z > 0.014 show phases where the abundance of fluorine is enhanced. This occurs when the star is a WR star of the WC type. WC stars can show surface abundances of fluorine ten times larger than their initial abundance. However, we obtained that the winds of massive stars at metallicities larger than Z=0.006 do not significantly contribute to fluorine production, confirming previous findings. In contrast, very metal-poor rapidly rotating massive star models may be important sources of fluorine through the mass expelled at the time of their SN explosion. Observations of WC stars at solar or super-solar metallicities may provide very interesting indications on the nuclear pathways that lead to fluorine production in massive stars. The possibility of observing fluorine-rich CEMPs is also a way to put constrains in present models at very low metallicities.

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arXiv2025

SAEBench: A Comprehensive Benchmark for Sparse Autoencoders in Language Model Interpretability

Sparse autoencoders (SAEs) are a popular technique for interpreting language model activations, and there is extensive recent work on improving SAE effectiveness. However, most prior work evaluates progress using unsupervised proxy metrics with unclear practical relevance. We introduce SAEBench, a comprehensive evaluation suite that measures SAE performance across eight diverse metrics, spanning interpretability, feature disentanglement and practical applications like unlearning. To enable systematic comparison, we open-source a suite of over 200 SAEs across eight recently proposed SAE architectures and training algorithms. Our evaluation reveals that gains on proxy metrics do not reliably translate to better practical performance. For instance, while Matryoshka SAEs slightly underperform on existing proxy metrics, they substantially outperform other architectures on feature disentanglement metrics; moreover, this advantage grows with SAE scale. By providing a standardized framework for measuring progress in SAE development, SAEBench enables researchers to study scaling trends and make nuanced comparisons between different SAE architectures and training methodologies. Our interactive interface enables researchers to flexibly visualize relationships between metrics across hundreds of open-source SAEs at: www.neuronpedia.org/sae-bench

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arXiv2024

Applying sparse autoencoders to unlearn knowledge in language models

We investigate whether sparse autoencoders (SAEs) can be used to remove knowledge from language models. We use the biology subset of the Weapons of Mass Destruction Proxy dataset and test on the gemma-2b-it and gemma-2-2b-it language models. We demonstrate that individual interpretable biology-related SAE features can be used to unlearn a subset of WMDP-Bio questions with minimal side-effects in domains other than biology. Our results suggest that negative scaling of feature activations is necessary and that zero ablating features is ineffective. We find that intervening using multiple SAE features simultaneously can unlearn multiple different topics, but with similar or larger unwanted side-effects than the existing Representation Misdirection for Unlearning technique. Current SAE quality or intervention techniques would need to improve to make SAE-based unlearning comparable to the existing fine-tuning based techniques.

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arXiv2024

Toward a Comprehensive Grid of Cepheid Models with MESA I. Uncertainties of the Evolutionary Tracks of Intermediate-Mass Stars

Helium-burning stars, in particular Cepheids, are especially difficult to model, as the choice of free parameters can greatly impact the shape of the blue loops - the part of the evolutionary track at which instability strip is crossed. Contemporary one-dimensional stellar evolution codes, like MESA (Modules for Experiments in Stellar Astrophysics), come with a large number of free parameters that allow to model the physical processes in stellar interiors under many assumptions. The uncertainties that arise from this freedom are rarely discussed in the literature despite their impact on the evolution of the model. We calculate a grid of evolutionary models with MESA, varying several controls, like solar mixture of heavy elements, mixing length theory prescription, nuclear reaction rates, scheme to determine convective boundaries, atmosphere model, temporal and spatial resolution, and quantify their impact on age and location of the evolutionary track on the HR diagram from the main sequence till the end of core-helium burning. Our investigation was conducted for a full range of masses and metallicities expected for classical Cepheids. The uncertainties are significant, especially during core-helium burning, reaching or exceeding the observational uncertainties of log Teff and log L for detached eclipsing binary systems. For models below 9 solar masses, thin convective shells develop and evolve erratically, not allowing the models to converge. A careful inspection of Kippenhahn diagrams and convergence study is advised for a given mass and metallicity, to assess how severe this problem is and to what extent it may affect the evolution.

