The formation and evolution of binary stars are critical components of several fields in astronomy. The most numerous sources for gravitational wave observatories are inspiraling or merging compact binaries, while binary stars are present in nearly every electromagnetic survey regardless of the target population. Simulations of large binary populations serve to both predict and inform observations of electromagnetic and gravitational wave sources. Binary population synthesis is a tool that balances physical modeling with simulation speed to produce large binary populations on timescales of days. We present a community-developed binary population synthesis suite, COSMIC, which is designed to simulate compact-object binary populations and their progenitors. As a proof of concept, we simulate the Galactic population of compact binaries and their gravitational wave signals observable by the Laser Interferometer Space Antenna.

The recent detections of GW150914 and GW151226 imply an abundance of stellar-mass binary-blackhole mergers in the local universe. While ground-based gravitational-wave detectors are limited to observing the final moments before a binary merges, space-based detectors, such as the Laser Interferometer Space Antenna (LISA), can observe binaries at lower orbital frequencies where such systems may still encode information about their formation histories. In particular, the orbital eccentricity and mass of binary black holes in the LISA frequency band can be used together to discriminate between binaries formed in isolation in galactic fields and those formed in dense stellar environments such as globular clusters. In this letter, we explore the orbital eccentricity and mass of binary-black-hole populations as they evolve through the LISA frequency band. Overall we find that there are two distinct populations discernible by LISA. We show that up to ∼ 90% of binaries formed either dynamically or in isolation have eccentricities measurable by LISA. Finally, we note how measured eccentricities of low-mass binary black holes evolved in isolation could provide detailed constraints on the physics of black-hole natal kicks and common-envelope evolution.

We estimate the population of black holes with luminous stellar companions (BH-LCs) in the Milky Way (MW) observable by Gaia. We evolve a realistic distribution of BH-LC progenitors from zero-age to the current epoch taking into account relevant physics, including binary stellar evolution, BH-formation physics, and star formation rate, to estimate the BH-LC population in the MW today. We predict that Gaia will discover between 3, 800 and 12, 000 BHLCs by the end of its 5 yr mission, depending on BH natal kick strength and observability constraints. We find that the overall yield, and distributions of eccentricities and masses of observed BH-LCs can provide important constraints on the strength of BH natal kicks. Gaia-detected BH-LCs are expected to have very different orbital properties compared to those detectable via radio, X-ray, or gravitational wave observations.

The Galactic Center (GC) is dominated by the gravity of a super-massive black hole (SMBH), Sagittarius A * , and is suspected to contain a sizable population of binary stars. Such binaries form hierarchical triples with the SMBH, undergoing Eccentric Kozai-Lidov (EKL) evolution, which can lead to high eccentricity excitations for the binary companions' mutual orbit. This effect can lead to stellar collisions or Roche-lobe crossings, as well as orbital shrinking due to tidal dissipation. In this work we investigate the dynamical and stellar evolution of such binary systems, especially with regards to the binaries' post-main-sequence evolution. We find that the majority of binaries (∼ 75%) is eventually separated into single stars, while the remaining binaries (∼ 25%) undergo phases of common-envelope evolution and/or stellar mergers. These objects can produce a number of different exotic outcomes, including rejuvenated stars, G2-like infrared-excess objects, stripped giant stars, Type Ia supernovae (SNe), cataclysmic variables (CVs), symbiotic binaries (SBs), or compact object binaries. We estimate that, within a sphere of 250 Mpc radius, about 7.5 to 15 Type Ia SNe per year should occur in galactic nuclei due to this mechanism, potentially detectable by ZTF and ASAS-SN. Likewise we estimate that, within a sphere of 1 Gpc 3 volume, about 10 to 20 compact object binaries form per year that could become gravitational wave sources. Based on results of EKL-driven compact object binary mergers in galactic nuclei by Hoang et al. (2018), this compact object binary formation rate translates to about 15 to 30 events per year detectable by Advanced LIGO. nature of gravity, stellar cluster dynamics, and general relativity (GR) over the last decades (e.g., Ghez et al.