arXiv subjects

Giuliano Iorio

Publications and source records attributed to Giuliano Iorio.

At least 55 records · Page 3Linked to original sources

arXiv2024

Binary black hole mergers from Population III star clusters

Binary black holes (BBHs) born from the evolution of Population III (Pop. III) stars are one of the main high-redshift targets for next-generation ground-based gravitational-wave (GW) detectors. Their predicted initial mass function and lack of metals make them the ideal progenitors of black holes above the upper edge of the pair-instability mass gap, i.e. with a mass higher than $\approx{}134$ (241) M$_\odot$ for stars that become (do not become) chemically homogeneous during their evolution. Here, we investigate the effects of cluster dynamics on the mass function of BBHs born from Pop. III stars, by considering the main uncertainties on Pop. III star mass function, orbital properties of binary systems, star cluster's mass and disruption time. In our dynamical models, at least $\sim$5% and up to 100% BBH mergers in Pop. III star clusters have primary mass $m_1$ above the upper edge of the pair-instability mass gap. In contrast, only $\lesssim {} 3$% isolated BBH mergers have primary mass above the gap, unless their progenitors evolved as chemically homogeneous stars. The lack of systems with primary and/or secondary mass inside the gap defines a zone of avoidance with sharp boundaries in the primary mass - mass ratio plane. Finally, we estimate the merger rate density of BBHs and, in the most optimistic case, we find a maximum of $\mathcal{R}\approx200\,{\rm Gpc^{-3}\,yr^{-1}}$ at $z\sim15$ for BBHs formed via dynamical capture. For comparison, the merger rate density of isolated Pop. III BBHs is $\mathcal{R}\leq{}10\,{\rm Gpc^{-3}\,yr^{-1}}$, for the same model of Pop. III star formation history.

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arXiv2024

Binary black hole mergers from Population III stars: uncertainties from star formation and binary star properties

Population III (Pop. III) binary stars likely produced the first stellar-born binary black hole (BBH) mergers in the Universe. Here, we quantify the main sources of uncertainty for the merger rate density evolution and mass spectrum of Pop. III BBHs by considering four different formation histories and 11 models of the initial orbital properties of Pop. III binary stars. The uncertainty on the orbital properties affects the BBH merger rate density by up to two orders of magnitude, models with shorter orbital periods leading to higher BBH merger rates. The uncertainty on the star formation history has a substantial impact on both the shape and the normalisation of the BBH merger rate density: the peak of the merger rate density shifts from $z\sim{8}$ up to $z\sim{16}$ depending on the assumed star formation rate, while the maximum BBH merger rate density for our fiducial binary population model spans from $\sim{2}$ to $\sim{30}$ Gpc$^{-3}$ yr$^{-1}$. The typical BBH masses are not affected by the star formation rate model and only mildly influenced by the binary population parameters. The primary black holes born from Pop. III stars tend to be rather massive ($30-40$ M$_\odot$) with respect to those born from metal-rich stars ($8-10$ M$_\odot$). We estimate that the Einstein Telescope will detect $10-10^4$ Pop. III BBH mergers per year, depending on the star formation history and binary star properties.

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arXiv2024

RR Lyrae From Binary Evolution: Abundant, Young and Metal-Rich

RR Lyrae are a well-known class of pulsating horizontal branch stars widely used as tracers of old, metal-poor stellar populations. However, mounting observational evidence shows that a significant fraction of these stars may be young and metal-rich. Here, through detailed binary stellar evolution modelling, we show that all such metal-rich RR Lyrae can be naturally produced through binary interactions. Binary companions of these RR Lyrae stars formed through binary interactions partly strip their progenitor's envelopes during a preceding red giant phase. As a result, stripped horizontal branch stars become bluer than their isolated stellar evolution counterparts and thus end up in the instability strip. In contrast, in the single evolution scenario, the stars can attain such colours only at large age and low metallicity. While binary-made RR Lyrae can possess any ages and metallicities, their Galactic population is relatively young (1 to 9 Gyr) and dominated by the Thin Disc and the Bulge. We show that Galactic RR Lyrae from binary evolution are produced at rates compatible with the observed metal-rich population and have consistent G-band magnitudes, Galactic kinematics and pulsation properties. Furthermore, these systems dominate the RR Lyrae population in the Solar Neighbourhood. We predict that all metal-rich RR Lyrae have an A, F, G or K-type companion with a long orbital period (P > 1000 d). Observationally characterising the orbital periods and masses of such stellar companions will provide valuable new constraints on mass and angular momentum-loss efficiency for Sun-like accretors and the nature of RR Lyrae populations.

