Studying the formation and evolution of black hole binaries (BHBs) is essential for the interpretation of current and forthcoming gravitational wave (GW) detections. We investigate the statistics of BHBs that form from isolated binaries, by means of a new version of the SEVN population-synthesis code. SEVN integrates stellar evolution by interpolation over a grid of stellar evolution tracks. We upgraded SEVN to include binary stellar evolution processes and we used it to evolve a sample of 1.5 × 10 8 binary systems, with metallicity in the range 10 −4 ; 4 × 10 −2 . From our simulations, we find that the mass distribution of black holes (BHs) in double compact-object binaries is remarkably similar to the one obtained considering only single stellar evolution. The maximum BH mass we obtain is ∼ 30, 45 and 55 M at metallicity Z = 2×10 −2 , 6×10 −3 , and 10 −4 , respectively. A few massive single BHs may also form ( < ∼ 0.1% of the total number of BHs), with mass up to ∼ 65, 90 and 145 M at Z = 2 × 10 −2 , 6 × 10 −3 , and 10 −4 , respectively. These BHs fall in the mass gap predicted from pair-instability supernovae. We also show that the most massive BHBs are unlikely to merge within a Hubble time. In our simulations, merging BHs like GW151226 and GW170608, form at all metallicities, the high-mass systems (like GW150914, GW170814 and GW170104) originate from metal poor (Z < ∼ 6 × 10 −3 ) progenitors, whereas GW170729-like systems are hard to form, even at Z = 10 −4 . The BHB merger rate in the local Universe obtained from our simulations is ∼ 90Gpc −3 yr −1 , consistent with the rate inferred from LIGO-Virgo data.

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 ρh ≥ 3.4 × 104 and ≥1.5 × 102 M⊙ pc−3 in dense and loose star clusters, respectively). Metal-poor clusters tend to form more massive BHs than metal-rich ones. We find ∼6, ∼2, and &lt;1 % of BHs with mass mBH &gt; 60 M⊙ at Z = 0.0002, 0.002 and 0.02, respectively. In metal-poor clusters, we form intermediate-mass BHs with mass up to ∼320 M⊙. 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 ∼140 M⊙ (∼80 M⊙). The most massive BBH merger in our simulations has primary mass ∼88 M⊙, inside the pair-instability mass gap, and a mass ratio of ∼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 ∼110 and ∼55 Gpc−3 yr−1, if we assume that all stars form in loose and dense star clusters, respectively.

We present the merger rate density of Population III binary black holes (BHs) by means of a widely used binary population synthesis code BSE with extensions to very massive and extreme metal-poor stars. We consider not only low-mass BHs (lBHs: 5–50M ⊙) but also high-mass BHs (hBHs: 130–200M ⊙), where lBHs and hBHs are below and above the pair-instability mass gap (50–130M ⊙), respectively. Population III BH–BHs can be categorized into three subpopulations: BH–BHs without hBHs (hBH0s: m tot ≲ 100M ⊙), with one hBH (hBH1s: m tot ∼ 130–260M ⊙), and with two hBHs (hBH2s: m tot ∼ 270–400M ⊙), where m tot is the total mass of a BH–BH. Their merger rate densities at the current universe are ∼0.1 yr−1 Gpc−3 for hBH0s, and ∼0.01 yr−1 Gpc−3 for the sum of hBH1s and hBH2s, provided that the mass density of Population III stars is ∼1013 M ⊙ Gpc−3. These rates are modestly insensitive to initial conditions and single star models. The hBH1 and hBH2 mergers can dominate BH–BHs with hBHs discovered in the near future. They have low effective spins ≲0.2 in the current universe. The number ratio of hBH2s to hBH1s is high, ≳0.1. We also find that BHs in the mass gap (up to ∼85M ⊙) merge. These merger rates can be reduced to nearly zero if Population III binaries are always wide (≳100R ⊙), and if Population III stars always enter into chemically homogeneous evolution. The presence of close Population III binaries (∼10R ⊙) is crucial for avoiding the worst scenario.

We study chaos and Lévy flights in the general gravitational three-body problem. We introduce new metrics to characterize the time evolution and final lifetime distributions, namely Scramble Density $\mathcal {S}$ and the LF index $\mathcal {L}$, that are derived from the Agekyan-Anosova maps and homology radius $R_{\mathcal {H}}$. Based on these metrics, we develop detailed procedures to isolate the ergodic interactions and Lévy flight interactions. This enables us to study the three-body lifetime distribution in more detail by decomposing it into the individual distributions from the different kinds of interactions. We observe that ergodic interactions follow an exponential decay distribution similar to that of radioactive decay. Meanwhile, Lévy flight interactions follow a power-law distribution. Lévy flights in fact dominate the tail of the general three-body lifetime distribution, providing conclusive evidence for the speculated connection between power-law tails and Lévy flight interactions. We propose a new physically-motivated model for the lifetime distribution of three-body systems and discuss how it can be used to extract information about the underlying ergodic and Lévy flight interactions. We discuss ejection probabilities in three-body systems in the ergodic limit and compare it to previous ergodic formalisms. We introduce a novel mechanism for a three-body relaxation process and discuss its relevance in general three-body systems.

