DEMOBLACK Publications

Since November 2018


Binary neutron star populations in the Milky Way

Sgalletta, C.; Iorio, G.; Mapelli, M.; Artale, M.C.; Boco, L.; Chattopadhyay, D.; Lapi, A.; Possenti, A.; Rinaldi, S.& Spera, M.

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 modelling the spin up and down of pulsars self-consistently. We analyse the merger rate RMW, orbital period Porb, eccentricity e, spin period P, and spin period derivative  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 (RMW30 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 (Porb, e, P, ) space, by comparing our radio-selected simulated pulsars against Galactic BNSs. Our analysis favours an uniform initial distribution for both the magnetic field (1010-13 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 ~20 new BNSs in the Milky Way.

Dynamical formation of Gaia BH1 in a yound star cluster

Rastello, S.; Iorio, G.; Mapelli, M.; Arca-Sedda, M.; Di Carlo U. N.; Escobar, G. J.; Shenar, T. & Torniamenti, S.

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 ≈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 × 102-3 × 104 M at solar metallicity, by means of 3.5 × 104 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 and initial conditions 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 match 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.

Massive binary black holes from population II and III stars

Costa, G.; Mapelli, M.; Iorio, G.; Santoliquido, F.; Escobar, G. J.; Klessen, R. S. & Bressan, A.

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 ≈86 (≈91) M 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. Only up to ≈3.3 per cent (≈0.09 per cent) 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.

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

Iorio, G.; Mapelli, M.; Costa, G.; Spera, M.; Escobar, G.J.; Sgalleta, C.; Trani, A.A.; Korb, E.; Santoliquido, F.; Dall’Amico, M.; Gaspari, N. & Bressan, A.

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 × 109 binaries in the metallicity range 0.0001 ≤ Z ≤ 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 (λov = 0.4, 0.5) and the pair-instability model, the maximum mass of a black hole can vary from ≈60 to ≈100 M. 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 ≳ 0.01) only <20% of the merging binary black holes and binary neutron stars form via this channel, while other authors found fractions >70%.

Binary black hole mergers from population III stars: Uncertainties from star formation and binary star properties

Santoliquido, F.; Mapelli, M.; Iorio, G.; Costa, G.; Glover, S.C.O.; Hartwig, T.; Klessen, R.S. & Merli, L.

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 ~ 8 up to z ~ 16 depending on the assumed star formation rate, while the maximum BBH merger rate density for our fiducial binary population model spans from ~2 to ~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) with respect to those born from metal-rich stars (8 – 10 M). We estimate that the Einstein Telescope will detect 10 – 104 Pop. III BBH mergers per year, depending on the star formation history and binary star properties.

Formation of black holes in the pair-instability mass gap: hydrodynamical simulations of a head-on massive star collision

Ballone, A.; Costa, G.; Mapelli, M.; MacLeod, M.; Torniamenti, S. & Pacheco-Arias, J.M.

The detection of the binary black hole merger GW190521, with primary black hole mass 85-0.14+21 M proved the existence of black holes in the theoretically predicted pair-instability gap (~60 − 120 M) of their mass spectrum. Some recent studies suggest that such massive black holes could be produced by the collision of an evolved star with a carbon-oxygen core and a main sequence star. Such a post-coalescence star could end its life avoiding the pair-instability regime and with a direct collapse of its very massive envelope. It is still not clear, however, how the collision shapes the structure of the newly produced star and how much mass is actually lost in the impact. We investigated this issue by means of hydrodynamical simulations with the smoothed particle hydrodynamics code STARSMASHER, finding that a head-on collision can remove up to 12 per cent of the initial mass of the colliding stars. This is a non-negligible percentage of the initial mass and could affect the further evolution of the stellar remnant, particularly in terms of the final mass of a possibly forming black hole. We also found that the main sequence star can plunge down to the outer boundary of the core of the primary, changing the inner chemical composition of the remnant. The collision expels the outer layers of the primary, leaving a remnant with an helium-enriched envelope (reaching He fractions of about 0.4 at the surface). These more complex abundance profiles can be directly used in stellar evolution simulations of the collision product.


Formation of black holes in the pair-instability mass gap: Evolution of a post-collision star

Costa, G.; Ballone, A.; Mapelli, M. & Bressan, A.

