arXiv subjects

Onno R. Pols

Publications and source records attributed to Onno R. Pols.

18 recordsLinked to original sources

arXiv2026

Ongoing and Post-Mass-Transfer Binaries: A Living Catalog and Unified Review of Binary Mass Transfer Products

Mass transfer is arguably the most defining interaction in binary stellar systems, yet many aspects of its physics remain poorly understood, from stability to endpoints and observable products. Comparing theory and observations is challenging because post-mass-transfer systems are studied across largely independent communities with different methods, nomenclature, and evolutionary frameworks. % We present a unified review and catalog of ongoing and post-mass-transfer binaries spanning the full stellar mass range, but restricted to systems likely to have experienced only a single episode of mass transfer (i.e., only one component is evolved). We review 16 observational classes of binary interaction products and compile a curated sample of 5,452 systems into a publicly available, community-driven catalog at https://binary-observations.github.io/post_mt_catalog/. % Using this catalog, we investigate global trends in orbital periods, eccentricities, masses, and mass ratios across post-mass-transfer binaries. We find I) non-zero eccentricities are common at all periods and system classes, with both median values and scatter increasing with period, II) the $e(\log P)$ relation depends on donor progenitor mass, with neutron-star and black-hole binaries showing the highest median eccentricities, likely reflecting effects of natal kicks, III) period distributions are broad and overlapping across evolutionary channels, and IV) the Gaia BH and NS systems are extreme in mass ratio but otherwise consistent with the general post-mass-transfer population. Together, these results support a unified empirical view of post-mass-transfer binaries that highlights several tensions between theory and observations.

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arXiv2025

Blue Stragglers and Friends: Initial Evolutionary Pathways in Close Low-Mass Binaries: Literature Review

The scope of this literature review is observations of the products of first-stage evolution for binaries having components with M < 2 $M_\odot$. A taxonomy for these products comprises dwarfs ("blue stragglers"), giants ("yellow stragglers"), subdwarf B stars, and giant-like stars ("sub-subgiants" and "red stragglers"). This literature review is organized according to this taxonomy within three distinct environments: open star clusters, globular star clusters, and the Galactic field. This literature review is the Supplemental Material for Blue Stragglers and Friends: Initial Evolutionary Pathways in Close Low-Mass Binaries (Mathieu & Pols, 2025, ARAA, 63:467-512, doi: www.annualreviews.org/content/journals/10.1146/annurev-astro-071221-054402). It is intended to support and expand on Section 3 of the main text of this Annual Review of Astronomy and Astrophysics, where an integrated perspective on the common and contrasting astrophysical properties of these binary evolution products is provided. Figures used in the main text to highlight key observational results are referenced in this literature review. The closing date of this review is January 17, 2025, with some citations subsequently updated.

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arXiv2025

Good things come to those who wait: Watching donor stars evolve towards a mass-transfer instability

Unstable mass transfer in binary systems can lead to transients such as luminous red novae (LRNe). Observations of such transients are valuable for understanding and testing models of mass transfer. For donor stars in the Hertzsprung gap, there can be a long phase of mass-transfer evolution before instability sets in. Only few case studies of such delayed dynamical instability (DDI) mass transfer exist. None consider the full pre-instability evolution and the effects thereof on the observable properties of a binary. We systematically analyse detailed models of stable and unstable mass transfer for Hertzsprung-gap donors. We focus on identifying observable evolutionary features characteristic of ultimately unstable mass transfer and not found in stable mass-transfer binaries. Our binary evolution models, calculated with the MESA code, cover initial donor masses between $2.5 M_{\odot}$ and $10 M_{\odot}$ and initial accretor-to-donor mass ratios between $0.1$ and $1$. We find that the pre-instability evolution is qualitatively the same for all DDI donor stars, consisting of a long slow dimming phase followed by a shorter phase of rapid brightening. The latter phase is powered by recombination of hydrogen and accompanied by a strong increase in effective temperature, unique to unstable mass-transfer binaries. We estimate that a significant fraction of the rapid brighteners should be detectable by Gaia throughout the Galaxy. We model the progenitors of LRNe M31-2015 and V838 Mon, find a higher initial donor mass for M31 2015 than past estimates, and propose a new scenario for V838 Mon in which the known tertiary star dominates the pre-outburst photometry and the outburst results from the DDI of a more massive primary star. This work provides a comprehensive framework linking theory to observations of transients and enables improved classification and prediction of mass-transfer events.

