Demographics of neutron stars in young massive and open clusters

Abstract

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 $\sim 3\times 10^{-2}$--$3\times 10^{-3}$ 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.

7 pages, 3 figures, accepted by ApJ Lett

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Cited by in corpus (25)

• Observation of gravitational waves from two neutron star-black hole coalescences
• Rates of Compact Object Coalescences
• Impact of Massive Binary Star and Cosmic Evolution on Gravitational Wave Observations I: Black Hole-Neutron Star Mergers
• The Science of the Einstein Telescope
• Repeated mergers, mass-gap black holes, and formation of intermediate-mass black holes in nuclear star clusters
• On the formation of GW190814
• Modelling Neutron Star-Black Hole Binaries: Future Pulsar Surveys and Gravitational Wave Detectors
• Preparing the next gravitational million-body simulations: Evolution of single and binary stars in Nbody6++GPU, MOCCA and McLuster
• Black hole-neutron star mergers are unlikely multi-messenger sources
• Compact Binary Coalescences: Astrophysical Processes and Lessons Learned
• Black Hole Mergers from Star Clusters with Top-Heavy Initial Mass Functions
• GW200115: a non-spinning black hole -- neutron star merger
• Neutron Star-Black Hole Mergers from Gravitational Wave Captures
• Stellar-mass black holes in young massive and open stellar clusters V: comparisons with LIGO-Virgo merger rate densities
• Black hole - black hole total merger mass and the origin of LIGO/Virgo sources
• Dynamical formation of the GW190814 merger
• Impact of natal kicks on merger rates and spin-orbit misalignments of black hole -- neutron star mergers
• Explodability fluctuations of massive stellar cores enable asymmetric compact object mergers such as GW190814
• Hunting intermediate-mass black holes with LISA binary radial velocity measurements
• Orbital Eccentricity and Spin-Orbit Misalignment Are Evidence that Neutron Star-Black Hole Mergers Form through Triple Star Evolution
• Black Holes merging with Low Mass Gap Objects inside Globular Clusters
• Constraining neutron star radii in black hole-neutron star mergers from their electromagnetic counterparts
• Challenges in Forming Millisecond Pulsar-Black Holes from Isolated Binaries
• Evolutionary tracks of binary neutron star progenitors across cosmic times
• Modelling the formation of the first two neutron star-black hole mergers, GW200105 and GW200115: metallicity, chirp masses and merger remnant spins