Unravelling the progenitors of merging black hole binaries

The recent detection of gravitational waves has proven the existence of massive stellar black hole binaries (BBHs), but the formation channels of BBHs are still an open question. Here, we investigate the demography of BBHs by using our new population-synthesis code MOBSE. MOBSE is an updated version of the widely used binary population-synthesis code BSE [6, 7] and includes the key ingredients to determine the fate of massive stars: up-to-date stellar wind prescriptions and supernova models. With MOBSE, we form BBHs with total mass up to ~ 120 M&09 at low metallicity, but only systems with total mass up to ~ 80 M&09 merge in less than a Hubble time. Our results show that only massive metal-poor stars (Z 72 0.002) can be the progenitors of gravitational wave events like GW150914. Moreover, we predict that merging BBHs form much more efficiently from metal-poor than from metal-rich stars

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.Comment: 17 pages, 4 figure

Black-hole remnants from black-hole-neutron-star mergers

Observations of gravitational waves and their electromagnetic counterparts may soon uncover the existence of coalescing compact binary systems formed by a stellar-mass black hole and a neutron star. These mergers result in a remnant black hole, possibly surrounded by an accretion disk. The mass and spin of the remnant black hole depend on the properties of the coalescing binary. We construct a map from the binary components to the remnant black hole using a sample of numerical-relativity simulations of different mass ratios q, (anti)aligned dimensionless spins of the black hole aBH, and several neutron star equations of state. Given the binary total mass, the mass and spin of the remnant black hole can therefore be determined from the three parameters (q,aBH,Λ), where Λ is the tidal deformability of the neutron star. Our models also incorporate the binary black hole and test-mass limit cases and we discuss a simple extension for generic black-hole spins. We combine the remnant characterization with recent population synthesis simulations for various metallicities of the progenitor stars that generated the binary system. We predict that black-hole-neutron-star mergers produce a population of remnant black holes with masses distributed around 7 M and 9 M. For isotropic spin distributions, nonmassive accretion disks are favored: no bright electromagnetic counterparts are expected in such mergers

Merging black holes in young star clusters

Searching for distinctive signatures, which characterize different formation channels of binary black holes (BBHs), is a crucial step towards the interpretation of current and future gravitationalwave detections.Here, we investigate the demography ofmerging BBHs in young star clusters (SCs), which are the nursery of massive stars. We performed 4 7 103 N-body simulations of SCs with metallicity Z = 0.002, initial binary fraction 0.4, and fractal initial conditions, to mimic the clumpiness of star-forming regions. Our simulations include a novel population-synthesis approach based on the code MOBSE. We find that SC dynamics does not affect the merger rate significantly, but leaves a strong fingerprint on the properties of merging BBHs. More than 50 per cent of merging BBHs in young SCs form by dynamical exchanges in the first few Myr. Dynamically formed merging BBHs are significantly heavier than merging BBHs in isolated binaries: merging BBHs with total mass up to ~120M 99 form in young SCs, while the maximum total mass of merging BBHs in isolated binaries with the same metallicity is only ~70 M 99. Merging BBHs born via dynamical exchanges tend to have smaller mass ratios than BBHs in isolated binaries. Furthermore, SC dynamics speeds up the merger: the delay time between star formation and coalescence is significantly shorter in young SCs. In our simulations, massive systems such as GW170729 form only via dynamical exchanges. Finally ~2 per cent of merging BBHs in young SCs have mass in the pair-instability mass gap (~60-120 M 99). This represents a unique fingerprint of merging BBHs in SCs

Host galaxies of merging compact objects: mass, star formation rate, metallicity, and colours

