BSE versus StarTrack: Implementations of new wind, remnant-formation, and natal-kick schemes in NBODY7 and their astrophysical consequences

Abstract

<jats:p><jats:italic>Context.</jats:italic> As a result of their formation via massive single and binary stellar evolution, the masses of stellar-remnant black holes (BH) are subjects of great interest in this era of gravitational-wave detection from binary black hole (BBH) and binary neutron star merger events.</jats:p> <jats:p><jats:italic>Aims.</jats:italic> In this work, we present new developments in the stellar-remnant formation and related schemes of the current <jats:italic>N</jats:italic>-body evolution program <jats:monospace>NBODY7</jats:monospace>. We demonstrate that the newly implemented stellar-wind and remnant-formation schemes in the stellar-evolutionary sector or <jats:monospace>BSE</jats:monospace> of the <jats:monospace>NBODY7</jats:monospace> code, such as the “rapid” and the “delayed” supernova (SN) schemes along with an implementation of pulsational-pair-instability and pair-instability supernova (PPSN/PSN), now produce neutron star (NS) and BH masses that agree nearly perfectly, over large ranges of zero-age-main-sequence (ZAMS) mass and metallicity, with those from the widely recognised <jats:monospace>StarTrack</jats:monospace> population-synthesis program. We also demonstrate the new, recipe-based implementations of various widely debated mechanisms of natal kicks on NSs and BHs, such as “convection-asymmetry-driven”, “collapse-asymmetry-driven”, and “neutrino-emission-driven” kicks, in addition to a fully consistent implementation of the standard, fallback-dependent, momentum-conserving natal kick.</jats:p> <jats:p><jats:italic>Methods.</jats:italic> All the above newly implemented schemes are also shared with the standalone versions of <jats:monospace>SSE</jats:monospace> and <jats:monospace>BSE</jats:monospace>. All these demonstrations are performed with both the updated standalone <jats:monospace>BSE</jats:monospace> and the updated <jats:monospace>NBODY7</jats:monospace>/<jats:monospace>BSE</jats:monospace>.</jats:p> <jats:p><jats:italic>Results.</jats:italic> When convolved with stellar and primordial-binary populations as observed in young massive clusters, such remnant-formation and natal-kick mechanisms crucially determine the accumulated number, mass, and mass distribution of the BHs retained in young massive, open, and globular clusters (GCs); these BHs would eventually become available for long-term dynamical processing.</jats:p> <jats:p><jats:italic>Conclusions.</jats:italic> Among other conclusions, we find that although the newer, delayed SN remnant formation model gives birth to the largest number (mass) of BHs, the older remnant-formation schemes cause the largest number (mass) of BHs to survive in clusters, when incorporating SN material fallback onto the BHs. The SN material fallback also causes the convection-asymmetry-driven SN kick to effectively retain similar numbers and masses of BHs in clusters as for the standard, momentum-conserving kick. The collapse-asymmetry-driven SN kick would cause nearly all BHs to be retained in clusters irrespective of their mass, remnant-formation model, and metallicity, whereas the inference of a large population of BHs in GCs would potentially rule out the neutrino-driven SN kick mechanism. Pre-SN mergers of massive primordial binaries would potentially cause BH masses to deviate from the theoretical, single-star ZAMS to mass-remnant mass relation unless a substantial of the total merging stellar mass of up to ≈40% is lost during a merger process. In particular, such mergers, at low metallicities, have the potential to produce low-spinning BHs within the PSN mass gap that can be retained in a stellar cluster and be available for subsequent dynamical interactions. As recent studies indicate, the new remnant-formation modelling reassures us that young massive and open clusters would potentially contribute to the dynamical BBH merger detection rate to a similar extent as their more massive GC counterparts.</jats:p>

On the survival of resonant and non-resonant planetary systems in star clusters

“…The reader interested in more details could have a look into the DRAGON (million body) simulations (Wang et al 2016). In recent years the stellar evolution prescriptions for N-body simulations are undergoing considerable changes, see for example Khalaj & Baumgardt (2015), Spera, Mapelli & Bressan (2015), and Banerjee et al (2020) for an overview. The updates according to that paper are now also available in NBODY6++GPU, but have not yet been used for the simulations of this paper.…”

Dynamically formed black hole binaries: In-cluster versus ejected mergers

“…The version of NBODY6 we use was forked from the official branch in 2015 and customised for increased accuracy in the sphere of influence of a BH (see Trenti et al 2010 for further details). Note that our version does not include the recently implemented upgrades to some stellar evolution prescriptions for massive stars (stellar wind, mass fallback, and pair-instability supernova) presented in Banerjee et al (2019). While we do not expect significant changes to our key results as these are relatively minor changes to an For each simulation (identified by a unique ID) we report (from left to right) the initial number of stars; the initial IMBH mass in M ò ;…”

Flipping spins in mass transferring binaries and origin of spin-orbit misalignment in binary black holes

“…4, we compute R MS =FðqÞ and R RG =FðqÞ, where R MS is the maximum stellar radius during the main sequence, and R GB is the radius at the start of He burning, as a function of the mass of the star at that evolutionary stage. The stellar radii were obtained with the fast binary stellar evolution code BSE [8,50]. Hence, R MS =FðqÞ and R RG =FðqÞ represent the maximum value of the binary semimajor axis that will still allow a mass transfer event to happen during the main sequence or before He burning starts, respectively.…”

“…We evolve this binary population in time by means of the latest version of the binary stellar evolution code BSE [8,50]. BSE simulates the stellar evolution, including binary features such as mass transfer, mass accretion, common-envelope evolution, supernova kicks, and angular momentum losses.…”

“…7, we take the latter effect into account by implementing the tilts ofĥ due to the natal kicks at the first and second supernovae. That is, for each supernova, we adopt the widely held assumption that the magnitude of the natal kick velocity v kick of the black hole follows the one observed for neutron stars scaled down by some fallbackfraction f fb [50],…”