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Nicola Giacobbo

Publications and source records attributed to Nicola Giacobbo.

At least 19 recordsLinked to original sources

Emergent Models: Intelligence from Tiny Substrates

Emergent Models (EMs) are a machine learning paradigm based on simple yet open-ended substrates, such as cellular automata, in which modeling is treated not as the learning of a closed-form input-output map but as the emergence, within simple dynamical systems, of computational behaviors that solve external tasks. Such substrates typically iterate a fixed local rule over a latent space for an adaptive number of steps, with an interface linking the latent state to external input/output signals. Training proceeds by evolutionary search. We hypothesize that some instances of this framework are biased toward global generalization: capturing the rule generating the data over its full domain, and therefore extrapolating beyond the training range. Theoretically, we prove that some EMs are latent-universal: with the update rule and interface held fixed, they can realize any partial computable function by varying only the initial condition of the latent state. Empirically, we study a zoo of minimal EM instantiations across discrete and continuous substrates, showing that local-recursive computation at a tiny scale (tens to hundreds of parameters) can extrapolate exactly on simple arithmetic functions, can support control behaviour and online adaptation, while still exposing several limitations. This work is foundational: it does not propose a competitive architecture, but a framework meant to widen the design space of machine learning beyond differentiable feed-forward maps.

cs.NE↗

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.

astro-ph.SR↗

Astrometric mass measurement of compact companions in binary systems with Gaia

For binary systems with an unseen primary and a luminous secondary, the astrometric wobble of the secondary could be used to study the primary. With Gaia, it is possible to measure the mass of the black hole or neutron star with a luminous companion (hereafter BH/NS-LC). Our aim is to provide a method for predicting Gaia's ability in measuring the mass of BH/NS-LCs. We also tried to estimate the number of solvable BH/NS-LCs using Gaia. We used a realistic Markov chain Monte Carlo simulation of mock Gaia observations to obtain a relation between the uncertainty of mass measurement of the primary in BH/NS-LCs with the observable variables of the secondary astrometric orbit. Furthermore, we used the MOBSE code to evolve a Galactic BH/NS-LC sample with a combined Milky Way model. Our relation is applied to this sample to estimate the number of solvable BH/NS-LCs. We derived a good relation between the mass uncertainty and the binary parameters. For the first time, we show the quantitive influence of the period P, inclination i, eccentricity e, and ecliptic latitude $β$ to the mass measurement. Our results suggest that $48^{+7}_{-7}$ BH-LCs and $102^{+11}_{10}$ NS-LCs are solvable during a 5 yr Gaia mission. We also give the distribution of the distance and apparent magnitude of the Gaia solvable BH/NS-LCs. This solvable sample would be increased by additional spectroscopic data or a prolonged Gaia mission. The mass uncertainty relation could be used in future simulations of BH/NS-LCs observed by Gaia. The prediction of the solvable BH/NS-LCs is not only influenced by the process in generating the Galactic BH/NS-LC sample, but is also affected by our uncertainty relation. In particular, the relations of parameters such as $[P, e, i, β]$ are very useful to correct the selection effect in the statistic results of the future BH/NS-LC sample observed by Gaia.

astro-ph.GA↗

Gravitational background from dynamical binaries and detectability with 2G detectors

We study the impact of young clusters on the gravitational wave background from compact binary coalescence. We simulate a catalog of sources from population I/II isolated binary stars and stars born in young clusters, corresponding to one year of observations with second-generation (2G) detectors. Taking into account uncertainties on the fraction of dynamical binaries and star formation parameters, we find that the background is dominated by the population of binary black holes, and we obtain a value of $Ω_{gw}(25 \rm{Hz}) = 1.2^{+1.38}_{-0.65} \times 10^{-9}$ for the energy density, in agreement with the actual upper limits derived from the latest observation run of LIGO--Virgo. We demonstrate that a large number of sources in a specific corrected mass range yields to a bump in the background. This background could be detected with 8 years of coincident data by a network of 2G detectors.

astro-ph.CO↗

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.

astro-ph.HE↗

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.