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arXiv2018

Understanding the orbital periods of CEMP-s stars

The chemical enrichments detected in CEMP-$s$ stars are believed to be the consequence of a mass-transfer episode from a now extinct AGB primary star. This hypothesis is borne out by the fact that most CEMP-$s$ stars exhibit RV variations suggesting that they are in binary systems. We use the period distribution of an observed sample of CEMP-$s$ stars to investigate the constraints it imposes on our models of binary evolution and on the properties of the metal-poor binary population in the Galactic halo. We simulate binary populations using different assumptions about the initial period distribution and the mass-transfer process, and we compare the predicted period distributions of our synthetic CEMP-$s$ stars with the observed one. With a set of default assumptions often made in binary population-synthesis studies, the percentage of observed CEMP-$s$ systems with periods shorter than 2000 days is underestimated by almost a factor of 3, and by a factor of 2 between 3000 and 10000 days, while 40% of the simulated systems have periods longer than 10000 days, which is approximately the longest measured period among CEMP-$s$ stars. To reconcile the results of the models with the orbital properties of observed CEMP-$s$ stars, one or both of the following conditions are necessary: ($i$) the specific angular momentum carried away by the material that escapes the binary system is 2-5 times higher than currently predicted by analytical models and hydrodynamical simulations of wind mass transfer, and ($ii$) the initial period distribution of very metal-poor binary stars is significantly different from that observed in the solar vicinity and weighted towards periods shorter than about 10000 days. Our simulations show that some, perhaps all, of the observed CEMP-$s$ stars with apparently constant RV could be undetected binaries with periods longer than 10000 days, but (...)

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arXiv2016

Origin of central abundances in the hot intra-cluster medium - II. Chemical enrichment and supernova yield models

The hot intra-cluster medium (ICM) is rich in metals, which are synthesised by supernovae (SNe) and accumulate over time into the deep gravitational potential well of clusters of galaxies. Since most of the elements visible in X-rays are formed by type Ia (SNIa) and/or core-collapse (SNcc) supernovae, measuring their abundances gives us direct information on the nucleosynthesis products of billions of SNe since the epoch of the star formation peak (z~2-3). In this study, we compare the most accurate average X/Fe abundance ratios (compiled in a previous work from XMM-Newton EPIC and RGS observations of 44 galaxy clusters, groups, and ellipticals), representative of the chemical enrichment in the nearby ICM, to various SNIa and SNcc nucleosynthesis models found in the literature. The use of a SNcc model combined to any favoured standard SNIa model (deflagration or delayed-detonation) fails to reproduce our abundance pattern. In particular, the Ca/Fe and Ni/Fe ratios are significantly underestimated by the models. We show that the Ca/Fe ratio can be reproduced better, either by taking a SNIa delayed-detonation model that matches the observations of the Tycho supernova remnant, or by adding a contribution from the Ca-rich gap transient SNe, whose material should easily mix into the hot ICM. On the other hand, the Ni/Fe ratio can be reproduced better by assuming that both deflagration and delayed-detonation SNIa contribute in similar proportions to the ICM enrichment. In either case, the fraction of SNIa over the total number of SNe (SNIa+SNcc) contributing to the ICM enrichment ranges within 29-45%. This fraction is found to be systematically higher than the corresponding SNIa/SNe fraction contributing to the enrichment of the proto-solar environnement (15-25%). We also discuss and quantify two useful constraints on both SNIa and SNcc that can be inferred from the ICM abundance ratios.

