arXiv subjects

Robert G. Izzard

Publications and source records attributed to Robert G. Izzard.

At least 19 recordsLinked to original sources

arXiv2026

A method for constructing the joint mass function of binary stars

The initial mass function (IMF) describes the distribution of stellar masses in a population of newly born stars and is amongst the most fundamental concepts in astrophysics. It is not only the direct result of the star formation process but it also explains the evolution of galaxies' luminosities, metal yields, star-formation efficiencies, and supernova production rates. Because most stars exist in binary systems, however, a full statistical account of stellar mass requires not the IMF but rather the joint distribution of a binary population's primary- and secondary-star masses. This joint distribution must respect the IMF of the stars from which the population has been assembled as well as the distribution of mass ratios that results from the assembly mechanism. Despite its importance, this joint distribution is known only in the case of random pairing. Here we present a method for constructing it in the general case. We also illustrate the use of our method by recovering the known result for random pairing and by finding the previously unknown result for uniform pairing.

astro-ph.SR↗

arXiv2025

Stellar velocity distributions in binary-rich ultrafaint dwarf galaxies

Ultrafaint dwarf (UFD) galaxies are dominated by dark matter, the distribution of which may be inferred from the kinematics of that galaxy's stellar population. Star-by-star observations are available for the satellite UFD galaxies of the Milky Way, making them uniquely good laboratories in which to test cosmological predictions at the smallest scales. However, the kinematics of these galaxies are complicated by the presence of binary stars, which alter the stellar velocity distribution. In particular these binary stars increase the galaxy's stellar velocity dispersion, which is related to the total galactic mass by the virial theorem. Without correctly eliminating or accounting for binary stars we may therefore overestimate the masses of UFD galaxies or even confuse globular clusters for UFD galaxies. Here we write down the probability density function for the observed line-of-sight (LOS) velocity of a stellar population containing both visual and spectroscopic binary stars, which we then use to determine the effect of those binary stars on the observed LOS velocity dispersion. For the coldest UFD galaxies the fractional increase in LOS velocity dispersion is of order one and for the coldest globular clusters is of order 100. However, if the stellar initial mass function is bottom light, as it may be for UFD galaxies and globular clusters, then both of these values increase by half a dex.

astro-ph.GA↗

arXiv2025

A stellar evolutionary grid for binary population synthesis: from the main sequence to helium ignition

Mass changes due to strong stellar winds and binary mass transfer have a dramatic impact on the consequent evolution of stars. This is generally not accounted for in population synthesis codes which are built using single star evolution models from full stellar evolution codes. We produce a new grid of models using the 1D stellar evolution code \textit{MESA} which includes models with a range of core mass fractions, at each total stellar mass, evolved from core hydrogen exhaustion to the onset of core helium burning. The model grid is used to produce an interpolation table, designed for use in population synthesis codes such as \textit{binary\_c}. We test the interpolation table with a simple integration algorithm to evaluate its capability of reproducing accurate evolutionary tracks. We test our method for stellar masses in the range $M=1-16~\mathrm{M}_\odot$ and show that it successfully reproduces the Hertzpsrung-gap and giant branch lifetime, the core mass at helium ignition and the HR diagram.

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arXiv2024

Anomalously low-mass core-He-burning star in NGC 6819 as a post-common-envelope phase product

