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BPASS Predictions of Compact Object – Luminous Companion Binaries in the Milky Way

Published online by Cambridge University Press:  15 October 2026

Erin Eastep Open the ORCID record for Erin Eastep [Opens in a new window]  and
Jan Eldridge
Erin Eastep*
Affiliation:
Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand
Jan Eldridge
Affiliation:
Department of Physics, University of Auckland, Private Bag 92019, Auckland, New Zealand
*
email: erin.eastep@auckland.ac.nz
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Abstract

We present a study using the (BPASS) code suite that predicts the Galactic population of binaries that contain a black hole or neutron star. By incorporating the stellar evolution models from BPASS with a Milky Way analogue galaxy from the Feedback in Realistic Environment (FIRE) simulation, we can generate a theoretical population of quiescent compact remnant binary systems as well as x-ray faint binaries within our Galaxy. The study of how compact object masses evolve over time plays a key role for time domain astronomy and gravitational wave progenitors. We find that the agreement between our synthetic and observed systems is reasonable, but there are strong indications that new physical processes should be explored within BPASS to be able to accurately reproduce the observed compact remnant distributions.

Keywords

starsbinariesblack holesneutron starspopulation synthesis

Information

Type
Contributed Paper
Information
Proceedings of the International Astronomical Union , Volume 21 , Symposium S398: Compact Objects and Binaries in Dense Star Clusters , September 2025 , pp. 243 - 246
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Creative Common License - CCCreative Common License - BYCreative Common License - SA
This is an Open Access article, distributed under the terms of the Creative Commons Attribution-ShareAlike licence (https://creativecommons.org/licenses/by-sa/4.0/), which permits re-use, distribution, and reproduction in any medium, provided the same Creative Commons licence is used to distribute the re-used or adapted article and the original article is properly cited.
Copyright
© The Author(s), 2026. Published by Cambridge University Press on behalf of The International Astronomical Union

1. Introduction

It is well known that more than 50% of stars are in a binary or higher systems and two thirds of high mass stars undergo some sort of interaction in their lifetimes. (Sana et al. Reference Sana and de Mink2012) Such interacting systems are typically observed as X-ray binaries (XBs), where a donor star accretes material onto a compact object (CO) such as a neutron star (NS), or a black hole (BH). Conversely, binary systems predominantly exist in a state of quiescence, characterized by little to no mass transfer. Alternatively, certain systems may appear to be quiescent while engaging in mass transfer; however, this transfer is of a magnitude such that it remains primarily optically dominant. While it has been previously theorized that there should be a significant population of quiescent binary (QB) systems in our galaxy (Breivik et al. Reference Breivik and Coughlin2020), their nature has made them difficult to detect. In recent times however, astrometric measurements from cutting-edge space telescopes have led to the discovery of systems such as the Gaia BH systems. (El-Badry et al. Reference El-Badry and Rix2022; El-Badry et al. Reference El-Badry and Rix2023; Panuzzo et al. Reference Panuzzo and Mazeh2024) As the field develops further with new observatories coming online, many more are expected to be discovered. With this in mind, we set out to create a theoretical population of systems within our Galaxy.

2. Methodology

The core part of our work is the creation of a pipeline to take stellar population predictions on the binaries with compact remnants as different stages of evolutions and combine them with a model galaxy to predict the expected populations in our Galaxy. In this work we use the BPASS to make the model stellar populations and a FIRE simulation to provide us with the model galaxy.

We use the results of BPASS v2.2 binary evolution models. (Eldridge et al. Reference Eldridge and Stanway2017; Stanway and Eldridge Reference Stanway and Eldridge2018) BPASS is a customized version of the Cambridge STARS code which allows us to model binary interactions in detail. This is accomplished by modeling both the primary and secondary, but one at a time in detail. The primary is evolved in detail with full internal structure with the secondary being approximated based on the physics described in Hurley et al. (Reference Hurley and Pols2000). At the end of primary evolution, the secondary is recalculated in detail, accounting for rejuvenation effects as well as mass transfer.

Additionally, we make use of the m12i model from the Feedback In Realistic Environments (FIRE, Hopkins et al. (Reference Hopkins and Kereš2014)) suite of code. FIRE can be described as a collection of “star particles” each with an initial mass, metallicity, age, and spatial distribution within the Milky Way-like galaxy. Building off the work of Tang et al. (Reference Tang and Eldridge2024), we assume that each star particle is a star cluster. We can then match those parameters with BPASS models with shared mass, age, and metallicity. Our BPASS methodology presents a predicted number of binary systems per stellar mass, which is then multiplied by the star particle mass to yield the number of binaries per particle. Integer reminders are carried over to the next particle in order of star particle age. These are all then randomly distributed throughout the model’s output bin with a uniform random distribution. This is done to make sure all of the binaries don’t have the same parameters. At the end of this process, we then have a list of the binaries distributed within FIRE’s galaxy. Through a set of post-hoc calculations, we can then classify the outputs into pre-interaction systems and X-ray faint Roche lobe Overflow (RLOF) systems.

