Compact object mergers: exploring uncertainties from stellar and binary evolution with SEVN

Population-synthesis codes are an unique tool to explore the parameter space of massive binary star evolution and binary compact object (BCO) formation. Most population-synthesis codes are based on the same stellar evolution model, limiting our ability to explore the main uncertainties. Our code SEVN overcomes this issue by interpolating the main stellar properties from a set of pre-computed evolutionary tracks. With SEVN, we evolved $1.2\times10^9$ binaries in the metallicity range $0.0001\leq Z \leq 0.03$, exploring a number of models for electron-capture, core-collapse and pair-instability supernovae, different assumptions for common envelope, stability of mass transfer, quasi-homogeneous evolution and stellar tides. We find that stellar evolution has a dramatic impact on the formation of single and binary compact objects. Just by slightly changing the overshooting parameter ($\lambda_{\rm ov}=0.4,0.5$) and the pair-instability model, the maximum mass of a black hole can vary from $\approx{60}$ to $\approx{100}\ \mathrm{M}_\odot$. Furthermore, the formation channels of BCOs and the merger efficiency we obtain with SEVN show significant differences with respect to the results of other population-synthesis codes, even when the same binary-evolution parameters are used. For example, the main traditional formation channel of BCOs is strongly suppressed in our models: at high metallicity ($Z\gtrsim{0.01}$) only $<20$% of the merging binary black holes and binary neutron stars form via this channel, while other authors found fractions $>70$%. The local BCO merger rate density of our fiducial models is consistent with the most recent estimates by the LIGO--Virgo--KAGRA collaboration.Comment: Submitted to MNRAS, comments welcome! The SEVN code is available at https://gitlab.com/sevncodes/sevn.git. All the data underlying this article are available in Zenodo at the link https://doi.org/10.5281/zenodo.7260771. All the Jupyter notebooks used to produce the plots in the paper are available in the gitlab repository https://gitlab.com/iogiul/iorio22_plot.gi

Compact Binary Coalescences in the Band of Ground-based Gravitational-Wave Detectors

As the ground-based gravitational-wave telescopes LIGO, Virgo, and GEO 600 approach the era of first detections, we review the current knowledge of the coalescence rates and the mass and spin distributions of merging neutron-star and black-hole binaries. We emphasize the bi-directional connection between gravitational-wave astronomy and conventional astrophysics. Astrophysical input will make possible informed decisions about optimal detector configurations and search techniques. Meanwhile, rate upper limits, detected merger rates, and the distribution of masses and spins measured by gravitational-wave searches will constrain astrophysical parameters through comparisons with astrophysical models. Future developments necessary to the success of gravitational-wave astronomy are discussed.Comment: Replaced with version accepted by CQG

STROOPWAFEL: Simulating rare outcomes from astrophysical populations, with application to gravitational-wave sources

Gravitational-wave observations of double compact object (DCO) mergers are providing new insights into the physics of massive stars and the evolution of binary systems. Making the most of expected near-future observations for understanding stellar physics will rely on comparisons with binary population synthesis models. However, the vast majority of simulated binaries never produce DCOs, which makes calculating such populations computationally inefficient. We present an importance sampling algorithm, STROOPWAFEL, that improves the computational efficiency of population studies of rare events, by focusing the simulation around regions of the initial parameter space found to produce outputs of interest. We implement the algorithm in the binary population synthesis code COMPAS, and compare the efficiency of our implementation to the standard method of Monte Carlo sampling from the birth probability distributions. STROOPWAFEL finds $\sim$25-200 times more DCO mergers than the standard sampling method with the same simulation size, and so speeds up simulations by up to two orders of magnitude. Finding more DCO mergers automatically maps the parameter space with far higher resolution than when using the traditional sampling. This increase in efficiency also leads to a decrease of a factor $\sim$3-10 in statistical sampling uncertainty for the predictions from the simulations. This is particularly notable for the distribution functions of observable quantities such as the black hole and neutron star chirp mass distribution, including in the tails of the distribution functions where predictions using standard sampling can be dominated by sampling noise.Comment: Accepted. Data and scripts to reproduce main results is publicly available. The code for the STROOPWAFEL algorithm will be made publicly available. Early inquiries can be addressed to the lead autho

Formation of the first three gravitational-wave observations through isolated binary evolution

During its first 4 months of taking data, Advanced LIGO has detected gravitational waves from two binary black hole mergers, GW150914 and GW151226, along with the statistically less significant binary black hole merger candidate LVT151012. We use our rapid binary population synthesis code COMPAS to show that all three events can be explained by a single evolutionary channel -- classical isolated binary evolution via mass transfer including a common envelope phase. We show all three events could have formed in low-metallicity environments (Z = 0.001) from progenitor binaries with typical total masses $\gtrsim 160 M_\odot$, $\gtrsim 60 M_\odot$ and $\gtrsim 90 M_\odot$, for GW150914, GW151226, and LVT151012, respectively.Comment: Published in Nature Communication