Precise masses of red-giant stars enable a robust inference of their ages, but there are cases where these age estimates are highly precise yet very inaccurate. Examples are core-helium-burning (CHeB) stars that have lost more mass than predicted by standard single-star evolutionary models. Members of star clusters in the ${\it Kepler}$ database represent a unique opportunity to identify such stars, because they combine exquisite asteroseismic constraints with independent age information. In this work we focus on the single, metal-rich, Li-rich, low-mass, CHeB star KIC4937011, which is a member of the open cluster NGC 6819 (turn-off mass of $\approx 1.6 \, M_\odot$, i.e. age of $\approx 2.4$ Gyr). This star has $\approx 1 \, M_\odot$ less mass than expected for its age and metallicity, which could be explained by binary interactions or mass-loss along the red-giant branch (RGB). To infer formation scenarios for this object, we perform a Bayesian analysis by combining the binary stellar evolutionary framework $\texttt{binary_c v2.2.3}$ with the dynamic nested sampling approach contained in the $\texttt{dynesty v2.1.1}$ package. We find that this star is likely the result of a common-envelope evolution (CEE) phase during the RGB stage of the primary star in which the low-mass ($<0.71 \, M_\odot$) main sequence companion does not survive. During the CEE phase $\approx 1 \, M_\odot$ of material is ejected from the system, and the final star reaches the CHeB stage after helium flashes as if it were a single star of mass $\approx 0.7 \, M_\odot$, which is what we observe today. Although the proposed scenario is consistent with photometric and spectroscopic observations, a quantitative comparison with detailed stellar evolution calculations is needed to quantify the systematic skewness of radius, luminosity, and effective temperature distributions towards higher values than observations.

astro-ph.SR↗

arXiv2024

Nova contributions to the chemical evolution of the Milky Way

Context. The explosive burning that drives nova eruptions results in unique nucleosynthesis that heavily over-produces certain isotopes relative to the solar abundance. However, novae are often ignored when considering the chemical evolution of our Galaxy due to their low ejecta masses. Aims. In this work, we use previously computed synthetic nova populations and the galactic chemical evolution code OMEGA+ to assess the impact that novae have on the evolution of stable elemental and isotopic abundances. Methods. We combine populations of novae computed using the binary population synthesis code binary_c with the galactic chemical evolution code OMEGA+ and detailed, white dwarf mass-dependent nova yields to model the nucleosynthetic contributions of novae to the evolution of the Milky Way. We consider three different nova yield profiles, each corresponding to a different set of nova yield calculations. Results. Despite novae from low-mass white dwarfs (WDs) dominating nova ejecta contributions, we find that novae occurring on massive WDs are still able to contribute significantly to many isotopes, particularly those with high mass numbers. We find that novae can produce up to 35% of the Galactic 13C and 15N mass by the time the model Galaxy reaches [Fe/H] = 0, and earlier in the evolution of the Galaxy (between [Fe/H] = -2 and -1) novae may have been the dominant source of 15N. Predictions for [13C/Fe], [15N/Fe], 12C/13C, and 14N/15N abundances ratios vary by up to 0.2 dex at [Fe/H] = 0 and by up to 0.7 dex in [15N/Fe] and 14N/15N between [Fe/H] = -2 and -1 (corresponding approximately to Galactic ages of 170 Myr and 1 Gyr in our model). The Galactic evolution of other stable isotopes (excluding Li) is not noticeably affected by including novae.

astro-ph.GA↗

arXiv2024

Circumbinary discs for stellar population models

We develop a rapid algorithm for the evolution of stable, circular, circumbinary discs suitable for parameter estimation and population synthesis modelling. Our model includes disc mass and angular momentum changes, accretion on to the binary stars, and binary orbital eccentricity pumping. We fit our model to the post-asymptotic giant branch (post-AGB) circumbinary disc around IRAS 08544-4431, finding reasonable agreement despite the simplicity of our model. Our best-fitting disc has a mass of about $0.01\, \mathrm{M}_{\odot }$ and angular momentum $2.7\times 10^{52}\, \mathrm{g}\, \mathrm{cm}^{2}\, \mathrm{s}^{-1}\simeq 9 \,\mathrm{M}_{\odot }\, \mathrm{km}\, \mathrm{s}^{-1}\, \mathrm{au}$, corresponding to 0.0079 and 0.16 of the common-envelope mass and angular momentum, respectively. The best-fitting disc viscosity is $α_\mathrm{disc} = 5 \times 10^{-3}$ and our tidal torque algorithm can be constrained such that the inner edge of the disc $R_{\mathrm{in}}\sim 2a$. The inner binary eccentricity reaches about 0.13 in our best-fitting model of IRAS 08544-4431, short of the observed 0.22. The circumbinary disc evaporates quickly when the post-AGB star reaches a temperature of $\sim \! 6\times 10^4\, \mathrm{K}$, suggesting that planetismals must form in the disc in about $10^{4}\, \mathrm{yr}$ if secondary planet formation is to occur, while accretion from the disc on to the stars at about 10 times the inner-edge viscous rate can double the disc lifetime.