The detection of GW190521 by the LIGO-Virgo collaboration has revealed the existence of black holes (BHs) in the pair-instability (PI) mass gap. Here, we investigate the formation of BHs in the PI mass gap via star-star collisions in young stellar clusters. To avoid PI, the stellar-collision product must have a relatively small core and a massive envelope. We generate our initial conditions from the outputs of a hydrodynamical simulation of the collision between a core helium burning star (~58 M) and a main-sequence star (~42 M). The hydrodynamical simulation allows us to take into account the mass lost during the collision (~12 M) and to build the chemical composition profile of the post-collision star. We then evolve the collision product with the stellar evolution codes PARSEC and MESA. We find that the post-collision star evolves through all the stellar burning phases until core collapse, avoiding PI. At the onset of core collapse, the post-collision product is a blue supergiant star. We estimate a total mass-loss of about 1 M during the post-collision evolution, due to stellar winds and shocks induced by neutrino emission in a failed supernova. The final BH mass is ≈87 M. Therefore, we confirm that the collision scenario is a suitable formation channel to populate the PI mass gap.

Modelling the host galaxies of binary compact object mergers with observational scaling relations

Santoliquido, F.; Mapelli, M.; Artale, M.C. & Boco, L.

The merger rate density evolution of binary compact objects and the properties of their host galaxies carry crucial information to understand the sources of gravitational waves. Here, we present GALAXYRATE, a new code that estimates the merger rate density of binary compact objects and the properties of their host galaxies, based on observational scaling relations. We generate our synthetic galaxies according to the galaxy stellar mass function. We estimate the metallicity according to both the mass-metallicity relation (MZR) and the fundamental metallicity relation (FMR). Also, we take into account galaxy-galaxy mergers and the evolution of the galaxy properties from the formation to the merger of the binary compact object. We find that the merger rate density changes dramatically depending on the choice of the star-forming galaxy main sequence, especially in the case of binary black holes (BBHs) and black hole neutron star systems (BHNSs). The slope of the merger rate density of BBHs and BHNSs is steeper if we assume the MZR with respect to the FMR, because the latter predicts a shallower decrease of metallicity with redshift. In contrast, binary neutron stars (BNSs) are only mildly affected by both the galaxy main sequence and metallicity relation. Overall, BBHs and BHNSs tend to form in low-mass metal-poor galaxies and merge in high-mass metal-rich galaxies, while BNSs form and merge in massive galaxies. We predict that passive galaxies host at least ~5-10 per cent, ~15-25 per cent, and ~15-35 per cent of all BNS, BHNS, and BBH mergers in the local Universe.

Host galaxies and electromagnetic counterparts to binary neutron star mergers across the cosmic time: detectability of GW170817-like events

Perna, R.; Artale, M.C.; Wang, Y.; Mapelli, M.; Lazzati, D.; Sgalletta, C. & Santoliquido, F.

The association of GRB170817A with a binary neutron star (BNS) merger has revealed that BNSs produce at least a fraction of short gamma-ray bursts (SGRBs). As gravitational wave (GW) detectors push their horizons, it is important to assess coupled electromagnetic (EM)/GW probabilities and maximize observational prospects. Here, we perform BNS population synthesis calculations with the code MOBSE, seeding the binaries in galaxies at three representative redshifts, z= 0.01, 0.1, and 1 of the Illustris TNG50 simulation. The binaries are evolved and their locations numerically tracked in the host galactic potentials until merger. Adopting the microphysics parameters of GRB170817A, we numerically compute the broad-band light curves of jets from BNS mergers, with the afterglow brightness dependent on the local medium density at the merger site. We perform Monte Carlo simulations of the resulting EM population assuming either a random viewing angle with respect to the jet, or a jet aligned with the orbital angular momentum of the binary, which biases the viewing angle probability for GW-triggered events. We find a gamma-ray detection probability of 2 per cent, 10 per cent, and 40 per cent for BNSs at z = 1, 0.1, and 0.01, respectively, for the random case, rising to 75per cent for the z = 0.01, GW-triggered aligned case. Afterglow detection probabilities of GW-triggered BNS mergers vary in the range of 0.30.5 per cent, with higher values for aligned jets, and are comparable across the high- and low-energy bands, unlike gamma-ray-triggered events (cosmological SGRBs) which are significantly brighter at higher energies. We further quantify observational biases with respect to host galaxy masses.