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arXiv2023

Impact of gas hardening on the population properties of hierarchical black hole mergers in AGN disks

Hierarchical black hole (BH) mergers in active galactic nuclei (AGNs) are unique among formation channels of binary black holes (BBHs) because they are likely associated with electromagnetic counterparts and can efficiently lead to the mass growth of BHs. Here, we explore the impact of gas accretion and migration traps on the evolution of BBHs in AGNs. We have developed a new fast semi-analytic model, which allows us to explore the parameter space while capturing the main physical processes involved. We find that effective exchange of energy and angular momentum between the BBH and the surrounding gas (hereafter, gas hardening) during inspiral greatly enhances the efficiency of hierarchical mergers, leading to the formation of intermediate-mass BHs (up to 10.000 solar masses) and triggering spin alignment. Moreover, our models with efficient gas hardening show both an anti-correlation between BBH mass ratio and effective spin, and a correlation between primary BH mass and effective spin. In contrast, if gas hardening is inefficient, the hierarchical merger chain is already truncated after the first two or three generations. We compare the BBH population in AGNs with other dynamical channels as well as isolated binary evolution.

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arXiv2023

Massive binary black holes from Population II and III stars

Population III stars, born from the primordial gas in the Universe, lose a negligible fraction of their mass via stellar winds and possibly follow a top-heavy mass function. Hence, they have often been regarded as the ideal progenitors of massive black holes (BHs), even above the pair instability mass gap. Here, we evolve a large set of Population III binary stars (metallicity $Z=10^{-11}$) with our population-synthesis code SEVN, and compare them with Population II binary stars ($Z=10^{-4}$). In our models, the lower edge of the pair-instability mass gap corresponds to a BH mass of $\approx{86}$ ($\approx{91}$) M$_\odot$ for single Population III (II) stars. Overall, we find only mild differences between the properties of binary BHs (BBHs) born from Population III and II stars, especially if we adopt the same initial mass function and initial orbital properties. Most BBH mergers born from Population III and II stars have primary BH mass below the pair-instability gap, and the maximum secondary BH mass is $ < 50$ M$_\odot$. Only up to $\approx{3.3}$% ($\approx{0.09}$%) BBH mergers from Population III (II) progenitors have primary mass above the gap. Unlike metal-rich binary stars, the main formation channel of BBH mergers from Population III and II stars involves only stable mass transfer episodes in our fiducial model.

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arXiv2023

Dynamical formation of $Gaia$ BH1 in a young star cluster

$Gaia$ BH1, the first quiescent black hole (BH) detected from $Gaia$ data, poses a challenge to most binary evolution models: its current mass ratio is $\approx{0.1}$, and its orbital period seems to be too long for a post-common envelope system and too short for a non-interacting binary system. Here, we explore the hypothesis that $Gaia$ BH1 formed through dynamical interactions in a young star cluster (YSC). We study the properties of BH-main sequence (MS) binaries formed in YSCs with initial mass $3\times{}10^2-3\times{}10^4$ M$_\odot$ at solar metallicity, by means of $3.5\times{}10^4$ direct $N$-body simulations coupled with binary population synthesis. For comparison, we also run a sample of isolated binary stars with the same binary population synthesis code used in the dynamical models. We find that BH-MS systems that form via dynamical exchanges populate the region corresponding to the main orbital properties of $Gaia$ BH1 (period, eccentricity, and masses). In contrast, none of our isolated binary systems matches the orbital period and MS mass of $Gaia$ BH1. Our best matching $Gaia$ BH1--like system forms via repeated dynamical exchanges and collisions involving the BH progenitor star, before it undergoes core collapse. YSCs are at least two orders of magnitude more efficient in forming $Gaia$ BH1--like systems than isolated binary evolution.

