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Star clusters appear to be the ideal environment for the assembly of neutron star-neutron star (NS-NS) and black hole-neutron star (BH-NS) binaries. These binaries are among the most interesting astrophysical objects, being potential sources of gravitational waves (GWs) and gamma-ray bursts. We use for the first time high-precision N-body simulations of young massive and open clusters to study the origin and dynamical evolution of NSs, within clusters with different initial masses, metallicities, primordial binary fractions, and prescriptions for the compact object natal kicks at birth. We find that the radial profile of NSs is shaped by the BH content of the cluster, which partially quenches the NS segregation due to the BH-burning process. This leaves most of the NSs out of the densest cluster regions, where NS-NS and BH-NS binaries could potentially form. Due to a large velocity kick that they receive at birth, most of the NSs escape the host clusters, with the bulk of their retained population made up of NSs of $sim 1.3$ M$_odot$ coming from the electron-capture supernova process. The details of the primordial binary fraction and pairing can smear out this trend. Finally, we find that a subset of our models produce NS-NS mergers, leading to a rate of $sim 0.01$--$0.1$ Gpc$^{-3}$ yr$^{-1}$ in the local Universe, and compute an upper limit of $sim 3times 10^{-2}$--$3times 10^{-3}$ Gpc$^{-3}$ yr$^{-1}$ for the BH-NS merger rate. Our estimates are several orders of magnitude smaller than the current empirical merger rate from LIGO/Virgo, in agreement with the recent rate estimates for old globular clusters.
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The young star clusters we observe today are the building blocks of a new generation of stars and planets in our Galaxy and beyond. Despite their fundamental role we still lack knowledge about the conditions under which star clusters form and the impact of these often harsh environments on the evolution of their stellar and substellar members. We demonstrate the vital role numerical simulations play to uncover both key issues. Using dynamical models of different star cluster environments we show the variety of effects stellar interactions potentially have. Moreover, our significantly improved measure of mass segregation reveals that it can occur rapidly even for star clusters without substructure. This finding is a critical step to resolve the controversial debate on mass segregation in young star clusters and provides strong constraints on their initial conditions.
Even though plenty of symbiotic stars (SySts) have been found in the Galactic field and nearby galaxies, not a single one has ever been confirmed in a Galactic globular cluster (GC). We investigate the lack of such systems in GCs for the first time by analysing 144 GC models evolved with the MOCCA code, which have different initial properties and are roughly representative of the Galactic GC population. We focus here on SySts formed through the wind-accretion channel, which can be consistently modelled in binary population synthesis codes. We found that the orbital periods of the majority of such SySts are sufficiently long (${gtrsim10^3}$ d) so that, for very dense GC models, dynamical interactions play an important role in destroying their progenitors before the present day (${sim11-12}$ Gyr). In less dense GC models, some SySts are still predicted to exist. However, these systems tend to be located far from the central parts (${gtrsim70}$ per cent are far beyond the half-light radius) and are sufficiently rare (${lesssim1}$ per GC per Myr), which makes their identification rather difficult in observational campaigns. We propose that future searches for SySts in GCs should be performed in the outskirts of nearby low-density GCs with sufficiently long half-mass relaxation times and relatively large Galactocentric distances. Finally, we obtained spectra of the candidate proposed in $omega$ Cen (SOPS IV e-94) and showed that this object is most likely not a SySt.
Depending on the stellar type, more than $sim 50$% and $sim 15%$ of stars in the field have at least one and two stellar companions, respectively. Hierarchical systems can be assembled dynamically in dense star clusters, as a result of few-body encounters among stars and/or compact remnants in the cluster core. In this paper, we present the demographics of stellar and compact-object triples formed via binary--binary encounters in the texttt{CMC Cluster Catalog}, a suite of cluster simulations with present-day properties representative of the globular clusters (GCs) observed in the Milky Way. We show how the initial properties of the host cluster set the typical orbital parameters and formation times of the formed triples. We find that a cluster typically assembles hundreds of triples with at least one black hole (BH) in the inner binary, while only clusters with sufficiently small virial radii are efficient in producing triples with no BHs, as a result of the BH-burning process. We show that a typical GC is expected to host tens of triples with at least one luminous component at present day. We discuss how the Lidov-Kozai mechanism can drive the inner binary of the formed triples to high eccentricities, whenever it takes place before the triple is dynamically reprocessed by encountering another cluster member. Some of these systems can reach sufficiently large eccentricities to form a variety of transients and sources, such as blue stragglers, X-ray binaries, Type Ia Supernovae, Thorne-Zytkow objects, and LIGO/Virgo sources.
We study the populations of massive stars in the Carina region and their energetic feedback and ejection of $^{26}$Al. We did a census of the stellar populations in young stellar clusters within a few degrees of the Carina Nebula. For each star we estimated the mass, based on the spectral type and the host cluster age. We used population synthesis to calculate the energetic feedback and ejection of $^{26}$Al from the winds of the massive stars and their supernova explosions. We used 7 years of INTEGRAL observations to measure the $^{26}$Al signal from the region. The INTEGRAL $^{26}$Al signal is not significant with a best-fit value of about 1.5e-5 ph/cm^2/s, approximately half of the published Compton Gamma Ray Observatory (CGRO) result, but in agreement with the latest CGRO estimates. Our analysis of the stellar populations in the young clusters leads to an expected signal of half the observed value, but the results are consistent within 2 sigma. We find that the fraction of $^{26}$Al ejected in Wolf-Rayet winds is high, and the observed signal is unlikely to be caused by $^{26}$Al ejected in supernovae alone, indicating a strong wind ejection of $^{26}$Al. Due to the lack of prominent O stars, regions with ages $gtrsim$10 Myr are often neglected in studies of OB associations. We find that in the Carina region such clusters contribute significantly to the stellar mass and the energetics of the region.
The LIGO-Virgo-Kagra collaboration (LVC) discovered recently GW190521, a gravitational wave (GW) source associated with the merger between two black holes (BHs) with mass $66$ M$_odot$ and $>85$ M$_odot$. GW190521 represents the first BH binary (BBH) merger with a primary mass falling in the upper mass-gap and the first leaving behind a $sim 150$ M$_odot$ remnant. So far, the LVC reported the discovery of four further mergers having a total mass $>100$ M$_odot$, i.e. in the intermediate-mass black holes (IMBH) mass range. Here, we discuss results from a series of 80 $N$-body simulations of young massive clusters (YMCs) that implement relativistic corrections to follow compact object mergers. We discover the development of a GW190521-like system as the result of a 3rd-generation merger, and four IMBH-BH mergers with total mass $~(300-350)$ M$_odot$. We show that these IMBH-BH mergers are low-frequency GW sources detectable with LISA and DECIGO out to redshift $z=0.01-0.1$ and $z>100$, and we discuss how their detection could help unravelling IMBH natal spins. For the GW190521 test case, we show that the 3rd-generation merger remnant has a spin and effective spin parameter that matches the $90%$ credible interval measured for GW190521 better than a simpler double merger and comparably to a single merger. Due to GW recoil kicks, we show that retaining the products of these mergers require birth-sites with escape velocities $gtrsim 50-100$ km s$^{-1}$, values typically attained in galactic nuclei and massive clusters with steep density profiles.
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