Gravitational background from dynamical binaries and detectability with 2G detectors


Périgois, C.; Santoliquido, F.; Bouffanais, Y.; Di Carlo, U.; Giacobbo, N.; Rastello, S.; Mapelli, M. & Regimbau, T.

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 Ωgw(25 Hz )=1. 2-0.65+1.38×10-9 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.

The cosmic evolution of binary black holes in young, globular, and nuclear star clusters: rates, masses, spins, and mixing fractions

Mapelli, M.; Bouffanais, Y.; Santoliquido, F.; Arca Sedda, M. & Artale, M.C.

The growing population of binary black holes (BBHs) observed by gravitational wave (GW) detectors is a potential Rosetta stone for understanding their formation channels. Here, we use an upgraded version of our semi-analytical codes FASTCLUSTER and COSMO RATE to investigate the cosmic evolution of four different BBH populations: isolated BBHs and dynamically formed BBHs in nuclear star clusters (NSCs), globular clusters (GCs), and young star clusters (YSCs). With our approach, we can study different channels assuming the same stellar and binary input physics. We find that the merger rate density of BBHs in GCs and NSCs is barely affected by stellar metallicity (Z), while the rate of isolated BBHs changes wildly with Z. BBHs in YSCs behave in an intermediate way between isolated and GC/NSC BBHs. The local merger rate density of Nth-generation black holes (BHs), obtained by summing up hierarchical mergers in GCs, NSCs, and YSCs, ranges from ~1 to ~4 Gpc-3 yr-1 and is mostly sensitive to the spin parameter. We find that the mass function of primary BHs evolves with redshift in GCs and NSCs, becoming more top-heavy at higher z. In contrast, the primary BH mass function almost does not change with redshift in YSCs and in the field. This signature of the BH mass function has relevant implications for Einstein Telescope and Cosmic Explorer. Finally, our analysis suggests that multiple channels contribute to the BBH population of the second GW transient catalogue.

Clustering of Gravitational Wave and Supernovae events: a multitracer analysis in Luminosity Distance Space

Libanore, S.; Artale, M.C.; Karagiannis, D.; Liguori, M.; Bartolo, N.; Bouffanais, Y.; Mapelli, M. & Matarrese, S.

We study the clustering of Gravitational Wave (GW) merger events and Supernovae IA (SN), as cosmic tracers in Luminosity Distance Space. We modify the publicly available CAMB code to numerically evaluate auto- and cross- power spectra for the different sources, including Luminosity Distance Space distortion effects generated by peculiar velocities and lensing convergence. We perform a multitracer Fisher analysis to forecast expected constraints on cosmological and GW bias coefficients, using outputs from hydrodynamical N-body simulations to determine the bias fiducial model and considering future observations from the Vera Rubin Observatory and Einstein Telescope (ET), both single and in a 3 detector network configuration. We find that adding SN to the GW merger dataset considerably improves the forecast, mostly by breaking significant parameter degeneracies, with final constraints comparable to those obtainable from a Euclid-like survey. GW merger bias is forecasted to be detectable with good significance even in the single ET case.

Hierarchical generative models for star clusters from hydrodynamical simulations

Torniamenti, S.; Pasquato, M.; Di Cintio, P.; Ballone, A.; Iorio, G.; Artale, M.C. & Mapelli, M.

Star formation in molecular clouds is clumpy, hierarchically subclustered. Fractal structure also emerges in hydrodynamical simulations of star-forming clouds. Simulating the formation of realistic star clusters with hydrodynamical 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 hydrodynamical 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 hydrodynamical simulation. We use hierarchical clustering in phase space to inform 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 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 hydrodynamical 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.


GW190521 formation via three-body encounters in young massive star clusters

Dall’Amico, M.; Mapelli, M.; Di Carlo, U.N.; Bouffanais, Y.; Rastello, S.; Santoliquido, F.; Ballone, A. & Arca Sedda, M.