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arXiv2023

The Dragon-II simulations -- I. Evolution of single and binary compact objects in star clusters with up to 1 million stars

We present the first results of the \textsc{Dragon-II} simulations, a suite of 19 $N$-body simulations of star clusters with up to $10^6$ stars, with up to $33\%$ of them initially paired in binaries. In this work, we describe the main evolution of the clusters and their compact objects (COs). All \textsc{Dragon-II} clusters form in their centre a black hole (BH) subsystem with a density $10-100$ times larger than the stellar density, with the cluster core containing $50-80\%$ of the whole BH population. In all models, the BH average mass steeply decreases as a consequence of BH burning, reaching values $\langle m_{\rm BH}\rangle < 15$ M$_\odot$ within $10-30$ relaxation times. Generally, our clusters retain only BHs lighter than $30$ M$_\odot$ over $30$ relaxation times. Looser clusters retain a higher binary fraction, because in such environments binaries are less likely disrupted by dynamical encounters. We find that BH-main sequence star binaries have properties similar to recently observed systems. Double CO binaries (DCOBs) ejected from the cluster exhibit larger mass ratios and heavier primary masses than ejected binaries hosting a single CO (SCOBs). Ejected SCOBs have BH masses $m_{\rm BH} = 3-20$ M$_\odot$, definitely lower than those in DCOBs ($m_{\rm BH} = 10-100$ M$_\odot$).

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arXiv2023

Compact object mergers: exploring uncertainties from stellar and binary evolution with SEVN

Population-synthesis codes are an unique tool to explore the parameter space of massive binary star evolution and binary compact object (BCO) formation. Most population-synthesis codes are based on the same stellar evolution model, limiting our ability to explore the main uncertainties. Here, we present the new version of the code SEVN, which overcomes this issue by interpolating the main stellar properties from a set of pre-computed evolutionary tracks. We describe the new interpolation and adaptive time-step algorithms of SEVN, and the main upgrades on single and binary evolution. With SEVN, we evolved $1.2\times10^9$ binaries in the metallicity range $0.0001\leq Z \leq 0.03$, exploring a number of models for electron-capture, core-collapse and pair-instability supernovae, different assumptions for common envelope, stability of mass transfer, quasi-homogeneous evolution and stellar tides. We find that stellar evolution has a dramatic impact on the formation of single and binary compact objects. Just by slightly changing the overshooting parameter ($λ_{\rm ov}=0.4,0.5$) and the pair-instability model, the maximum mass of a black hole can vary from $\approx{60}$ to $\approx{100}\ \mathrm{M}_\odot$. Furthermore, the formation channels of BCOs and the merger efficiency we obtain with SEVN show significant differences with respect to the results of other population-synthesis codes, even when the same binary-evolution parameters are used. For example, the main traditional formation channel of BCOs is strongly suppressed in our models: at high metallicity ($Z\gtrsim{0.01}$) only $<20$% of the merging binary black holes and binary neutron stars form via this channel, while other authors found fractions $>70$%. The local BCO merger rate density of our fiducial models is consistent with the most recent estimates by the LIGO--Virgo--KAGRA collaboration.

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arXiv2023

Binary neutron star populations in the Milky Way

Galactic binary neutron stars (BNSs) are a unique laboratory to probe the evolution of BNSs and their progenitors. Here, we use a new version of the population synthesis code SEVN to evolve the population of Galactic BNSs, by modeling the spin up and down of pulsars self-consistently. We analyze the merger rate $\mathcal{R}_{\rm MW}$, orbital period $P_{\rm orb}$, eccentricity $e$, spin period $P$, and spin period derivative $\dot{P}$ of the BNS population. Values of the common envelope parameter $α=1 - 3$ and an accurate model of the Milky Way star formation history best reproduce the BNS merger rate in our Galaxy ($\mathcal{R}_{\rm MW}\approx{}30$ Myr$^{-1}$). We apply radio-selection effects to our simulated BNSs and compare them to the observed population. Using a Dirichlet process Gaussian mixture method, we evaluate the four-dimensional likelihood in the $(P_{\rm orb}, e, P, \dot{P})$ space, by comparing our radio-selected simulated pulsars against Galactic BNSs. Our analysis favours an uniform initial distribution for both the magnetic field ($10^{10-13}$ G) and the spin period ($10-100$ ms). The implementation of radio selection effects is critical to match not only the spin period and period derivative, but also the orbital period and eccentricity of Galactic BNSs. According to our fiducial model, the Square Kilometre Array will detect $\sim 20$ new BNSs in the Milky Way.