astro-ph.HE↗

Modeling the outcome of supernova explosions in binary population synthesis using the stellar compactness

Following the collapse of their cores, some of the massive binary stars that populate our Universe are expected to form merging binaries composed of black holes and neutron stars. Gravitational-wave observations of the resulting compact binaries can reveal precious details on the inner workings of the supernova mechanism and the subsequent formation of compact objects. Within the framework of the population-synthesis code MOBSE, we present the implementation of a new supernova model that relies on the compactness of the collapsing star. The model has two free parameters, namely the compactness threshold that separates the formation of black holes and that of neutron stars, and the fraction of the envelope that falls back onto the newly formed black holes. We compare this model extensively against other prescriptions that are commonly used in binary population synthesis. We find that the cleanest signatures of the role of the pre-supernova stellar compactness are (i) the relative formation rates of the different kinds of compact binaries, which mainly depend on the compactness threshold parameter, and (ii) the location of the upper edge of the mass gap between the lightest black holes and the heaviest neutron stars, which mainly depends on the fallback fraction.

astro-ph.HE↗

High mass but low spin: an exclusion region to rule out hierarchical black-hole mergers as a mechanism to populate the pair-instability mass gap

The occurrence of pair-instability supernovae is predicted to prevent the formation of black holes with masses $\gtrsim 50 M_\odot$. Recent gravitational-wave detections in this mass range require an explanation beyond that of standard stellar collapse. Current modeling strategies include the hierarchical assembly of previous generations of black-hole mergers as well as other mechanisms of astrophysical nature (lowered nuclear-reaction rates, envelope retention, stellar mergers, accretion, dredge-up episodes). In this paper, we point out the occurrence of an exclusion region that cannot be easily populated by hierarchical black-hole mergers. A future gravitational-wave detection of a black hole with mass $\gtrsim 50M_\odot$ and spin $\lesssim 0.2$ will indicate that the pair-instability mass gap is polluted in some other way. Such a putative outlier can be explained using hierarchical mergers only with considerable fine-tuning of both mass ratio and spins of the preceding black-hole merger -- an assumption that can then be cross-checked against the bulk of the gravitational-wave catalog.

astro-ph.HE↗

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.

astro-ph.HE↗

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.

astro-ph.GA↗

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.

astro-ph.GA↗

The impact of binaries on the evolution of star clusters from turbulent molecular clouds

Most of massive stars form in binary or higher-order systems in clumpy, sub-structured clusters. In the very first phases of their life, these stars are expected to interact with the surrounding environment, before being released to the field when the cluster is tidally disrupted by the host galaxy. We present a set of N-body simulations to describe the evolution of young stellar clusters and their binary content in the first phases of their life. To do this, we have developed a method that generates realistic initial conditions for binary stars in star clusters from hydrodynamical simulations. We considered different evolutionary cases to quantify the impact of binary and stellar evolution. Also, we compared their evolution to that of King and fractal models with different length scales. Our results indicate that the global expansion of the cluster from hydrodynamical simulations is initially balanced by the sub-clump motion and accelerates when a monolithic shape is reached, as in a post-core collapse evolution. Compared to the spherical initial conditions, the ratio of the 50% to 10% Lagrangian radius shows a very distinctive trend, explained by the formation of a hot core of massive stars triggered by the high initial degree of mass segregation. As for its binary population, each cluster shows a self-regulating behaviour by creating interacting binaries with binding energies of the order of its energy scales. Also, in absence of original binaries, the dynamically formed binaries present a mass dependent binary fraction, that mimics the trend of the observed one.

astro-ph.GA↗

The cosmic merger rate density of compact objects: impact of star formation, metallicity, initial mass function and binary evolution