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arXiv2024

libcdict: fast dictionaries in C

A common requirement in science is to store and share large sets of simulation data in an efficient, nested, flexible and human-readable way. Such datasets contain number counts and distributions, i.e. histograms and maps, of arbitrary dimension and variable type, e.g. floating-point number, integer or character string. Modern high-level programming languages like Perl and Python have associated arrays, knowns as dictionaries or hashes, respectively, to fulfil this storage need. Low-level languages used more commonly for fast computational simulations, such as C and Fortran, lack this functionality. We present libcdict, a C dictionary library, to solve this problem. Libcdict provides C and Fortran application programming interfaces (APIs) to native dictionaries, called cdicts, and functions for cdicts to load and save these as JSON and hence for easy interpretation in other software and languages like Perl, Python and R.

cs.DS↗

arXiv2023

Population synthesis of Be X-ray binaries: metallicity dependence of total X-ray outputs

X-ray binaries (XRBs) are thought to regulate cosmic thermal and ionization histories during the Epoch of Reionization and Cosmic Dawn ($z\sim 5-30$).Theoretical predictions of the X-ray emission from XRBs are important for modelling such early cosmic evolution. Nevertheless, the contribution from Be-XRBs, powered by accretion of compact objects from decretion disks around rapidly rotating O/B stars, has not been investigated systematically. Be-XRBs are the largest class of high-mass XRBs (HMXBs) identified in local observations and are expected to play even more important roles in metal-poor environments at high redshifts. In light of this, we build a physically motivated model for Be-XRBs based on recent hydrodynamic simulations and observations of decretion disks. Our model is able to reproduce the observed population of Be-XRBs in the Small Magellanic Cloud with appropriate initial conditions and binary stellar evolution parameters. We derive the X-ray output from Be-XRBs as a function of metallicity in the (absolute) metallicity range $Z\in [10^{-4},0.03]$ with a large suite of binary population synthesis (BPS) simulations. The simulated Be-XRBs can explain a non-negligible fraction ($\gtrsim 30\%$) of the total X-ray output from HMXBs observed in nearby galaxies for $Z\sim 0.0003-0.02$. The X-ray luminosity per unit star formation rate from Be-XRBs in our fiducial model increases by a factor of $\sim 8$ from $Z=0.02$ to $Z=0.0003$, which is similar to the trend seen in observations of all types of HMXBs. We conclude that Be-XRBs are potentially important X-ray sources that deserve greater attention in BPS of XRBs.

astro-ph.HE↗

arXiv2023

The impact of binary stars on the dust and metal evolution of galaxies

We present detailed implementations of (a) binary stellar evolution (using binary_c) and (b) dust production and destruction into the cosmological semi-analytic galaxy evolution simulation, L-Galaxies. This new version of L-Galaxies is compared to a version assuming only single stars and to global and spatially-resolved observational data across a range of redshifts ($z$). We find that binaries have a negligible impact on the stellar masses, gas masses, and star formation rates of galaxies only if the total mass ejected by massive stars is unchanged. This is because massive stars determine the strength of supernova (SN) feedback, which in turn regulates galaxy growth. Binary effects, such as common envelope ejection and novae, affect carbon and nitrogen enrichment in galaxies, however heavier alpha elements are more affected by the choice of SN and wind yields. Unlike many other simulations, the new L-Galaxies reproduces observed dust-to-metal (DTM) and dust-to-gas (DTG) ratios at $z\sim{}0-4$. This is mainly due to shorter dust accretion timescales in dust-rich environments. However, dust masses are under-predicted at $z>4$, highlighting the need for enhanced dust production at early times in simulations, possibly accompanied by increased star formation. On sub-galactic scales, there is very good agreement between L-Galaxies and observed dust and metal radial profiles at $z=0$. A drop in DTM ratio is also found in diffuse, low-metallicity regions, contradicting the assumption of a universal value. We hope that this work serves as a useful template for binary stellar evolution implementations in other cosmological simulations in future.