Characterizing the properties of the host galaxies of merging compact objects provides essential clues to interpret current and future gravitational-wave detections. Here, we investigate the stellar mass, star formation rate (SFR), metallicity, and colours of the host galaxies of merging compact objects in the local Universe by combining the results of MOBSE population-synthesis models together with galaxy catalogues from the EAGLE simulation. We predict that the stellar mass of the host galaxy is an excellent tracer of the merger rate per galaxy n(GW) of double neutron stars (DNSs), double black holes (DBHs), and black hole-neutron star binaries (BHNSs). We find a significant correlation also between n(GW) and SFR. As a consequence, n(GW) correlates also with the r-band luminosity and with the g-r colour of the host galaxies. Interestingly, greater than or similar to 60 per cent, greater than or similar to 64 per cent, and greater than or similar to 73 per cent of all the DNSs, BHNSs, and DBHs merging in the local Universe lie in early-type galaxies, such as NGC 4993. We predict a local DNS merger rate density of similar to 238 Gpc(-3) yr(-1) and a DNS merger rate similar to 16-121 Myr(-1) for Milky Way-like galaxies. Thus, our results are consistent with both the DNS merger rate inferred from GW170817 and the one inferred from Galactic DNSs

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 MOBSE interfaced with the N-body code NBODY6++GPU. BHNSs formed in young star clusters (dynamical BHNSs) are significantly more massive than BHNSs formed from isolated binaries (isolated BHNSs): ~40 per cent of the dynamical BHNS mergers have a total mass of > 15 M0, while only ~0.01 per cent 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 of ~26 M0, 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

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

Using simulations performed with the population synthesis code MOBSE, we compute the merger rate densities and detection rates of compact binary mergers formed in isolation for second- and third-generation gravitational-wave detectors. We estimate how rates are affected by uncertainties on key stellar-physics parameters, namely common envelope evolution and natal kicks. We estimate how future upgrades will increase the size of the available catalog of merger events, and we discuss features of the merger rate density that will become accessible with third-generation detectors.Comment: 9 pages, 4 figures, 2 tables Matches the published versio

Genetic diversity of the genus Prunus based on per se evaluation of peach clonal rootstocks.

A aplicação de análises multivariadas e quantificação da divergência genética fornecem parâmetros que favorecem a seleção de porta-enxertos superiores. O objetivo deste trabalho foi avaliar a viabilidade técnica do uso de parte da diversidade genética do gênero Prunus como porta-enxerto clonal do pessegueiro ?BRS-Libra?. O pomar experimental faz parte de uma rede nacional de avaliação de porta-enxertos para prunáceas, sob a coordenação geral da Embrapa Clima Temperado. O delineamento experimental foi em blocos ao acaso e a coleta de dados ocorreu no ciclo produtivo 2016/2017. Foram realizadas análises físicas (área de secção do tronco, massa média de frutos) e química (sólidos solúveis), além da quantificação da produtividade estimada das plantas. Os dados obtidos foram submetidos ao teste de normalidade de Shapiro-Wilk, ao nível 5% de probabilidade, sendo posteriormente realizado o agrupamento dos porta-enxertos através do método hierárquico UPGMA. A realização do agrupamento resultou na formação de cinco grupos. As cultivares do grupo I, Mirabolano 29C e Marianna 2624, apresentaram incompatibilidade de enxertia com a cultivar BRS-Libra. Semelhantemente, as cultivares do grupo II apresentaram sintomas característicos de incompatibilidade de enxertia, resultando em pouco desenvolvimento. Os grupos III e IV são compostos por cultivares que apresentaram baixo e médio vigor, com boas perspectivas de uso para formar pomares em alta densidade, enquanto as cultivares do grupo V destacaram-se por apresentar alto vigor, recomendadas para pomares de baixa densidade. O uso de diferentes porta-enxertos influencia no comportamento da cultivar BRS-Libra no vigor e produção de frutos, havendo divergência genética entre eles

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 $\sigma_{\text{Z}}$ and the parameter $\sigma_{\text{sp}}$, describing the distribution of spin magnitudes. We find that the dynamical model is slightly favoured with a median value of $f=0.26$, when $\sigma_{\text{sp}}=0.1$ and $\sigma_{\text{Z}}=0.4$. Models with higher spin magnitudes tend to strongly favour dynamically formed BBHs ($f\le{}0.1$ if $\sigma_{\text{sp}}=0.3$). Furthermore, we show that hyper-parameters controlling the rates of the model, such as $\sigma_{\rm Z}$, have a large impact on the inference of the mixing fraction, which rises from $0.18$ to $0.43$ when we increase $\sigma_{\text{Z}}$ from 0.2 to 0.6, for a fixed value of $\sigma_{\text{sp}}=0.1$. 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.Comment: 13 pages, 10 figures, 2 tables, published in MNRA