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arXiv2024

Rapidly rotating Population III stellar models as a source of primary nitrogen

The first stars might have been fast rotators. This would have important consequences for their radiative, mechanical and chemical feedback. We discuss the impact of fast initial rotation on the evolution of massive Population III models and on their nitrogen and oxygen stellar yields. We explore the evolution of Population III stars with initial masses in the range of 9Msol < Mini < 120Msol starting with an initial rotation on the Zero Age Main Sequence equal to 70% of the critical one. We find that with the physics of rotation considered here, our rapidly-rotating Population III stellar models do not follow a homogeneous evolution. They lose very little mass in case mechanical winds are switched on when the surface rotation becomes equal or larger than the critical velocity. Impact on the ionising flux appears modest when compared to moderately-rotating models. Fast rotation favours, in models with initial masses above ~20Msol, the appearance of a very extended intermediate convective zone around the H-burning shell during the core He-burning phase. This shell has important consequences on the sizes of the He- and CO-cores and thus impacts the final fate of stars. Moreover, it has a strong impact on nucleosynthesis boosting the production of primary 14N. Fast initial rotation impacts significantly the chemical feedback of Population III stars. Observations of extremely metal-poor stars and/or starbursting regions are essential to provide constraints on the properties of the first stars.

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arXiv2024

A method for non-linear inversion of the stellar structure applied to gravity-mode pulsators

We present a method for a non-linear asteroseismic inversion suitable for gravity-mode pulsators and apply it to slowly pulsating B-type (SPB) stars. Our inversion method is based on the iterative improvement of a parameterised static stellar structure model, which in turn is based on constraints from the observed oscillation periods. We present tests to demonstrate that the method is successful in recovering the properties of artificial targets both inside and outside the parameter space. We also present a test of our method on the well-studied SPB star KIC 7760680. We believe that this method is promising for carrying out detailed analyses of observations of SPB and $\gamma$ Dor stars and will provide complementary information to evolutionary models.

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arXiv2024

Explaining the high nitrogen abundances observed in high-z galaxies via population III stars of a few thousand solar masses

The chemical enrichment of the early Universe is a crucial element in the formation and evolution of galaxies, and Population III (PopIII) stars must play a vital role in this process. In this study, we examine metal enrichment from massive stars in the early Universe's embryonic galaxies. Using radiation hydrodynamic simulations and stellar evolution modelling, we calculated the expected metal yield from these stars. Specifically, we applied accretion rates from a previous radiation-hydrodynamic simulation to inform our stellar evolution modelling, executed with the Geneva code, across 11 selected datasets, with final stellar masses between 500 and 9000 Msol. Our results demonstrate that the first generation of Pop III stars within a mass range of 2000 to 9000 Msol result in N/O, C/O and O/H ratios compatible with the values observed in very high-z galaxies GN-z11 and CEERS 1019. The ejecta of these Pop III stars are predominantly composed of He, H, and N. Our Pop III chemical enrichment model of the halo can accurately reproduce the observed N/O and C/O ratios, and, by incorporating a hundred times more zero-metallicity interstellar material with the stellar ejecta, it accurately attains the observed O/H ratio. Thus, a sub-population of extremely massive PopIII stars, with masses surpassing approximately 2000 Msol, effectively reproduces the CNO elemental abundances observed in high-z JWST galaxies to date.