astro-ph.GA↗

arXiv2023

Aluminium-26 production in low- and intermediate-mass binary systems

Aluminium-26 is a radioactive isotope which can be synthesized within asymptotic giant branch (AGB) stars, primarily through hot bottom burning. Studies exploring $^{26}$Al production within AGB stars typically focus on single-stars; however, observations show that low- and intermediate-mass stars commonly exist in binaries. We use the binary population synthesis code binary_c to explore the impact of binary evolution on $^{26}$Al yields at solar metallicity both within individual AGB stars and a low/intermediate-mass stellar population. We find the key stellar structural condition achieving most $^{26}$Al overproduction is for stars to enter the thermally-pulsing AGB (TP-AGB) phase with small cores relative to their total masses, allowing those stars to spend abnormally long times on the TP-AGB compared to single-stars of identical mass. Our population with a binary fraction of 0.75 has an $^{26}$Al weighted population yield increase of $25\%$ compared to our population of only single-stars. Stellar-models calculated from the Mt Stromlo/Monash Stellar Structure Program, which we use to test our results from binary_c and closely examine the interior structure of the overproducing stars, support our binary_c results only when the stellar envelope gains mass after core-He depletion. Stars which gain mass before core-He depletion still overproduce $^{26}$Al, but to a lesser extent. This introduces some physical uncertainty into our conclusions as $55\%$ of our $^{26}$Al overproducing stars gain envelope mass through stellar wind accretion onto pre-AGB objects. Our work highlights the need to consider binary influence on the production of $^{26}$Al.

astro-ph.SR↗

arXiv2023

Detailed equilibrium and dynamical tides: impact on circularization and synchronization in open clusters

Binary stars evolve into chemically-peculiar objects and are a major driver of the Galactic enrichment of heavy elements. During their evolution they undergo interactions, including tides, that circularize orbits and synchronize stellar spins, impacting both individual systems and stellar populations. Using Zahn's tidal theory and MESA main-sequence model grids, we derive the governing parameters $λ_{lm}$ and $E_2$, and implement them in the new MINT library of the stellar population code BINARY_C. Our MINT equilibrium tides are 2 to 5 times more efficient than the ubiquitous BSE prescriptions while the radiative-tide efficiency drops sharply with increasing age. We also implement precise initial distributions based on bias-corrected observations. We assess the impact of tides and initial orbital-parameter distributions on circularization and synchronization in eight open clusters, comparing synthetic populations and observations through a bootstrapping method. We find that changing the tidal prescription yields no statistically-significant improvement as both calculations typically lie within 0.5$σ$. The initial distribution, especially the primordial concentration of systems at $\log_{10}(P/{\rm d}) \approx 0.8, e\approx 0.05$ dominates the statistics even when artificially increasing tidal strength. This confirms the inefficiency of tides on the main sequence and shows that constraining tidal-efficiency parameters using the $e-\log_{10}(P/{\rm d})$ distribution alone is difficult or impossible. Orbital synchronization carries a more striking age-dependent signature of tidal interactions. In M35 we find twice as many synchronized rotators in our MINT calculation as with BSE. This measure of tidal efficiency is verifiable with combined measurements of orbital parameters and stellar spins.

