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

TL;DR

BPASS v2.1 delivers an expanded, physically motivated binary population and spectral synthesis framework that integrates binary interactions into stellar evolution and population synthesis. It introduces broader metallicity coverage, updated O-star atmospheres, and a comprehensive data release of SEDs and population properties, validated against both resolved stars (eclipsing binaries, WR/BSG/RSG samples) and unresolved galaxies (local and high-redshift) with improvements in ionizing photon production and emission-line predictions. The work demonstrates binary models better reproduce observed galaxy properties across cosmic time, including spectral energy distributions, line ratios, and transient rates, while clearly outlining limitations (e.g., old populations, AGB treatment, and non-stellar emission components) and proposing targeted future enhancements. Overall, the BPASS v2.1 release provides a self-consistent, publicly accessible framework to interpret both resolved and distant stellar populations, with direct implications for studies of star formation, reionization, and galaxy evolution.

Abstract

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.

Figures (45)

• Figure 1: The fraction of primary stars in our fiducial population that experience a binary interaction at two metallicities within the age of the Universe versus the initial mass of the primary star. The solid line represents the total fraction that experience Roche lobe overflow (RLOF). While the dashed line represents the number when RLOF progresses to common envelope evolution (CEE).
• Figure 2: The mean times that binary interactions last for in our fiducial simulation versus the initial mass of the primary star. The solid lines are for RLOF and the dashed lines for CEE. The thick lines are for the mean, the thin lines are at $\pm1\sigma$. As discussed below, the CEE time are significantly overestimated due to our method of including CEE in our detailed evolution models.
• Figure 3: The evolution of the fiducial stellar population mass (the mass contained in stars only) with time. We assume formation of an initial population with total mass $10^6$ M$_\odot$ within the first Myr. Solid lines are for binary populations and dashed lines are for the single-star populations, and results are shown at two metallicities.
• Figure 4: Contour plots showing how the stellar mass function evolves with age. The greyscale contours represent a binary population with each contour being a change in number by an order of magnitude. The lines represent the maximum mass at any given time, the solid line for the initial mass of the star and the dashed line the final mass of the star with that lifetime.
• Figure 5: The synthetic spectra produced for a co-eval population (i.e. instantaneous starburst) at times of 1, 3, 10, 30, 100, 300, 1000 and 3000 Myr after star formation. Spectra are shown for binary populations (bold, coloured lines) and single stars (pale, greyscale line), and at metallicities of Z=0.002 (top) and Z=0.020 (centre). In the bottom panel we compare BPASS binary models at the two metallicities directly, at ages of 3, 30 and 300 Myr.