We investigate the formation of merging binary black holes (BHs) through isolated binary evolution, performing binary population synthesis calculations covering an unprecedentedly wide metallicity range of Population (Pop) I, II, III, and extremely metal-poor (EMP) binary stars. We find that the predicted merger rate density and primary BH mass (m 1) distribution are consistent with the gravitational wave (GW) observations. Notably, Population III and EMP (<10−2 Z ⊙) binary stars yield most of the pair instability (PI) mass gap events with m 1 = 65–130 M ⊙. Population III binary stars contribute more to the PI mass gap events with increasing redshift, and all the PI mass gap events have the Population III origin at redshifts ≳8. Our result can be assessed by future GW observations in the following two points. First, there are no binary BHs with m 1 = 100–130 M ⊙ in our result, and thus the m 1 distribution should suddenly drop in the range of m 1 = 100–130 M ⊙. Second, the PI mass gap event rate should increase toward higher redshift up to ∼11, since those events mainly originate from the Population III binary stars. We find that the following three assumptions are needed to reproduce the current GW observations: a top-heavy stellar initial mass function and the presence of close binary stars for Population III and EMP binary stars, and inefficient convective overshoot in the main-sequence phase of stellar evolution. Without any of the above, the number of PI mass gap events becomes too low to reproduce current GW observations.

Recent spectroscopic analysis has set an upper limit to the age of the S-stars, the ∼30 B-type stars in highly eccentric orbits around the supermassive black hole (SMBH) in the Galactic center. The inferred age (<15 Myr) is in tension with the binary break-up scenario proposed to explain their origin. However, the new estimate is compatible with the age of the disk of O-type stars that lies at a farther distance from the SMBH. Here we investigate a new formation scenario, assuming that both S-stars and the O-type stars were born in the same disk around SgrA*. We simulate encounters between binaries of the stellar disk and stellar black holes from a dark cusp around SgrA*. We find that B-type binaries can be easily broken up by the encounters and their binary components are kicked into highly eccentric orbits around the SMBH. In contrast, O-type binaries are less frequently disrupted and their members remain in low eccentricity orbits. This mechanism can reproduce 12 S-stars just by assuming that the binaries initially lie within the stellar disk as observed nowadays. To reproduce all the S-stars, the original disk must have been extended down to 0.006 pc. However in this case many B-and O-type stars remain in low eccentricity orbits below 0.03 pc, in contrast with the observations. Therefore, some other mechanism is necessary to disrupt the disk below 0.03 pc. This scenario can also explain the high eccentricity of the G-objects, if they have a stellar origin.

We report the first discovery of a thick-disk planet, LHS 1815b (TOI-704b, TIC 260004324), detected in the TESS survey. LHS 1815b transits a bright (V = 12.19 mag, K = 7.99 mag) and quiet M dwarf located 29.87 ± 0.02 pc away with a mass of 0.502 ± 0.015 M and a radius of 0.501 ± 0.030 R . We validate the planet by combining space and ground-based photometry, spectroscopy, and imaging. The planet has a radius of 1.088 ± 0.064 R ⊕ with a 3σ mass upper-limit of 8.7 M ⊕ . We analyze the galactic kinematics and orbit of the host star LHS 1815 and find that it has a large probability (P thick /P thin = 6482) to be in the thick disk 2 with a much higher expected maximal height (Z max = 1.8 kpc) above the Galactic plane compared with other TESS planet host stars. Future studies of the interior structure and atmospheric properties of planets in such systems using for example the upcoming James Webb Space Telescope (JWST), can investigate the differences in formation efficiency and evolution for planetary systems between different Galactic components (thick and thin disks, and halo).

Recent studies indicate that the progenitors of merging black hole (BH) binaries from young star clusters can undergo a common envelope phase just like isolated binaries. If the stars emerge from the common envelope as naked cores, tidal interactions can efficiently synchronize their spins before they collapse into BHs. Contrary to the isolated case, these binary BHs can also undergo dynamical interactions with other BHs in the cluster before merging. The interactions can tilt the binary orbital plane, leading to spin-orbit misalignment. We estimate the spin properties of merging binary BHs undergoing this scenario by combining up-to-date binary population synthesis and accurate few-body simulations. We show that post-common envelope binary BHs are likely to undergo only a single encounter, due to the high binary recoil velocity and short coalescence times. Adopting conservative limits on the binary-single encounter rates, we obtain a local BH merger rate density of ${\sim } 6.6 {-1} \, \rm Gpc^{-3}$. Assuming low (≲ 0.2) natal BH spins, this scenario reproduces the trends in the distributions of effective spin χeff and precession parameters χp inferred from GWTC-2, including the peaks at (χeff, χp) ∼ (0.1, 0.2) and the tail at negative χeff values.