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arXiv2023

Impact of different approaches for computing rotating stellar models I. The solar metallicity case

The physics of stellar rotation plays a crucial role in the evolution of stars, their final fate and the properties of compact remnants. Diverse approaches have been adopted to incorporate the effects of rotation in stellar evolution models. This study seeks to explore the consequences of these various prescriptions for rotation on essential outputs of massive star models. We compute a grid of 15 and 60 M$_{\odot}$ stellar evolution models with the Geneva Stellar Evolution Code (GENEC), accounting for both hydrodynamical and magnetic instabilities induced by rotation. In both the 15 and 60 M$_{\odot}$ models, the choice of the vertical and horizontal diffusion coefficients for the non magnetic models strongly impacts the evolution of the chemical structure, but has a weak impact on the angular momentum transport and the rotational velocity of the core. In the 15 M$_{\odot}$ models, the choice of diffusion coefficient impacts the convective core size during the core H-burning phase, whether the model begins core He-burning as a blue or red supergiant and the core mass at the end of He-burning. In the 60 M$_{\odot}$ models, the evolution is dominated by mass loss and is less affected by the choice of diffusion coefficient. In the magnetic models, magnetic instability dominates the angular momentum transport and such models are found to be less mixed when compared to their rotating non-magnetic counterparts. Stellar models with the same initial mass, chemical composition, and rotation may exhibit diverse characteristics depending on the physics applied. By conducting thorough comparisons with observational features, we can ascertain which method(s) produce the most accurate results in different cases.

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arXiv2023

Critical accretion rates for rapidly growing massive Population III stars

Efforts to understand the origin and growth of massive black holes observed in the early Universe have spurred a strong interest in the evolution and fate of rapidly-accreting primordial (metal-free) stars. Here, we investigate the evolution of such Population III stars under variable accretion rates, focusing on the thermal response and stellar structure, the impact of the luminosity wave encountered early in the pre-main sequence phase, and the influence of accretion on their subsequent evolution. We employ the Geneva stellar evolution code and simulate ten models with varying accretion histories, covering a final mass range from 491 M$_{\odot}$ to 6127 M$_{\odot}$. Our findings indicate that the critical accretion rate delineating the red and blue supergiant regimes during the pre-main sequence evolution is approximately $2.5\times10^{-2} $M$_{\odot}$/yr. Once core hydrogen burning commences, the value of this critical accretion rate drops to $7.0\times10^{-3}$M$_{\odot}$/yr. Moreover, we also confirm that the Kelvin-Helmholtz timescale in the outer surface layers is the more relevant timescale for determining the transition between red and blue phases. Regarding the luminosity wave, we find that it affects only the early pre-main sequence phase of evolution and does not directly influence the transition between red and blue phases, which primarily depends on the accretion rate. Finally, we demonstrate that variable accretion rates significantly impact the lifetimes, surface enrichment, final mass and time spent in the red phase. Our study provides a comprehensive understanding of the intricate evolutionary patterns of Population III stars subjected to variable accretion rates.

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arXiv2022

The Initial Magnetic Field Distribution in AB Stars

Stars are born with magnetic fields, but the distribution of their initial field strengths remains uncertain. We combine observations with theoretical models of magnetic field evolution to infer the initial distribution of magnetic fields for AB stars in the mass range of 1.6 - 3.4 M$_{\odot}$. We tested a variety of distributions with different shapes and found that a distribution with a mean of $\sim$800 G and a full width of $\sim$600 G is most consistent with the observed fraction of strongly magnetized stars as a function of mass. Our most-favored distribution is a Gaussian with a mean of $\mu$ = 770 G and standard deviation of $\sigma$ = 146 G. Independent approaches to measure the typical field strength suggest values closer to 2 - 3 kG, a discrepancy that could suggest a mass-dependent and bimodal initial field distribution, or an alternative theoretical picture for the origin of these magnetic fields.