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arXiv2013

AGB nucleosynthesis at low metallicity: what can we learn from carbon- and s-elements-enhanced metal-poor stars

CEMP-s stars are very metal-poor stars with enhanced abundances of carbon and s-process elements. They form a significant proportion of the very metal-poor stars in the Galactic halo and are mostly observed in binary systems. This suggests that the observed chemical anomalies are due to mass accretion in the past from an asymptotic giant branch (AGB) star. Because CEMP-s stars have hardly evolved since their formation, the study of their observed abundances provides a way to probe our models of AGB nucleosynthesis at low metallicity. To this end we included in our binary evolution model the results of the latest models of AGB nucleosynthesis and we simulated a grid of 100,000 binary stars at metallicity Z=0.0001 in a wide range of initial masses and separations. We compared our modelled stars with a sample of 60 CEMP-s stars from the SAGA database of metal-poor stars. For each observed CEMP-s star of the sample we found the modelled star that reproduces best the observed abundances. The result of this comparison is that we are able to reproduce simultaneously the observed abundance of the elements affected by AGB nucleosynthesis (e.g. C, Mg, s-elements) for about 60% of the stars in the sample.

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arXiv2013

Structure and evolution of high-mass stellar mergers

In young dense clusters repeated collisions between massive stars may lead to the formation of a very massive star (above 100 Msun). In the past the study of the long-term evolution of merger remnants has mostly focussed on collisions between low-mass stars (up to about 2 Msun) in the context of blue-straggler formation. The evolution of collision products of more massive stars has not been as thoroughly investigated. In this paper we study the long-term evolution of a number of stellar mergers formed by the head-on collision of a primary star with a mass of 5-40 Msun with a lower mass star at three points in its evolution in order to better understand their evolution. We use smooth particle hydrodynamics (SPH) calculations to model the collision between the stars. The outcome of this calculation is reduced to one dimension and imported into a stellar evolution code. We follow the subsequent evolution of the collision product through the main sequence at least until the onset of helium burning. We find that little hydrogen is mixed into the core of the collision products, in agreement with previous studies of collisions between low-mass stars. For collisions involving evolved stars we find that during the merger the surface nitrogen abundance can be strongly enhanced. The evolution of most of the collision products proceeds analogously to that of normal stars with the same mass, but with a larger radius and luminosity. However, the evolution of collision products that form with a hydrogen depleted core is markedly different from that of normal stars with the same mass. They undergo a long-lived period of hydrogen shell burning close to the main-sequence band in the Hertzsprung-Russell diagram and spend the initial part of core helium burning as compact blue supergiants.

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arXiv2013

Massive Binary Stars and Self-Enrichment of Globular Clusters

Globular clusters contain many stars with surface abundance patterns indicating contributions from hydrogen burning products, as seen in the anti-correlated elemental abundances of e.g. sodium and oxygen, and magnesium and aluminium. Multiple generations of stars can explain this phenomenon, with the second generation forming from a mixture of pristine gas and ejecta from the first generation. We show that massive binary stars may be a source of much of the material that makes this second generation of stars. Mass transfer in binaries is often non-conservative and the ejected matter moves slowly enough that it can remain inside a globular cluster and remain available for subsequent star formation. Recent studies show that there are more short-period massive binaries than previously thought, hence also more stars that interact and eject nuclear-processed material.