Distinguishing compact binary population synthesis models using gravitational-wave observations of coalescing binary black holes

The coalescence of compact binaries containing neutron stars or black holes is one of the most promising signals for advanced ground-based laser interferometer gravitational-wave detectors, with the first direct detections expected over the next few years. The rate of binary coalescences and the distribution of component masses is highly uncertain, and population synthesis models predict a wide range of plausible values. Poorly constrained parameters in population synthesis models correspond to poorly understood astrophysics at various stages in the evolution of massive binary stars, the progenitors of binary neutron star and binary black hole systems. These include effects such as supernova kick velocities, parameters governing the energetics of common envelope evolution and the strength of stellar winds. Observing multiple binary black hole systems through gravitational waves will allow us to infer details of the astrophysical mechanisms that lead to their formation. Here we simulate gravitational-wave observations from a series of population synthesis models including the effects of known selection biases, measurement errors and cosmology. We compare the predictions arising from different models and show that we will be able to distinguish between them with observations (or the lack of them) from the early runs of the advanced LIGO and Virgo detectors. This will allow us to narrow down the large parameter space for binary evolution models.Comment: 16 pages, 8 figures, updated to match version published in Ap

Binary Population and Spectral Synthesis Version 2.1: construction, observational verification and new results

The Binary Population and Spectral Synthesis (BPASS) suite of binary stellar evolution models and synthetic stellar populations provides a framework for the physically motivated analysis of both the integrated light from distant stellar populations and the detailed properties of those nearby. We present a new version 2.1 data release of these models, detailing the methodology by which BPASS incorporates binary mass transfer and its effect on stellar evolution pathways, as well as the construction of simple stellar populations. We demonstrate key tests of the latest BPASS model suite demonstrating its ability to reproduce the colours and derived properties of resolved stellar populations, including well- constrained eclipsing binaries. We consider observational constraints on the ratio of massive star types and the distribution of stellar remnant masses. We describe the identification of supernova progenitors in our models, and demonstrate a good agreement to the properties of observed progenitors. We also test our models against photometric and spectroscopic observations of unresolved stellar populations, both in the local and distant Universe, finding that binary models provide a self-consistent explanation for observed galaxy properties across a broad redshift range. Finally, we carefully describe the limitations of our models, and areas where we expect to see significant improvement in future versions.Comment: 69 pages, 45 figures. Accepted for publication in PASA. Accompanied by a full, documented data release at http://bpass.auckland.ac.nz and http://warwick.ac.uk/bpas

Are Supernova Kicks Responsible for X-ray Binary Ejection from Young Clusters?

Recent Chandra observations of interacting and starburst galaxies have led us to investigate the apparent correlation between the positions of young star clusters and Chandra point sources. Assumed to be X-ray binaries (XRBs), these point sources do not seem to coincide with the massive (~1e5 Msun), young (1-50 Myr) stellar clusters that can easily form systems capable of such emission. We use a sophisticated binary evolution and population synthesis code (StarTrack) and a simplified cluster model to track both the X-ray luminosity and position of XRBs as a function of time. These binaries are born within the cluster potential with self-consistent positions and velocities and we show that a large fraction (~70%) can be ejected from the parent due to supernova explosions and associated systemic velocities. For brighter sources and cluster masses below ~1e6 Msun, we find that the average number of bright XRBs per cluster remains near or below unity, consistent with current observations.Comment: 5 pages, 1 figure. Accepted for publication in Astrophysical Journal Letter

Constraining Compact Object Formation with 2M0521

We show that the recently discovered binary 2M05215658+4359220 (2M0521), comprised of a giant star (GS) orbiting a suspected black hole (BH) in a ~80 day orbit, may be instrumental in shedding light on uncertain BH-formation physics and can be a test case for studying wind accretion models. Using binary population synthesis with a realistic prescription for the star formation history and metallicity evolution of the Milky Way, we analyze the evolution of binaries containing compact objects (COs) in orbit around GSs with properties similar to 2M0521. We find ~100-1000 CO-GS binaries in the Milky Way observable by Gaia, and 0-12 BH-GS and 0-1 neutron star-GS binaries in the Milky Way with properties similar to 2M0521. We find that all CO-GSs with Porb<5 yr, including 2M0521, go through a common envelope (CE) and hence form a class of higher mass analogs to white dwarf post-CE binaries. We further show how the component masses of 2M0521-like binaries depend strongly on the supernova-engine model we adopt. Thus, an improved measurement of the orbit of 2M0521, imminent with Gaia's third data release, will strongly constrain its component masses and as a result inform supernova-engine models widely used in binary population synthesis studies. These results have widespread implications for the origins and properties of CO binaries, especially those detectable by LIGO and LISA. Finally, we show that the reported X-ray non-detection of 2M0521 is a challenge for wind accretion theory, making 2M0521-like CO-GS binaries a prime target for further study with accretion models.Comment: 7 pages, 5 figures, Accepted for Publication in ApJ