GW190521 is the most massive binary black hole (BBH) merger observed to date, and its primary component lies in the pair-instability (PI) mass gap. Here, we investigate the formation of GW190521-like systems via three-body encounters in young massive star clusters. We performed 2 × 105 simulations of binary-single interactions between a BBH and a massive ≥ 60  M black hole (BH), including post-Newtonian terms up to the 2.5 order and a prescription for relativistic kicks. In our initial conditions, we take into account the possibility of forming BHs in the PI mass gap via stellar collisions. If we assume that first-generation BHs have low spins, 0.17 per cent of all the simulated BBH mergers have component masses, effective and precessing spin, and remnant mass and spin inside the 90 per cent credible intervals of GW190521. Seven of these systems are first-generation exchanged binaries, while five are second-generation BBHs. We estimate a merger rate density of 0.03  Gpc-3  yr-1 for GW190521-like binaries formed via binary-single interactions in young star clusters. This rate is extremely sensitive to the spin distribution of first-generation BBHs. Stellar collisions, second-generation mergers and dynamical exchanges are the key ingredients to produce GW190521-like systems in young star clusters.

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 (300103 M) 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 105 direct N-body simulations with a high original binary fraction (100 % for stars with mass >5 M). Most BHs are ejected in the first 20 Myr by dynamical interactions. Dynamical exchanges are the main formation channel of BBHs, accounting for 4080 % of all the systems. Most BBH mergers in low-mass young star clusters involve primary BHs with mass <40 M and low mass ratios are extremely more common than in the field. Comparing our data with those of more massive star clusters (1033×104 M), 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.

Intermediate mass black holes from stellar mergers in young star clusters

Intermediate mass black holes (IMBHs) in the mass range 102105M 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 104 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 100140 M. This channel is strongly suppressed by the low escape velocity of our star clusters. In contrast, IMBHs with masses up to 438 M efficiently form via runaway stellar collisions, especially at low metallicity. Up to 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: 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: 8% (1%) of the simulated clusters with initial mass 1043×104 M (1035×103 M) host at least one IMBH.

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.

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 ∼103 to ∼20 Gpc-3 yr-1 if we change the common envelope efficiency parameter from αCE = 7 to 0.5, while the local merger rates of BBHs and BHNSs vary by a factor of ∼2-3. The BBH merger rate changes by one order of magnitude, when 1σ 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 αCE ≳ 2 and moderately low natal kicks (depending on the ejected mass and the supernovae mechanism), result in a local BNS merger rate density within the 90 per cent credible interval inferred from the second gravitational-wave transient catalogue.

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 2438 M (915 M) in metal-poor (metal-rich) NSCs. The maximum BH mass mainly depends on the escape velocity: BHs with mass up to several thousand M form in NSCs, while YSCs and GCs host BHs with mass up to several hundred M. Furthermore, we calculate the fraction of mergers with at least one component in the pair-instability mass gap (fPI) and in the intermediate-mass BH regime (fIMBH). In the fiducial model for dynamical BBHs with metallicity Z=0.002, we find fPI0.05, 0.02 and 0.007 (fIMBH0.01, 0.002 and 0.001) in NSCs, GCs and YSCs, respectively. Both fPI and fIMBH 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.

Formation of GW190521 from stellar evolution: the impact of the hydrogen-rich envelope, dredge-up, and 12C(α, γ)16O rate on the pair-instability black hole mass gap

Pair-instability (PI) is expected to open a gap in the mass spectrum of black holes (BHs) between ≍40-65 and ≍120 M. The existence of the mass gap is currently being challenged by the detection of GW190521, with a primary component mass of 85+2114 M. Here, we investigate the main uncertainties on the PI mass gap: the 12C(α, γ)16O reaction rate and the H-rich envelope collapse. With the standard 12C(α, γ)16O rate, the lower edge of the mass gap can be 70 M if we allow for the collapse of the residual H-rich envelope at metallicity Z ≤ 0.0003. Adopting the uncertainties given by the STARLIB database, for models computed with the 12C(α, γ)16O rate 1σ , we find that the PI mass gap ranges between ≍80 and ≍150 M. Stars with MZAMS > 110 M may experience a deep dredge-up episode during the core helium-burning phase, that extracts matter from the core enriching the envelope. As a consequence of the He-core mass reduction, a star with MZAMS = 160 M may avoid the PI and produce a BH of 150 M. In the 2σ case, the PI mass gap ranges from 92 to 110 M. Finally, in models computed with 12C(α, γ)16O 3σ , the mass gap is completely removed by the dredge-up effect. The onset of this dredge-up is particularly sensitive to the assumed model for convection and mixing. The combined effect of H-rich envelope collapse and low 12C(α, γ)16O rate can lead to the formation of BHs with masses consistent with the primary component of GW190521.