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arXiv2022

Modules for Experiments in Stellar Astrophysics (MESA): Time-Dependent Convection, Energy Conservation, Automatic Differentiation, and Infrastructure

We update the capabilities of the open-knowledge software instrument Modules for Experiments in Stellar Astrophysics (MESA). The new auto_diff module implements automatic differentiation in MESA, an enabling capability that alleviates the need for hard-coded analytic expressions or finite difference approximations. We significantly enhance the treatment of the growth and decay of convection in MESA with a new model for time-dependent convection, which is particularly important during late-stage nuclear burning in massive stars and electron degenerate ignition events. We strengthen MESA's implementation of the equation of state, and we quantify continued improvements to energy accounting and solver accuracy through a discussion of different energy equation features and enhancements. To improve the modeling of stars in MESA we describe key updates to the treatment of stellar atmospheres, molecular opacities, Compton opacities, conductive opacities, element diffusion coefficients, and nuclear reaction rates. We introduce treatments of starspots, an important consideration for low-mass stars, and modifications for superadiabatic convection in radiation-dominated regions. We describe new approaches for increasing the efficiency of calculating monochromatic opacities and radiative levitation, and for increasing the efficiency of evolving the late stages of massive stars with a new operator split nuclear burning mode. We close by discussing major updates to MESA's software infrastructure that enhance source code development and community engagement.

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arXiv2022

Grids of stellar models with rotation VI: Models from 0.8 to 120 $M_\odot$ at a metallicity Z = 0.006

Context: Grids of stellar models, computed with the same physical ingredients, allow one to study the impact of a given physics on a broad range of initial conditions and are a key ingredient for modeling the evolution of galaxies. Aims: We present a grid of single star models for masses between 0.8 and 120 $M_\odot$, with and without rotation for a mass fraction of heavy element Z=0.006, representative of the Large Magellanic Cloud (LMC). Methods: We used the Geneva stellar evolution code. The evolution was computed until the end of the central carbon-burning phase, the early asymptotic giant branch phase, or the core helium-flash for massive, intermediate, and low mass stars, respectively. Results: The outputs of the present stellar models are well framed by the outputs of the two grids obtained by our group for metallicities above and below the one considered here. The models of the present work provide a good fit to the nitrogen surface enrichments observed during the main sequence for stars in the LMC with initial masses around 15 $M_\odot$. They also reproduce the slope of the luminosity function of red supergiants of the LMC well, which is a feature that is sensitive to the time-averaged mass loss rate over the red supergiant phase. The most massive black hole that can be formed from the present models at Z=0.006 is around 55 $M_\odot$. No model in the range of mass considered will enter into the pair-instability supernova regime, while the minimal mass to enter the region of pair pulsation instability is around 60 $M_\odot$ for the rotating models and 85 $M_\odot$ for the nonrotating ones. Conclusions: The present models are of particular interest for comparisons with observations in the LMC and also in the outer regions of the Milky Way. We provide public access to numerical tables that can be used for computing interpolated tracks and for population synthesis studies.

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arXiv2022

Grids of stellar models with rotation VII: Models from 0.8 to 300 M$_\odot$ at super-solar metallicity (Z = 0.020)

We present a grid of stellar models at super-solar metallicity (Z = 0.020) extending the previous grids of Geneva models at solar and sub-solar metallicities. A metallicity of Z = 0.020 was chosen to match that of the inner Galactic disk. A modest increase of 43% (=0.02/0.014) in metallicity compared to solar models means that the models evolve similarly to solar models but with slightly larger mass loss. Mass loss limits the final total masses of the super-solar models to 35 M$_\odot$ even for stars with initial masses much larger than 100 M$_\odot$. Mass loss is strong enough in stars above 20 M$_\odot$ for rotating stars (25 M$_\odot$ for non-rotating stars) to remove the entire hydrogen-rich envelope. Our models thus predict SNII below 20 M$_\odot$ for rotating stars (25 M$_\odot$ for non-rotating stars) and SNIb (possibly SNIc) above that. We computed both isochrones and synthetic clusters to compare our super-solar models to the Westerlund 1 (Wd1) massive young cluster. A synthetic cluster combining rotating and non-rotating models with an age spread between log10 (age/yr) = 6.7 and 7.0 is able to reproduce qualitatively the observed populations of WR, RSG and YSG stars in Wd1, in particular their simultaneous presence at log10(L/L$_\odot$) = 5-5.5. The quantitative agreement is imperfect and we discuss the likely causes: synthetic cluster parameters, binary interactions, mass loss and their related uncertainties. In particular, mass loss in the cool part of the HRD plays a key role.