astro-ph.SR↗

arXiv2023

An optimal envelope ejection efficiency for merging neutron stars

We use the rapid binary stellar evolution code BINARY_C to estimate the rate of merging neutron stars with numerous combinations of envelope ejection efficiency and natal kick dispersion. We find a peak in the local rate of merging neutron stars around $α\approx 0.3$$-$$0.4$, depending on the metallicity, where $α$ is the efficiency of utilising orbital energy to unbind the envelope. The peak height decreases with increasing electron-capture supernova kick dispersion $σ_\mathrm{ECSN}$. We explain the peak as a competition between the total number of systems that survive the common-envelope phase increasing with $α$ and their separation, which increases with $α$ as well. Increasing $α$ reduces the fraction of systems that merge within a time shorter than the age of the Universe and results in different mass distributions for merging and non-merging double neutron stars. This offers a possible explanation for the discrepancy between the Galactic double neutron star mass distribution and the observed massive merging neutron star event GW190425. Within the $α$$-$$σ_\mathrm{ECSN}$ parameter space that we investigate, the rate of merging neutron stars spans several orders of magnitude up to more than $1\times 10^{3} \, \mathrm{Gpc}^{-3}\,\mathrm{yr}^{-1}$ and can be higher than the observed upper limit or lower than the observed lower limit inferred thus far from merging neutron stars detected by gravitational waves. Our results stress the importance of common-envelope physics for the quantitative prediction and interpretation of merging binary neutron star events in this new age of gravitational wave astronomy.

astro-ph.HE↗

arXiv2022

Viability of novae as sources of Galactic lithium

Of all the light elements, the evolution of lithium (Li) in the Milky Way is perhaps the most difficult to explain. Li is difficult to synthesize and is easily destroyed, making most stellar sites unsuitable for producing Li in sufficient quantities to account for the proto-solar abundance. For decades, novae have been proposed as a potential explanation to this 'Galactic Li problem', and the recent detection of 7Be in the ejecta of multiple nova eruptions has breathed new life into this theory. In this work, we assess the viability of novae as dominant producers of Li in the Milky Way. We present the most comprehensive treatment of novae in a galactic chemical evolution code to date, testing theoretical- and observationally-derived nova Li yields by integrating metallicity-dependent nova ejecta profiles computed using the binary population synthesis code binary c with the galactic chemical evolution code OMEGA+. We find that our galactic chemical evolution models which use observationally-derived Li yields account for the proto-solar Li abundance very well, while models relying on theoretical nova yields cannot reproduce the proto-solar observation. A brief exploration of physical uncertainties including single-stellar yields, the metallicity resolution of our nova treatment, common-envelope physics, and nova accretion efficiencies indicates that this result is robust to physical assumptions. Scatter within the observationally-derived Li yields in novae is identified as the primary source of uncertainty, motivating further observations of 7Be in nova ejecta.

astro-ph.SR↗

arXiv2021

The impact of metallicity on nova populations

The metallicity of a star affects its evolution in a variety of ways, changing stellar radii, luminosities, lifetimes, and remnant properties. In this work, we use the population synthesis code binary_c to study how metallicity affects novae in the context of binary stellar evolution. We compute a 16-point grid of metallicities ranging from $Z=10^{-4}$ to 0.03, presenting distributions of nova white dwarf masses, accretion rates, delay-times, and initial system properties at the two extremes of our 16-point metallicity grid. We find a clear anti-correlation between metallicity and the number of novae produced, with the number of novae at $Z=0.03$ roughly half that at $Z=10^{-4}$. The white dwarf mass distribution has a strong systematic variation with metallicity, while the shape of the accretion rate distribution is relatively insensitive. We compute a current nova rate of approximately 33 novae per year for the Milky Way, a result consistent with observational estimates relying on extra-Galactic novae but an under-prediction relative to observational estimates relying on Galactic novae. However, the shape of our predicted Galactic white dwarf mass distribution differs significantly to existing observationally derived distributions, likely due to our underlying physical assumptions. In M31, we compute a current nova rate of approximately 36 novae per year, under-predicting the most recent observational estimate of $65^{+15}_{-16}$. Finally, we conclude that when making predictions about currently observable nova rates in spiral galaxies, or stellar environments where star formation has ceased in the distant past, metallicity can likely be considered of secondary importance compared to uncertainties in binary stellar evolution.

astro-ph.SR↗

arXiv2021

Population synthesis of accreting white dwarfs: Rates and evolutionary pathways of H and He novae