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arXiv2013

Population synthesis of triple systems in the context of mergers of carbon-oxygen white dwarfs

Hierarchical triple systems are common among field stars yet their long-term evolution is poorly understood theoretically. In such systems Kozai cycles can be induced in the inner binary system during which the inner orbit eccentricity and the inclination between both binary orbits vary periodically. These cycles, combined with tidal friction and gravitational wave emission, can significantly affect the inner binary evolution. To investigate these effects quantitatively we perform a population synthesis study of triple systems and focus on evolutionary paths that lead to mergers of carbon-oxygen (CO) white dwarfs (WDs), which constitute an important candidate progenitor channel for type Ia supernovae (SNe Ia). We approach this problem by Monte Carlo sampling from observation-based distributions of systems within the primary mass range 1.0 - 6.5 M_Sun and inner orbit semi-major axes a_1 and eccentricities e_1 satisfying a_1 (1-e_1^2) > 12 AU, i.e. non-interacting in the absence of a tertiary component. We evolve these systems by means of a newly developed algorithm that couples secular triple dynamics with an existing binary population synthesis code. We find that the tertiary significantly alters the inner binary evolution in about 24% of all sampled systems. In particular, we find several channels leading to CO WD mergers. Amongst these is a novel channel in which a collision occurs in wide inner binary systems as a result of extremely high eccentricities induced by Kozai cycles. With assumptions on which CO WD mergers lead to a SN Ia explosion we estimate the SNe Ia delay time distribution resulting from triples and compare to a binary population synthesis study and to observations. Although we find that our triple rate is low, we have determined a lower limit of the triple-induced SNe Ia rate and further study is needed that includes triples with initially tighter inner orbits.

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arXiv2012

The occurrence of nitrogen-enhanced metal-poor stars: implications for the initial mass function in the early Galactic halo

Most carbon-enhanced metal-poor (CEMP) stars are thought to result from past mass transfer of He-burning material from an asymptotic giant branch (AGB) star to a low-mass companion star, which we now observe as a CEMP star. Because AGB stars of intermediate mass efficiently cycle carbon into nitrogen in their envelopes, the same evolution scenario predicts the existence of a population of nitrogen-enhanced metal-poor (NEMP) stars, with [N/Fe] > 1 and [N/C] > 0.5. Such NEMP stars are rare, although their occurrence depends on metallicity: they appear to be more common at [Fe/H] < -2.8 by about a factor of 10 compared to less metal-poor stars. We analyse the observed sample of metal-poor stars with measurements of both carbon and nitrogen to derive firm constraints on the occurrence of NEMP stars as a function of metallicity. We compare these constraints to binary population synthesis calculations in which we vary the initial distributions of mass, mass ratio and binary orbital periods. We show that the observed paucity of NEMP stars at [Fe/H] > -2.8 does not allow for large modifications in the initial mass function, as have been suggested in the literature to account for the high frequency of CEMP stars. The situation at lower metallicity is less clear, and we do not currently have stellar models to perform this comparison for [Fe/H] < -2.8. However, unless intermediate-mass AGB stars behave very differently at such low metallicity, the observed NEMP frequency at [Fe/H] < -2.8 appears incompatible with the top-heavy forms of the initial mass function suggested in the literature.

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arXiv2007

Carbon-enhanced metal-poor stars and thermohaline mixing

One possible scenario for the formation of carbon-enhanced metal-poor stars is the accretion of carbon-rich material from a binary companion which may no longer visible. It is generally assumed that the accreted material remains on the surface of the star and does not mix with the interior until first dredge-up. However, thermohaline mixing should mix the accreted material with the original stellar material as it has a higher mean molecular weight. We investigate the effect that this has on the surface abundances by modelling a binary system of metallicity Z=0.0001 with a 2 solar mass primary star and a 0.74 solar mass secondary star in an initial orbit of 4000 days. The accretion of material from the wind of the primary leads to the formation of a carbon-rich secondary. We find that the accreted material mixes fairly rapidly throughout 90% of the star, with important consequences for the surface composition. Models with thermohaline mixing predict very different surface abundances after first dredge-up compared to canonical models of stellar evolution.