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. We explore the effects of additional hyper-parameters of the model, such as the spread in metallicity σZ and the parameter σsp, describing the distribution of spin magnitudes. We find that the dynamical model is slightly favoured with a median value of f=0.26, when σsp=0.1 and σZ=0.4. Models with higher spin magnitudes tend to strongly favour dynamically formed BBHs (f0.1 if σsp=0.3). Furthermore, we show that hyper-parameters controlling the rates of the model, such as σZ, have a large impact on the inference of the mixing fraction, which rises from 0.18 to 0.43 when we increase σZ from 0.2 to 0.6, for a fixed value of σsp=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.

From hydrodynamics to N-body simulations of star clusters: mergers and rotation

We present a new method to obtain more realistic initial conditions for N-body simulations of young star clusters. We start from the outputs of hydrodynamical simulations of molecular cloud collapse, in which star formation is modelled with sink particles. In our approach, we instantaneously remove gas from these hydrodynamical simulation outputs to mock the end of the gas-embedded phase, induced by stellar feedback. We then enforce a realistic initial mass function by splitting or joining the sink particles based on their mass and position. Such initial conditions contain more consistent information on the spatial distribution and the kinematical and dynamical states of young star clusters, which are fundamental to properly study these systems. For example, by applying our method to a set of previously run hydrodynamical simulations, we found that the early evolution of young star clusters is affected by gas removal and by the early dry merging of sub-structures. This early evolution can either quickly erase the rotation acquired by our (sub-)clusters in their embedded phase or ‘fuel’ it by feeding of angular momentum by sub-structure mergers, before two-body relaxation acts on longer time-scales.

Gravitational Wave mergers as tracers of Large Scale Structures

Clustering measurements of Gravitational Wave (GW) mergers in Luminosity Distance Space can be used in the future as a powerful tool for Cosmology. We consider tomographic measurements of the Angular Power Spectrum of mergers both in an Einstein Telescope-like detector network and in some more advanced scenarios (more sources, better distance measurements, better sky localization). We produce Fisher forecasts for both cosmological (matter and dark energy) and merger bias parameters. Our fiducial model for the number distribution and bias of GW events is based on results from hydrodynamical simulations. The cosmological parameter forecasts with Einstein Telescope are less powerful than those achievable in the near future via galaxy clustering observations with, e.g., Euclid. However, in the more advanced scenarios we see significant improvements. Moreover, we show that bias can be detected at high statistical significance. Regardless of the specific constraining power of different experiments, many aspects make this type of analysis interesting anyway. For example, compact binary mergers detected by Einstein Telescope will extend up to very high redshifts, particularly for binary black holes. Furthermore, Luminosity Distance Space Distortions in the GW analysis have a different structure with respect to Redshift-Space Distortions in galaxy catalogues. Finally, measurements of the bias of GW mergers can provide useful insight into their physical nature and properties.

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 on the properties of binary BH (BBH) mergers, by means of population synthesis simulations. We model mass accretion efficiency with the parameter fMT[0.05,1], which represents the fraction of mass lost from the donor which is effectively accreted by the companion. Lower values of fMT 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 observing run of LIGO-Virgo, yields almost zero support for values of fMT0.3. This result holds for all the values of the common-envelope efficiency parameter we considered in this study (αCE=1, 5 and 10). The lower boundaries of the 95% credible intervals are equal to fMT=0.40,0.45 and 0.48 for αCE=1, 5 and 10, respectively. This confirms that future gravitational-wave data can be used to put constraints on several uncertain binary evolution processes.


Hierarchical mergers in young, globular and nuclear star clusters: black hole masses and merger rates

Hierarchical mergers are one of the distinctive signatures of binary black hole (BBH) formation through dynamical evolution. Here, we present a fast Monte Carlo approach to simulate hierarchical mergers in nuclear star clusters (NSCs), globular clusters (GCs) and young star clusters (YSCs). Hierarchical mergers are orders of magnitude more common in NSCs than they are in both GCs and YSCs, because of the different escape velocity. In our fiducial model, the fraction of hierarchical mergers over all mergers is 0.15, 6×103 and 104 in NSCs, GCs and YSCs, respectively. The mass distribution of hierarchical BBHs strongly depends on the properties of first-generation BBHs, such as their progenitor’s metallicity. In our fiducial model, we form black holes (BHs) with masses up to 103 M in NSCs and up to 102 M in both GCs and YSCs. When escape velocities in excess of 100 km s1 are considered, BHs with mass >103 M are allowed to form in NSCs. Hierarchical mergers lead to the formation of BHs in the pair instability mass gap and intermediate-mass BHs (IMBHs), but only in metal-poor environments. In our fiducial model, at metallicity Z0.0002, the fraction of BBH mergers with primary BH in the pair instability mass gap is 7×103, 3×104 and 5×106 in NSCs, GCs and YSCs, respectively. In metal-poor NSCs, the fraction of BBH mergers with primary mass in the IMBH regime is 5×104. The local BBH merger rate in our models ranges from 10 to 60 Gpc3 yr1; hierarchical BBHs in NSCs account for 1020.2 Gpc3 yr1, with a strong upper limit of 10 Gpc3 yr1