We evaluate the redshift distribution of binary black hole (BBH), black hole - neutron star binary (BHNS) and binary neutron star (BNS) mergers, exploring the main sources of uncertainty: star formation rate (SFR) density, metallicity evolution, common envelope, mass transfer via Roche lobe overflow, natal kicks, core-collapse supernova model and initial mass function. Among binary evolution processes, uncertainties on common envelope ejection have a major impact: the local merger rate density of BNSs varies from $\sim{}10^3$ to $\sim{}20$ Gpc$^{-3}$ yr$^{-1}$ if we change the common envelope efficiency parameter from $α_{\rm CE}=7$ to 0.5, while the local merger rates of BBHs and BHNSs vary by a factor of $\sim{}2-3$. The BBH merger rate changes by one order of magnitude, when $1 σ$ uncertainties on metallicity evolution are taken into account. In contrast, the BNS merger rate is almost insensitive to metallicity. Hence, BNSs are the ideal test bed to put constraints on uncertain binary evolution processes, such as common envelope and natal kicks. Only models assuming values of $α_{\rm CE}\gtrsim{}2$ and moderately low natal kicks (depending on the ejected mass and the SN mechanism), result in a local BNS merger rate density within the 90% credible interval inferred from the second gravitational-wave transient catalogue.

astro-ph.HE↗

The cosmic merger rate density evolution of compact binaries formed in young star clusters and in isolated binaries

Next generation ground-based gravitational-wave detectors will observe binary black hole (BBH) mergers up to redshift $z\gtrsim{}10$, probing the evolution of compact binary (CB) mergers across cosmic time. Here, we present a new data-driven model to estimate the cosmic merger rate density (MRD) evolution of CBs, by coupling catalogs of CB mergers with observational constraints on the cosmic star formation rate density and on the metallicity evolution of the Universe. We adopt catalogs of CB mergers derived from recent $N-$body and population-synthesis simulations, to describe the MRD of CBs formed in young star clusters (hereafter, dynamical CBs) and in the field (hereafter, isolated CBs). The local MRD of dynamical BBHs is $\mathcal{R}_{\rm BBH}=64^{+34}_{-20}$ Gpc$^{-3}$ yr$^{-1}$, consistent with the 90% credible interval from the first and second observing run (O1 and O2) of the LIGO-Virgo collaboration, and with the local MRD of isolated BBHs ($\mathcal{R}_{\rm BBH}=50^{+71}_{-37}$ Gpc$^{-3}$ yr$^{-1}$). The local MRD of dynamical and isolated black hole - neutron star binaries is $\mathcal{R}_{\rm BHNS}=41^{+33}_{-23}$ and $49^{+48}_{-34}$~Gpc$^{-3}$ yr$^{-1}$, respectively. Both values are consistent with the upper limit inferred from O1 and O2. Finally, the local MRD of dynamical binary neutron stars (BNSs, $\mathcal{R}_{\rm BNS}=151^{+59}_{-38}$ Gpc$^{-3}$ yr$^{-1}$) is a factor of two lower than the local MRD of isolated BNSs ($\mathcal{R}_{\rm BNS}=283^{+97}_{-75}$ Gpc$^{-3}$ yr$^{-1}$). The MRD for all CB classes grows with redshift, reaching its maximum at $z \in [1.5,2.5]$, and then decreases. This trend springs from the interplay between cosmic star formation rate, metallicity evolution and delay time of binary compact objects.

astro-ph.HE↗

Binary black holes in young star clusters: the impact of metallicity

Young star clusters are the most common birth-place of massive stars and are dynamically active environments. Here, we study the formation of black holes (BHs) and binary black holes (BBHs) in young star clusters, by means of 6000 N-body simulations coupled with binary population synthesis. We probe three different stellar metallicities (Z=0.02, 0.002 and 0.0002) and two initial density regimes (density at the half-mass radius $ρ_{\rm h}\ge{}3.4\times10^4$ and $\ge{1.5\times10^2}$ M$_\odot$ pc$^{-3}$ in dense and loose star clusters, respectively). Metal-poor clusters tend to form more massive BHs than metal-rich ones. We find $\sim{}6$, $\sim{}2$, and $<1$% of BHs with mass $m_{\rm BH}>60$ M$_\odot$ at Z=0.0002, 0.002 and 0.02, respectively. In metal-poor clusters, we form intermediate-mass BHs with mass up to $\sim{}320$ M$_\odot$. BBH mergers born via dynamical exchanges (exchanged BBHs) can be more massive than BBH mergers formed from binary evolution: the former (latter) reach total mass up to $\sim{}140$ M$_\odot$ ($\sim{}80$ M$_\odot$). The most massive BBH merger in our simulations has primary mass $\sim{}88$ M$_\odot$, inside the pair-instability mass gap, and a mass ratio of $\sim{}0.5$. Only BBHs born in young star clusters from metal-poor progenitors can match the masses of GW170729, the most massive event in O1 and O2, and those of GW190412, the first unequal-mass merger. We estimate a local BBH merger rate density $\sim{}110$ and $\sim{}55$ Gpc$^{-3}$ yr$^{-1}$, if we assume that all stars form in loose and dense star clusters, respectively.