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arXiv2022

Revisiting Common Envelope Evolution -- A New Semi-Analytic Model for N-body and Population Synthesis Codes

We present a novel way of modeling common envelope evolution in binary and few-body systems. We consider the common envelope inspiral as driven by a drag force with a power-law dependence in relative distance and velocity. The orbital motion is resolved either by direct N-body integration or by solving the set of differential equations for the orbital elements as derived using perturbation theory. Our formalism can model the eccentricity during the common envelope inspiral, and it gives results consistent with smoothed particles hydrodynamical simulations. We apply our formalism to common envelope events from binary population synthesis models and find that the final eccentricity distribution resembles the observed distribution of post-common-envelope binaries. Our model can be used for time-resolved common-envelope evolution in population synthesis calculations or as part of binary interactions in direct N-body simulations of star clusters.

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arXiv2022

The impact of gas disc flaring on rotation curve decomposition and revisiting baryonic and dark-matter relations for nearby galaxies

Gas discs of late-type galaxies are flared, with scale heights increasing with the distance from the galaxy centres and often reaching kpc scales. We study the effects of gas disc flaring on the recovered dark matter halo parameters from rotation curve decomposition. For this, we carefully select a sample of 32 dwarf and spiral galaxies with high-quality neutral gas, molecular gas, and stellar mass profiles, robust H\,{\sc i} rotation curves obtained via 3D kinematic modelling, and reliable bulge-disc decomposition. By assuming vertical hydrostatic equilibrium, we derive the scale heights of the atomic and molecular gas discs and fit dark matter haloes to the rotation curves self-consistently. We find that the effect of the gas flaring in the rotation curve decomposition can play an important role only for the smallest, gas-dominated dwarfs, while for most of the galaxies the effect is minor and can be ignored. We revisit the stellar- and baryon-to-halo mass relations ($M_\ast-M_{200}$ and $M_{\rm bar}-M_{200}$). Both relations increase smoothly up to $M_{200} \approx 10^{12}~\rm{ M_\odot}$, with galaxies at this end having high $M_\ast/M_{200}$ and $M_{\rm bar}/M_{200}$ ratios approaching the cosmological baryon fraction. At higher $M_{200}$ the relations show a larger scatter. Most haloes of our galaxy sample closely follow the concentration-mass ($c_{200}-M_{\rm 200}$) relation resulting from N-body cosmological simulations. Interestingly, the galaxies deviating above and below the relation have the highest and lowest stellar and baryon factions, respectively, which suggests that the departures from the $c_{200}-M_{\rm 200}$ law are regulated by adiabatic contraction and an increasing importance of feedback.

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arXiv2021

Hierarchical generative models for star clusters from hydro-dynamical simulations

Star formation in molecular clouds is clumpy, hierarchically subclustered. Fractal structure also emerges in hydro-dynamical simulations of star-forming clouds. Simulating the formation of realistic star clusters with hydro-dynamical simulations is a computational challenge, considering that only the statistically averaged results of large batches of simulations are reliable, due to the chaotic nature of the gravitational N-body problem. While large sets of initial conditions for N-body runs can be produced by hydro-dynamical simulations of star formation, this is prohibitively expensive in terms of computational time. Here we address this issue by introducing a new technique for generating many sets of new initial conditions from a given set of star masses, positions and velocities from a hydro-dynamical simulation. We use hierarchical clustering in phase space to learn a tree representation of the spatial and kinematic relations between stars. This constitutes the basis for the random generation of new sets of stars which share the same clustering structure of the original ones but have individually different masses, positions, and velocities. We apply this method to the output of a number of hydro-dynamical star-formation simulations, comparing the generated initial conditions to the original ones through a series of quantitative tests, including comparing mass and velocity distributions and fractal dimension. Finally, we evolve both the original and the generated star clusters using a direct N-body code, obtaining a qualitatively similar evolution.