Evolution of fractality and rotation in embedded star clusters

More and more observations indicate that young star clusters could retain imprints of their formation process. In particular, the degree of substructuring and rotation are possibly the direct result of the collapse of the parent molecular cloud from which these systems form. Such properties can, in principle, be washed-out, but they are also expected to have an impact on the relaxation of these systems. We ran and analysed a set of 10 hydrodynamical simulations of the formation of embedded star clusters through the collapse of turbulent massive molecular clouds. We systematically studied the fractality of our star clusters, showing that they are all extremely substructured (fractal dimension D = 1.0-1.8). We also found that fractality is slowly reduced, with time, on small scales, while it persists on large scales on longer time-scales. Signatures of rotation are found in different simulations at every time of the evolution, even for slightly supervirial substructures, proving that the parent molecular gas transfers part of its angular momentum to the new stellar systems.

Binary black hole mergers: formation and populations

We review the main physical processes that lead to the formation of stellar binary black holes (BBHs) and to their merger. BBHs can form from the isolated evolution of massive binary stars. The physics of core-collapse supernovae and the process of common envelope are two of the main sources of uncertainty about this formation channel. Alternatively, two black holes can form a binary by dynamical encounters in a dense star cluster. The dynamical formation channel leaves several imprints on the mass, spin and orbital properties of BBHs.

An astrophysically motivated ranking criterion for low-latency electromagnetic follow-up of gravitational wave events

We investigate the properties of the host galaxies of compact binary mergers across cosmic time. To this end, we combine population synthesis simulations together with galaxy catalogues from the hydrodynamical cosmological simulation EAGLE to derive the properties of the host galaxies of binary neutron star (BNS), black hole-neutron star (BHNS), and binary black hole (BBH) mergers. Within this framework, we derive the host galaxy probability, I.e. the probability that a galaxy hosts a compact binary coalescence as a function of its stellar mass, star formation rate, Ks magnitude, and B magnitude. This quantity is particularly important for low-latency searches of gravitational wave (GW) sources as it provides a way to rank galaxies lying inside the credible region in the sky of a given GW detection, hence reducing the number of viable host candidates. Furthermore, even if no electromagnetic counterpart is detected, the proposed ranking criterion can still be used to classify the galaxies contained in the error box. Our results show that massive galaxies (or equivalently galaxies with a high luminosity in Ks band) have a higher probability of hosting BNS, BHNS, and BBH mergers. We provide the probabilities in a suitable format to be implemented in future low-latency searches.

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

Santoliquido Filippo, Mapelli, Michela, Bouffanais Yann, Giacobbo Nicola, Di Carlo Ugo N., Rastello Sara, Artale M. Celeste, Ballone Alessandro, The cosmic merger rate density evolution of compact binaries formed in young star clusters and in isolated binaries, 2020, ApJ, 898, 152,…898..152S/abstract

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

Di Carlo Ugo N., Mapelli Michela, Giacobbo Nicola, Mario Spera, Yann Bouffanais, Sara Rastello, Filippo Santoliquido, Mario Pasquato, Alessandro Ballone, Alessandro A. Trani, Stefano Torniamenti, Francesco Haardt, Binary black holes in young star clusters: the impact of metallicity, 2020, MNRAS, 498, 495,


Fingerprints of binary black hole formation channels encoded in the mass and spin of merger remnants

Arca Sedda Manuel, Mapelli Michela, Spera Mario, Benacquista Matthew, Giacobbo Nicola, Fingerprints of binary black hole formation channels encoded in the mass and spin of merger remnants, 2020, ApJ, submitted,