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arXiv2021

Numerical experiments to help understand cause and effect in massive star evolution

The evolution of massive stars is affected by a variety of physical processes including convection, rotation, mass loss and binary interaction. Because these processes modify the internal chemical abundance profiles in multiple ways simultaneously, it can be challenging to determine which properties of the stellar interior are primarily driving the overall evolution. Building on previous work, we develop a new modelling approach called SNAPSHOT that allows us to isolate the key features of the internal abundance profile that drive the evolution of massive stars. Using our approach, we compute numerical stellar structure models in thermal equilibrium covering key phases of stellar evolution. For the main sequence, we demonstrate that models with the same mass and very similar surface properties can have different internal distributions of hydrogen and convective core masses. We discuss why massive stars expand after the main sequence and the fundamental reasons for why they become red, blue or yellow supergiants. For the post-main sequence, we demonstrate that small changes in the abundance profile can cause very large effects on the surface properties. We also discuss the effects that produce blue supergiants and the cause of blue loops. Our models show that massive stars with lower metallicity tend to be more compact due to the combined effect of lower CNO abundances in the burning regions and lower opacity in the envelope.

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arXiv2021

Ionizing photon production of Population III stars: effects of rotation, convection, and initial mass function

The first stars are thought to be one of the dominant sources of hydrogen reionization in the early Universe, with their high luminosities and surface temperatures expected to drive high ionizing photon production rates. In this work, we take our Geneva stellar evolution models of zero-metallicity stars and predict their production rates of photons capable to ionize H, He I and He II, based on a blackbody approximation. We present analytical fits in the range 1.7-500 solar masses. We then explore the impact of stellar initial mass, rotation, and convective overshooting for individual stars. We have found that ionizing photon production rates increase with increasing initial mass. For the rotational velocities considered we see changes of up to 25% to ionizing photons produced. This varies with initial mass and ionizing photon species and reflects changes to surface properties due to rotation. We have also found that higher convective overshooting increases ionizing photon production by approximately 20% for the change in overshooting considered here. For stellar populations, we explore how the production of ionizing photons varies as a function of the initial mass function (IMF) slope, and minimum and maximum initial masses. For a fixed population mass we have found changes of the order of 20-30% through varying the nature of the IMF. This work presents ionizing photon production predictions for the most up to date Geneva stellar evolution models of Population III stars, and provides insight into how key evolutionary parameters impact the contribution of the first stars to reionization.

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arXiv2020

Grids of stellar models with rotation: V. Models from 1.7 to 120 Msun at zero metallicity

Understanding the nature of the first stars is key to understanding the early universe. With new facilities such as JWST we may soon have the first observations of the earliest stellar populations, but to understand these observations we require detailed theoretical models. Here we compute a grid of stellar evolution models using the Geneva code with the aim to improve our understanding of the evolution of zero-metallicity stars, with particular interest in how rotation affects surface properties, interior structure, and metal enrichment. We produce a range of models of initial masses (Mini) from 1.7 Msun to 120 Msun, focusing on massive models of 9 Msun < Mini < 120 Msun. Our grid includes models with and without rotation, with rotating models having an initial velocity of 40% of the critical velocity. We find that rotation strongly impacts the evolution of the first stars, mainly through increased core size and stronger H-burning shells during core He-burning. Without radiative mass loss, angular momentum builds at the surface in rotating models, thus models of initial masses Mini > 60 Msun reach critical rotation on the main sequence and experience mass loss. We find that rotational mixing strongly affects metal enrichment, but does not always increase metal production as we see at higher metallicities. This is because rotation leads to an earlier CNO boost to the H shell during He-burning, which may hinder metal enrichment depending on initial mass and rotational velocity. Electronic tables of this new grid of Population III models are publicly available.

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