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arXiv2021

Constraining accretion efficiency in massive binary stars with LIGO-Virgo black holes

The growing sample of LIGO--Virgo black holes (BHs) opens new perspectives for the study of massive binary evolution. Here, we study the impact of mass accretion efficiency and common envelope on the properties of binary BH (BBH) mergers, by means of population synthesis simulations. We model mass accretion efficiency with the parameter $f_{\rm MT}\in[0.05,1]$, which represents the fraction of mass lost from the donor which is effectively accreted by the companion. Lower values of $f_{\rm MT}$ result in lower BBH merger rate densities and produce mass spectra skewed towards lower BH masses. Our hierarchical Bayesian analysis, applied to BBH mergers in the first and second gravitational wave transient catalogue, yields zero support for values of $f_{\rm MT}\lesssim{}0.6$, with a lower boundary of the 99\% credible intervals equal to $f_{\rm MT}= 0.59$. This result holds for all the values of the common-envelope efficiency parameter we considered in this study $α_{\rm CE} \in [1,10]$. This confirms that gravitational-wave data can be used to put constraints on several uncertain binary evolution processes.

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arXiv2021

New insights on binary black hole formation channels after GWTC-2: young star clusters versus isolated binaries

With the recent release of the second gravitational-wave transient catalogue (GWTC-2), which introduced dozens of new detections, we are at a turning point of gravitational wave astronomy, as we are now able to directly infer constraints on the astrophysical population of compact objects. Here, we tackle the burning issue of understanding the origin of binary black hole (BBH) mergers. To this effect, we make use of state-of-the-art population synthesis and N-body simulations, to represent two distinct formation channels: BBHs formed in the field (isolated channel) and in young star clusters (dynamical channel). We then use a Bayesian hierarchical approach to infer the distribution of the mixing fraction $f$, with $f=0$ ($f=1$) in the pure dynamical (isolated) channel. %that controls the proportion of isolated and dynamical BBHs. We explore the effects of additional hyper-parameters of the model, such as the spread in metallicity $σ_{\text{Z}}$ and the parameter $σ_{\text{sp}}$, describing the distribution of spin magnitudes. We find that the dynamical model is slightly favoured with a median value of $f=0.26$, when $σ_{\text{sp}}=0.1$ and $σ_{\text{Z}}=0.4$. Models with higher spin magnitudes tend to strongly favour dynamically formed BBHs ($f\le{}0.1$ if $σ_{\text{sp}}=0.3$). Furthermore, we show that hyper-parameters controlling the rates of the model, such as $σ_{\rm Z}$, have a large impact on the inference of the mixing fraction, which rises from $0.18$ to $0.43$ when we increase $σ_{\text{Z}}$ from 0.2 to 0.6, for a fixed value of $σ_{\text{sp}}=0.1$. Finally, our current set of observations is better described by a combination of both formation channels, as a pure dynamical scenario is excluded at the $99\%$ credible interval, except when the spin magnitude is high.

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arXiv2021

Hierarchical black hole mergers in young, globular and nuclear star clusters: the effect of metallicity, spin and cluster properties

We explore hierarchical black hole (BH) mergers in nuclear star clusters (NSCs), globular clusters (GCs) and young star clusters (YSCs), accounting for both original and dynamically assembled binary BHs (BBHs). We find that the median mass of both first- and nth-generation dynamical mergers is larger in GCs and YSCs with respect to NSCs, because the lighter BHs are ejected by supernova kicks from the lower-mass clusters. Also, first- and nth-generation BH masses are strongly affected by the metallicity of the progenitor stars: the median mass of the primary BH of a nth-generation merger is $\sim{}24-38$ M$_\odot$ ($\sim{}9-15$ M$_\odot$) in metal-poor (metal-rich) NSCs. The maximum BH mass mainly depends on the escape velocity: BHs with mass up to several thousand M$_\odot$ form in NSCs, while YSCs and GCs host BHs with mass up to several hundred M$_\odot$. Furthermore, we calculate the fraction of mergers with at least one component in the pair-instability mass gap ($f_{\rm PI}$) and in the intermediate-mass BH regime ($f_{\rm IMBH}$). In the fiducial model for dynamical BBHs with metallicity $Z=0.002$, we find $f_{\rm PI}\approx{}0.05$, $0.02$ and $0.007$ ($f_{\rm IMBH}\approx{}0.01$, $0.002$ and $0.001$) in NSCs, GCs and YSCs, respectively. Both $f_{\rm PI}$ and $f_{\rm IMBH}$ drop by at least one order of magnitude at solar metallicity. Finally, we investigate the formation of GW190521 by assuming that it is either a nearly equal-mass BBH or an intermediate-mass ratio inspiral.