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arXiv2005

A Complete N-body Model of the Old Open Cluster M67

The old open cluster M67 is an ideal testbed for current cluster evolution models because of its dynamically evolved structure and rich stellar populations that show clear signs of interaction between stellar, binary and cluster evolution. Here we present the first truly direct N-body model for M67, evolved from zero age to 4 Gyr taking full account of cluster dynamics as well as stellar and binary evolution. Our preferred model starts with 12000 single stars and 12000 binaries placed in a Galactic tidal field at 8.0 kpc from the Galactic Centre. Our choices for the initial conditions and for the primordial binary population are explained in detail. At 4 Gyr, the age of M67, the total mass has reduced by 90% as a result of mass loss and stellar escapes. The mass and half-mass radius of luminous stars in the cluster are a good match to observations although the model is more centrally concentrated than observations indicate. The stellar mass and luminosity functions are significantly flattened by preferential escape of low-mass stars. We find that M67 is dynamically old enough that information about the initial mass function is lost, both from the current luminosity function and from the current mass fraction in white dwarfs. The model contains 20 blue stragglers at 4 Gyr which is slightly less than the 28 observed in M67. Nine are in binaries. The blue stragglers were formed by a variety of means and we find formation paths for the whole variety observed in M67. Both the primordial binary population and the dynamical cluster environment play an essential role in shaping the population. A substantial population of short-period primordial binaries (with periods less than a few days) is needed to explain the observed number of blue stragglers in M67.

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arXiv2002

MODEST-1: Integrating Stellar Evolution and Stellar Dynamics

We summarize the main results from MODEST-1, the first workshop on MOdeling DEnse STellar systems. Our goal is to go beyond traditional population synthesis models, by introducing dynamical interactions between single stars, binaries, and multiple systems. The challenge is to define and develop a software framework to enable us to combine in one simulation existing computer codes in stellar evolution, stellar dynamics, and stellar hydrodynamics. With this objective, the workshop brought together experts in these three fields, as well as other interested astrophysicists and computer scientists. We report here our main conclusions, questions and suggestions for further steps toward integrating stellar evolution and stellar (hydro)dynamics.

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arXiv2002

Could Edge-Lit Type Ia Supernovae be Standard Candles

The progenitors of Type Ia supernovae (SNe Ia) have not been identified. Though they are no longer fashionable we investigate the consequences if a significant number of SNe Ia were edge-lit detonations (ELDs) of carbon/oxygen white dwarfs that have accreted a critical mass of helium. Our best understanding of the Phillips relation between light curve speed and peak luminosity assigns both these phenomena to the amount of $^{56}$Ni produced. In ELDs there are two sites of $^{56}$Ni synthesis. If the peak luminosity is determined primarily by the C/O ratio in the core it is primarily a function of its progenitor's initial mass. If the light curve decay speed is determined by the total mass of iron group elements ejected this is a function of the total mass of the ELD at the time of explosion. In general these two masses are correlated and an empirical relation between peak luminosity and light curve shape can be expected. However when we perform population synthesis for progenitors of different metallicities we find a systematic shift in this relation that would make distant SNe Ia fainter than those nearby. The abundances of alpha-rich isotopes, such as $^{44}$Ca, in the solar system indicate that only about 40 per cent of SNe Ia are edge-lit so any systematic effect that could be present would be correspondingly diluted. If we examine only the small subset of ELDs that accrete from a naked helium star, rather than a He white dwarf, the systematic effect disappears.

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arXiv2002

Helium-Star Mass Loss and its Implications for Black-Hole Formation and Supernova Progenitors

Recently the observationally derived stellar-wind mass-loss rates for Wolf-Rayet stars, or massive naked helium stars, have been revised downwards by a substantial amount. We present evolutionary calculations of helium stars incorporating such revised mass-loss rates, as well as mass transfer to a close compact binary companion. Our models reach final masses well in excess of 10 Msun, consistent with the observed masses of black holes in X-ray binaries. This resolves the discrepancy found with previously assumed high mass-loss rates between the final masses of stars which spend most of their helium-burning lifetime as Wolf-Rayet stars (about 3 Msun) and the minimum observed black hole masses (6 Msun). Our calculations also suggest that there are two distinct classes of progenitors for Type Ic supernovae: one with very large initial masses (> 35 Msun), which are still massive when they explode and leave black hole remnants, and one with moderate initial masses (about 12 - 20 Msun) undergoing binary interaction, which end up with small pre-explosion masses and leave neutron star remnants.