There are a couple of important notes to this methodology. We calculate the effective temperature of the accretion, and provided that upper T Subscript normal e normal f normal f Baseline greater than or equals 10 Superscript 6 Baseline upper K $T_{\rm eff}\geq10^{6}K$ and the system has an initial period less than 10 Superscript 4 $\lt10^{4}$ days, we classify the system as X-ray faint RLOF and therefore optically dominant.

We reviewed the currently available data and selected some of the more well known QBs. These notably include Gaia-BH1 (El-Badry et al. Reference El-Badry and Rix2022), Gaia BH-2 (El-Badry et al. Reference El-Badry and Rix2023), and Gaia-BH3 (Panuzzo et al. Reference Panuzzo and Mazeh2024). We also include the NSQB candidates from El-Badry et al. (Reference El-Badry and Rix2024). We also include data from Paduano et al. (Reference Paduano and Bahramian2021) for three sources located in NGC 3201. They are ACS ID #5132 (first reported in Giesers et al. (Reference Giesers and Kamann2019)), ACS ID #12560 (first reported in Giesers et al. (Reference Giesers and Dreizler2018)), and ACS ID #21859 (first reported in Giesers et al. (Reference Giesers and Kamann2019)). These are all binary systems that are thought to have invisible BH companions. Interestingly, Giesers et al. (Reference Giesers and Kamann2019) reported ACS ID #21859 to have partially filled H alpha $\alpha$ lines without X-ray or radio counterparts. Paduano et al. (Reference Paduano and Bahramian2021) reported this is a source where any accretion is wind caught by the black hole with low accretion rates. Assuming similar properties for the other two systems we place these systems on the pre-interaction systems plot.

3. Results

Starting with the systems immediately post-supernova, our predictions for the pre-interaction systems. In Figure 1, you can see the observations plotted over the BPASS results. In terms of population, we expect the vast majority of these systems to have a black hole alongside a low mass star ( less than 3 upper M Subscript circled dot $\lt3M_\odot$ ). While we do see some positive results in the mass distribution plot (upper left), the Gaia BHs do present themselves as outliers. We theories that this is due to the limited grid of masses used in creating the stellar populations. For example, Gaia BH3 features a main-sequence star of approximately 0.76 upper M Subscript circled dot $0.76M_\odot$ and a black hole of about 33 upper M Subscript circled dot $33M_\odot$ . The fiducial BPASS grid only goes to a minimum initial mass ratio of 0.1 $0.1$ and the minimum mass for a black hole formation is 20 upper M Subscript circled dot $20M_{\odot}$ . Therefore, BPASS’s minimum main sequence companion to a black hole is approximately 2 upper M Subscript circled dot $2M_{\odot}$ . Bespoke BPASS stellar evolution models have found that with a solar metallicity model with a suitable primary mass and secondary mass, BPASS can reproduce these systems.

Figure 1.

Pre-interaction quiescent binaries: The Gaia BH systems are the three black points and the Gaia NS systems presented in Cappelluti et al. (Reference Cappelluti and Pacucci2024) are the blue points. The NGC 3201 systems from Paduano et al. (Reference Paduano and Bahramian2021) are the magenta points while the ATNF MSPs with main sequence companions are in red.

Four scatter plots depict relationships between different properties of pre-interaction quiescent binaries.

Some observed systems that appear to be quiescent may fall under the faint-RLOF category so we elect to plot the systems also found in Figure 1. Figure 2 finds solid agreement with all observed systems and the BPASS data. It should also be noted that these faint systems have longer orbital periods than the bright ones. Additionally, these “faint” systems are more likely to house black holes rather than neutron stars. This is to be expected for these systems with a BH as a consequence of the difference with the innermost stable orbit.

Figure 2.

X-ray faint RLOF XRBs: BPASS systems where the effective temperature of the disk is less than or equals 10 Superscript 6 Baseline upper K $\le10^6K$ . We include the same observational data found in Figure 1.

Multiple graphs depict X-ray faint RLOF binaries with various parameters.

4. Conclusion

To summarize, we have combined the BPASS stellar models with the synthetic FIRE Milky Way like galaxy model to predict the binary systems of stars with compact objects. We calculate the total numbers of systems as well as mass distributions and periods of said systems. From this, we are able to also evaluate our models for their strengths and weaknesses. The new pipeline developed for this project will also be used to evaluate future developments of the BPASS project. In the near future, we plan to publish results that further studies quiescent binaries in different states of their evolution as well as expanding this framework to include X-ray binaries. This project additionally will allow us to predict yields of these systems in future Gaia data releases as well as future observatories such as the Nancy Grace Roman Space Telescope.

References

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Figure 0

Figure 1. Pre-interaction quiescent binaries: The Gaia BH systems are the three black points and the Gaia NS systems presented in Cappelluti et al. (2024) are the blue points. The NGC 3201 systems from Paduano et al. (2021) are the magenta points while the ATNF MSPs with main sequence companions are in red.

Figure 1

Figure 2. X-ray faint RLOF XRBs: BPASS systems where the effective temperature of the disk is ≤106K$\le10^6K$. We include the same observational data found in Figure 1.