Novae are some of the most commonly detected optical transients and have the potential to provide valuable information about binary evolution. Binary population synthesis codes have emerged as the most effective tool for modelling populations of binary systems, but such codes have traditionally employed greatly simplified nova physics, precluding detailed study. In this work, we implement a model treating H and He novae as individual events into the binary population synthesis code \binaryc. This treatment of novae represents a significant improvement on the `averaging' treatment currently employed in modern population synthesis codes. We discuss the evolutionary pathways leading to these phenomena and present nova event rates and distributions of several important physical parameters. Most novae are produced on massive white dwarfs, with approximately 70 and 55 per cent of nova events occurring on O/Ne white dwarfs for H and He novae respectively. Only 15 per cent of H-nova systems undergo a common-envelope phase, but these systems are responsible for the majority of H nova events. All He-accreting He-nova systems are considered post-common-envelope systems, and almost all will merge with their donor star in a gravitational-wave driven inspiral. We estimate the current annual rate of novae in M31 (Andromeda) to be approximately $41 \pm 4$ for H novae, underpredicting the current observational estimate of $65^{+15}_{-16}$, and $0.14\pm0.015$ for He novae. When varying common-envelope parameters, the H nova rate varies between 20 and 80 events per year.

astro-ph.SR↗

arXiv2018

Post-AGB discs from common-envelope evolution

Post-asymptotic giant branch (post-AGB) stars with discs are all binaries. Many of these binaries have orbital periods between 100 and 1000 days so cannot have avoided mass transfer between the AGB star and its companion, likely through a common-envelope type interaction. We report on preliminary results of our project to model circumbinary discs around post-AGB stars using our binary population synthesis code binary_c. We combine a simple analytic thin-disc model with binary stellar evolution to estimate the impact of the disc on the binary, and vice versa, fast enough that we can model stellar populations and hence explore the rather uncertain parameter space involved with disc formation. We find that, provided the discs form with sufficient mass and angular momentum, and have an inner edge that is relatively close to the binary, they can both prolong the life of their parent post-AGB star and pump the eccentricity of orbits of their inner binaries.

astro-ph.SR↗

arXiv2018

Population synthesis of binary stars

Many aspects of the evolution of stars, and in particular the evolution of binary stars, remain beyond our ability to model them in detail. Instead, we rely on observations to guide our often phenomenological models and pin down uncertain model parameters. To do this statistically requires population synthesis. Populations of stars modelled on computers are compared to populations of stars observed with our best telescopes. The closest match between observations and models provides insight into unknown model parameters and hence the underlying astrophysics. In this brief review, we describe the impact that modern big-data surveys will have on population synthesis, the large parameter space problem that is rife for the application of modern data science algorithms, and some examples of how population synthesis is relevant to modern astrophysics.

astro-ph.SR↗

arXiv2018

Post-common envelope binary systems experiencing helium-shell driven stable mass transfer

We evolve stellar models to study the common envelope (CE) interaction of an early asymptotic giant branch star of initial mass $5\,\rm M_{\odot}$ with a companion star of mass ranging from $0.1$ to $2\,\rm M_{\odot}$. We model the CE as a fast stripping phase in which the primary experiences rapid mass loss and loses about 80 per cent of its mass. The post-CE remnant is then allowed to thermally readjust during a Roche-lobe overflow (RLOF) phase and the final binary system and its orbital period are investigated. We find that the post-CE RLOF phase is long enough to allow nuclear burning to proceed in the helium shell. By the end of this phase, the donor is stripped of both its hydrogen and helium and ends up as carbon-oxygen white dwarf of mass about $0.8\,\rm M_{\odot}$. We study the sensitivity of our results to initial conditions of different companion masses and orbital separations at which the stripping phase begins. We find that the companion mass affects the final binary separation and that helium-shell burning causes the star to refill its Roche lobe leading to post-CE RLOF. Our results show that double mass transfer in such a binary interaction is able to strip the helium and hydrogen layers from the donor star without the need for any special conditions or fine tuning of the binary parameters.

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