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arXiv2002

Evolution of binary stars and the effect of tides on binary populations

We present a rapid binary evolution algorithm that enables modelling of even the most complex binary systems. In addition to all aspects of single star evolution, features such as mass transfer, mass accretion, common-envelope evolution, collisions, supernova kicks and angular momentum loss mechanisms are included. In particular, circularization and synchronization of the orbit by tidal interactions are calculated for convective, radiative and degenerate damping mechanisms. We use this algorithm to study the formation and evolution of various binary systems. We also investigate the effect that tidal friction has on the outcome of binary evolution. Using the rapid binary code, we generate a series of large binary populations and evaluate the formation rate of interesting individual species and events. By comparing the results for populations with and without tidal friction we quantify the hitherto ignored systematic effect of tides and show that modelling of tidal evolution in binary systems is necessary in order to draw accurate conclusions from population synthesis work. Tidal synchronism is important but because orbits generally circularize before Roche-lobe overflow the outcome of the interactions of systems with the same semi-latus rectum is almost independent of eccentricity. It is not necessary to include a distribution of eccentricities in population synthesis of interacting binaries, however, the initial separations should be distributed according to the observed distribution of semi-latera recta rather than periods or semi-major axes.

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arXiv2000

Direct N-body Modelling of Stellar Populations: Blue Stragglers in M67

We present a state-of-the-art N-body code which includes a detailed treatment of stellar and binary evolution as well as the cluster dynamics. This code is ideal for investigating all aspects relating to the evolution of star clusters and their stellar populations. It is applicable to open and globular clusters of any age. We use the N-body code to model the blue straggler population of the old open cluster M67. Preliminary calculations with our binary population synthesis code show that binary evolution alone cannot explain the observed numbers or properties of the blue stragglers. On the other hand, our N-body model of M67 generates the required number of blue stragglers and provides formation paths for all the various types found in M67. This demonstrates the effectiveness of the cluster environment in modifying the nature of the stars it contains and highlights the importance of combining dynamics with stellar evolution. We also perform a series of N = 10000 simulations in order to quantify the rate of escape of stars from a cluster subject to the Galactic tidal field.

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arXiv1999

The Spatial Distribution of Coalescing Neutron Star Binaries: Implications for Gamma-Ray Bursts

We find the distribution of coalescence times, birthrates, spatial velocities, and subsequent radial offsets of coalescing neutron stars (NSs) in various galactic potentials accounting for large asymmetric kicks introduced during a supernovae. The birthrates of bound NS-NS binaries are quite sensitive to the magnitude of the kick velocities but are, nevertheless, similar (~10 per Galaxy per Myr) to previous population synthesis studies. The distribution of merger times since zero-age main sequence is, however, relatively insensitive to the choice of kick velocities. With a median merger time of ~100 Myr, we find that compact binaries should closely trace the star formation rate in the Universe. In a range of plausible galactic potentials (M_galaxy > 3 x 10^10 M_solar) the median radial offset of a NS-NS mergers is less than 10 kpc. At a redshift of z=1 (with H_0 = 65 km/s/Mpc and Omega = 0.2), this means that half the coalescences should occur within ~1.3 arcsec from the host galaxy. In all but the most shallow potentials, ninety percent of NS-NS binaries merge within 30 kpc of the host. We find that although the spatial distribution of coalescing neutron star binaries is consistent with the close spatial association of known optical afterglows of gamma-ray bursts (GRBs) with faint galaxies, a non-negligible fraction (~15 percent) of GRBs should occur well outside (>30 kpc) dwarf galaxy hosts. Extinction due to dust in the host, projection of offsets, and a range in interstellar medium densities confound the true distribution of NS-NS mergers around galaxies with an observable set of optical transients/galaxy offsets.

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