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

Rastello Sara, Mapelli  Michela, Di Carlo Ugo N., Giacobbo Nicola, Santoliquido Filippo, Spera Mario, Ballone Alessandro, Dynamics of black hole – neutron star binaries in young star clusters, 2020, MNRAS, submitted, 


Impact of the Rotation and Compactness of Progenitors on the Mass of Black Holes

Mapelli Michela, Spera Mario, Montanari Enrico, Limongi Marco, Chieffi Alessandro, Giacobbo Nicola, Bressan Alessandro, Impact of the Rotation and Compactness of Progenitors on the Mass of Black Holes, 2020, ApJ, 888, 76,…888…76M/

Mass and star formation rate of the host galaxies of compact binary mergers across cosmic time

Artale M. Celeste, Mapelli Michela, Bouffanais Yann, Giacobbo Nicola, Spera Mario, Pasquato Mario, Mass and star formation rate of the host galaxies of compact binary mergers across cosmic time, 2020, MNRAS, 491, 3419,

Binary black holes in the pair-instability mass gap

Di Carlo Ugo N., Mapelli Michela, Bouffanais Yann, Giacobbo Nicola, Bressan Alessandro, Spera, Mario, Haardt, Francesco, Binary black holes in the pair-instability mass gap, MNRAS, submitted,


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

Bouffanais Yann, Mapelli Michela, Gerosa Davide, Di Carlo Ugo N., Giacobbo Nicola, Berti Emanuele, Baibhav Vishal, Constraining the fraction of binary black holes formed in isolation and young star clusters with gravitational-wave data, 2019, ApJ, in press,

Gravitational-wave detection rates for compact binaries formed in isolation: LIGO/Virgo O3 and beyond

Baibhav Vishal, Berti Emanuele, Gerosa Davide, Mapelli Michela, Giacobbo Nicola, Bouffanais Yann, Di Carlo Ugo N., Gravitational-wave detection rates for compact binaries formed in isolation: LIGO/Virgo O3 and beyond, 2019, PhRvD, 100, 4060 ,


Before 2019

The host galaxies of double compact objects merging in the local Universe

Mapelli Michela, Giacobbo Nicola, Toffano Mattia, Ripamonti Emanuele, Bressan Alessandro, Spera Mario, Branchesi Marica, The host galaxies of double compact objects merging in the local Universe, MNRAS, 2018, MNRAS, 481, 5324,

Merging black hole binaries: the effects of progenitor’s metallicity, mass-loss rate and Eddington factor

Giacobbo Nicola, Mapelli Michela, Spera Mario, Merging black hole binaries: the effects of progenitor’s metallicity, mass-loss rate and Eddington factor, 2018, MNRAS, 474, 2959,

The formation and coalescence sites of the first gravitational wave events

Schneider Raffaella, Graziani Luca, Marassi Stefania, Spera Mario, Mapelli Michela, Alparone Matteo, de Bennassuti Matteo, The formation and coalescence sites of the first gravitational wave events, 2017, MNRAS Letters, 471, L105,

Hierarchical black hole triples in young star clusters: impact of Kozai-Lidov resonance on mergers

Kimpson Thomas O., Spera Mario, Mapelli Michela, Ziosi Brunetto M., Hierarchical black hole triples in young star clusters: impact of Kozai-Lidov resonance on mergers, 2016, MNRAS, 463, 2443,

Dynamics of stellar black holes in young star clusters with different metallicities – II. Black hole-black hole binaries

Ziosi Brunetto M., Mapelli Michela, Branchesi Marica, Tormen Giuseppe, Dynamics of stellar black holes in young star clusters with different metallicities – II. Black hole-black hole binaries, 2014, MNRAS, 441, 3703

Dynamics of stellar black holes in young star clusters with different metallicities – I. Implications for X-ray binaries

Mapelli Michela, Zampieri Luca, Ripamonti Emanuele, Bressan Alessandro, Dynamics of stellar black holes in young star clusters with different metallicities – I. Implications for X-ray binaries, 2013, MNRAS, 429, 2298,

Dynamics of massive stellar black holes in young star clusters and the displacement of ultra-luminous X-ray sources

Mapelli Michela, Ripamonti Emanuele, Zampieri Luca, Colpi Monica, Dynamics of massive stellar black holes in young star clusters and the displacement of ultra-luminous X-ray sources, 2011, MNRAS, 416, 1756,