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arXiv2021

Dynamics of binary black holes in low-mass young star clusters

Young star clusters are dynamically active stellar systems and are a common birthplace for massive stars. Low-mass star clusters ($\sim{}300-10^3$ M$_\odot$) are more numerous than massive systems and are characterized by a two-body relaxation time scale of a few Myr: the most massive stars sink to the cluster core and dynamically interact with each other even before they give birth to compact objects. Here, we explore the properties of black holes (BHs) and binary black holes (BBHs) formed in low-mass young star clusters, by means of a suite of $10^5$ direct $N$-body simulations with a high original binary fraction (100 % for stars with mass $>5$ M$_\odot$). Most BHs are ejected in the first $\sim{}20$ Myr by dynamical interactions. Dynamical exchanges are the main formation channel of BBHs, accounting for $\sim{}40-80$ % of all the systems. Most BBH mergers in low-mass young star clusters involve primary BHs with mass $<40$ M$_\odot$ and low mass ratios are extremely more common than in the field. Comparing our data with those of more massive star clusters ($10^3-3\times{}10^4$ M$_\odot$), we find a strong dependence of the percentage of exchanged BBHs on the mass of the host star cluster. In contrast, our results show just a mild correlation between the mass of the host star cluster and the efficiency of BBH mergers.

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arXiv2021

Intermediate mass black holes from stellar mergers in young star clusters

Intermediate mass black holes (IMBHs) in the mass range $10^2-10^5\,\mathrm{M_{\odot}}$ bridge the gap between stellar black holes (BHs) and supermassive BHs. Here, we investigate the possibility that IMBHs form in young star clusters via runaway collisions and BH mergers. We analyze $10^4$ simulations of dense young star clusters, featuring up-to-date stellar wind models and prescriptions for core collapse and (pulsational) pair instability. In our simulations, only 9 IMBHs out of 218 form via binary BH mergers, with a mass $\sim{}100-140$ M$_\odot$. This channel is strongly suppressed by the low escape velocity of our star clusters. In contrast, IMBHs with masses up to $\sim{}438$ M$_{\odot}$ efficiently form via runaway stellar collisions, especially at low metallicity. Up to $\sim{}0.2$~% of all the simulated BHs are IMBHs, depending on progenitor's metallicity. The runaway formation channel is strongly suppressed in metal-rich ($Z=0.02$) star clusters, because of stellar winds. IMBHs are extremely efficient in pairing with other BHs: $\sim{}70$% of them are members of a binary BH at the end of the simulations. However, we do not find any IMBH-BH merger. More massive star clusters are more efficient in forming IMBHs: $\sim{}8$% ($\sim{}1$%) of the simulated clusters with initial mass $10^4-3\times{}10^4$ M$_\odot$ ($10^3-5\times{}10^3$ M$_\odot$) host at least one IMBH.

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arXiv2021

Effective N-body models of composite collisionless stellar systems

Gas-poor galaxies can be modelled as composite collisionless stellar systems, with a dark matter halo and one or more stellar components, representing different stellar populations. The dynamical evolution of such composite systems is often studied with numerical N-body simulations, whose initial conditions typically require realizations with particles of stationary galaxy models. We present a novel method to conceive these N-body realizations, which allows one to exploit at best a collisionless N-body simulation that follows their evolution. The method is based on the use of an effective N-body model of a composite system, which is in fact realized as a one-component system of particles that is interpreted a posteriori as a multi-component system, by assigning in post-processing fractions of each particle's mass to different components. Examples of astrophysical applications are N-body simulations that aim to reproduce the observed properties of interacting galaxies, satellite galaxies and stellar streams. As a case study we apply our method to an N-body simulation of tidal stripping of a two-component (dark matter and stars) satellite dwarf galaxy orbiting in the gravitational potential of the Milky Way.

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