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1. Nuclear star clusters

   https://doi.org/10.1007/s00159-020-00125-0  

   Neumayer, Nadine; Seth, Anil; Böker, Torsten (December 2020, The Astronomy and Astrophysics Review)

   Abstract We review the current knowledge about nuclear star clusters (NSCs), the spectacularly dense and massive assemblies of stars found at the centers of most galaxies. Recent observational and theoretical works suggest that many NSC properties, including their masses, densities, and stellar populations, vary with the properties of their host galaxies. Understanding the formation, growth, and ultimate fate of NSCs, therefore, is crucial for a complete picture of galaxy evolution. Throughout the review, we attempt to combine and distill the available evidence into a coherent picture of NSC evolution. Combined, this evidence points to a clear transition mass in galaxies of $$\sim 10^9\,M_\odot$$ ∼ 10 9 M ⊙ where the characteristics of nuclear star clusters change. We argue that at lower masses, NSCs are formed primarily from globular clusters that inspiral into the center of the galaxy, while at higher masses, star formation within the nucleus forms the bulk of the NSC. We also discuss the co-existence of NSCs and central black holes, and how their growth may be linked. The extreme densities of NSCs and their interaction with massive black holes lead to a wide range of unique phenomena including tidal disruption and gravitational-wave events. Finally, we review the evidence that many NSCs end up in the halos of massive galaxies stripped of the stars that surrounded them, thus providing valuable tracers of the galaxies’ accretion histories. 

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2. The Star Formation History of the Milky Way’s Nuclear Star Cluster

   https://doi.org/10.3847/1538-4357/aca8ad  

   Chen, Zhuo; Do, Tuan; Ghez, Andrea M.; Hosek, Jr., Matthew W.; Feldmeier-Krause, Anja; Chu, Devin S.; Bentley, Rory O.; Lu, Jessica R.; Morris, Mark R. (February 2023, The Astrophysical Journal)

   Abstract We report the first star formation history study of the Milky Ways nuclear star cluster (NSC), which includes observational constraints from a large sample of stellar metallicity measurements. These metallicity measurements were obtained from recent surveys from Gemini and the Very Large Telescope of 770 late-type stars within the central 1.5 pc. These metallicity measurements, along with photometry and spectroscopically derived temperatures, are forward modeled with a Bayesian inference approach. Including metallicity measurements improves the overall fit quality, as the low-temperature red giants that were previously difficult to constrain are now accounted for, and the best fit favors a two-component model. The dominant component contains 93% ± 3% of the mass, is metal-rich ( $\overline{\left[\mathrm{M}/\mathrm{H}\right]}\sim 0.45$), and has an age of $5_{-2}^{+3}$Gyr, which is ∼3 Gyr younger than earlier studies with fixed (solar) metallicity; this younger age challenges coevolutionary models in which the NSC and supermassive black holes formed simultaneously at early times. The minor population component has low metallicity ( $\overline{\left[\mathrm{M}/\mathrm{H}\right]}\sim -1.1$) and contains ∼7% of the stellar mass. The age of the minor component is uncertain (0.1–5 Gyr old). Using the estimated parameters, we infer the following NSC stellar remnant population (with ∼18% uncertainty): 1.5 × 10⁵neutron stars, 2.5 × 10⁵stellar-mass black holes (BHs), and 2.2 × 10⁴BH–BH binaries. These predictions result in 2–4 times fewer neutron stars compared to earlier predictions that assume solar metallicity, introducing a possible new path to understand the so-called “missing-pulsar problem”. Finally, we present updated predictions for the BH–BH merger rates (0.01–3 Gpc⁻³yr⁻¹). 

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3. Inferring Small Neutron Star Spins with Neutron Star–Black Hole Mergers

   https://doi.org/10.3847/1538-4357/ad49a0  

   Gupta, Ish (July 2024, The Astrophysical Journal)

   Abstract The precise measurement of neutron star (NS) spins can provide important insight into the formation and evolution of compact binaries containing NSs. While traditional methods of NS spin measurement rely on pulsar observations, gravitational-wave detections offer a complementary avenue. However, determining component spins with gravitational waves is hindered by the small dimensionless spins of the NSs and the degeneracy in the mass and spin parameters. This degeneracy can be addressed by the inclusion of higher-order modes in the waveform, which are important for systems with unequal masses. This study shows the suitability of NS–black hole mergers, which are naturally mass-asymmetric, for precise NS spin measurements. We explore the effects of the black hole masses and spins, higher-mode content, inclination angles, and detector sensitivity on the measurement of NS spin. We find that networks with next-generation observatories like the Cosmic Explorer and the Einstein Telescope can distinguish NS dimensionless spin of 0.04 (0.1) from zero at 1σconfidence for events within ∼350 (∼1000) Mpc. Networks with A+ and A^♯detectors achieve similar distinction within ∼30 (∼70) Mpc and ∼50 (∼110) Mpc, respectively. 

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4. Gravitational waveforms from spectral Einstein code simulations: Neutron star-neutron star and low-mass black hole-neutron star binaries

   https://doi.org/10.1103/PhysRevD.99.044008  

   Foucart, F.; Duez, M. D.; Hinderer, T.; Caro, J.; Williamson, Andrew R.; Boyle, M.; Buonanno, A.; Haas, R.; Hemberger, D. A.; Kidder, L. E.; et al (February 2019, Physical Review D)
5. Seeding the second star – II. CEMP star formation enriched from faint supernovae

   https://doi.org/10.1093/mnras/staa2144  

   Chiaki, Gen; Wise, John H; Marassi, Stefania; Schneider, Raffaella; Limongi, Marco; Chieffi, Alessandro (September 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Carbon-enhanced metal-poor (CEMP) stars are the living fossils holding records of chemical enrichment from early generations of stars. In this work, we perform a set of numerical simulations of the enrichment from a supernova (SN) of a first generation of metal-free (Pop III) star and the gravitational collapse of the enriched cloud, considering all relevant cooling/heating processes and chemical reactions as well as the growth of dust grains. We adopt faint SN models for the first time with progenitor masses MPopIII = 13–$$80 \ {\rm M_{\bigodot }}$$, which yield C-enhanced abundance patterns ([C/Fe] = 4.57–4.75) through mixing and fallback of innermost layers of the ejecta. This model also considers the formation and destruction of dust grains. We find that the metals ejected by the SN can be partly re-accreted by the same dark matter minihalo, and carbon abundance of the enriched cloud A(C) = 3.80–5.06 is lower than the abundance range of observed CEMP stars (A(C) ≳ 6) because the mass of the metals ejected by faint SNe is smaller than normal core-collapse SNe due to extensive fallback. We also find that cloud fragmentation is induced by gas cooling from carbonaceous grains for $$M_{\rm Pop III}= 13 \ {\rm M_{\bigodot }}$$ even with the lowest iron abundance [Fe/H] ∼ −9. This leads to the formation of low-mass stars, and these ‘giga metal-poor’ stars can survive until the present-day Universe and may be found by future observations. 

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