astro-ph.HE↗

Binary black holes in the pair-instability mass gap

Pair instability (PI) and pulsational PI prevent the formation of black holes (BHs) with mass $\gtrsim{}60$ M$_\odot$ from single star evolution. Here, we investigate the possibility that BHs with mass in the PI gap form via stellar mergers and multiple stellar mergers, facilitated by dynamical encounters in young star clusters. We analyze $10^4$ simulations, run with the direct N-body code nbody6++gpu coupled with the population synthesis code MOBSE. We find that up to $\sim{}6$~% of all simulated BHs have mass in the PI gap, depending on progenitor's metallicity. This formation channel is strongly suppressed in metal-rich (Z = 0.02) star clusters, because of stellar winds. BHs with mass in the PI gap are initially single BHs but can efficiently acquire companions through dynamical exchanges. We find that $\sim{}$21%, 10% and 0.5% of all binary BHs have at least one component in the PI mass gap at metallicity Z = 0.0002, 0.002 and 0.02, respectively. Based on the evolution of the cosmic star formation rate and metallicity, and under the assumption that all stars form in young star clusters, we predict that $\sim{}5$~% of all binary BH mergers detectable by advanced LIGO and Virgo at their design sensitivity have at least one component in the PI mass gap.

astro-ph.HE↗

Dynamics of black hole - neutron star binaries in young star clusters

Young star clusters are likely the most common birthplace of massive stars across cosmic time and influence the formation of compact binaries in several ways. Here, we simulate the formation of black hole -- neutron star binaries (BHNSs) in young star clusters, by means of the binary population synthesis code \texttt{MOBSE} interfaced with the $N$-body code \texttt{NBODY6++GPU}. BHNSs formed in young star clusters (dynamical BHNSs) are significantly more massive than BHNSs formed from isolated binaries (isolated BHNSs): $\sim{}40$~\% of the dynamical BHNS mergers have total mass $>15$ M$_\odot$, while only $\sim{}0.01$~\% of the isolated BHNS mergers have mass in excess of this value. Hence, our models strongly support a dynamical formation scenario for GW190814, given its total mass $\sim{}26$ M$_\odot$, if this event is a BHNS merger. All our dynamical BHNSs are ejected from their parent star cluster before they reach coalescence. Thus, a significant fraction of BHNS mergers occurring in the field might have originated in a young star cluster. The mass spectrum of BHNS mergers from gravitational-wave detections will provide a clue to differentiate between dynamical and isolated formation of BHNSs.

astro-ph.HE↗

Constraining the fraction of binary black holes formed in isolation and young star clusters with gravitational-wave data

Ten binary black-hole mergers have already been detected during the first two observing runs of advanced LIGO and Virgo, and many more are expected to be observed in the near future. This opens the possibility for gravitational-wave astronomy to better constrain the properties of black hole binaries, not only as single sources, but as a whole astrophysical population. In this paper, we address the problem of using gravitational-wave measurements to estimate the proportion of merging black holes produced either via isolated binaries or binaries evolving in young star clusters. To this end, we use a Bayesian hierarchical modeling approach applied to catalogs of merging binary black holes generated using state-of-the-art population synthesis and N-body codes. In particular, we show that, although current advanced LIGO/Virgo observations only mildly constrain the mixing fraction $f \in [0,1]$ between the two formation channels, we expect to narrow down the fractional errors on $f$ to $10-20\%$ after a few hundreds of detections.

astro-ph.HE↗