A case for Case A: Detailed look at binary black hole formation through stable mass transfer

¹ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, CH-1290 Versoix, Switzerland
² Gravitational Wave Science Center (GWSC), Université de Genève, CH-1211 Geneva, Switzerland
³ Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA
⁴ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
⁵ Center for Astrophysics | Harvard & Smithsonian, 60 Garden St., Cambridge, MA 02138, USA
⁶ Harvard Society of Fellows, 78 Mount Auburn Street, Cambridge, MA 02138, USA
⁷ The NSF-Simons AI Institute for the Sky (NSF-Simons SkAI), 172 E. Chestnut Street, Chicago, IL 60611, USA
⁸ The Adler Planetarium, 1300 South DuSable Lake Shore Drive, Chicago 60605, IL, USA
⁹ Department of Physics, University of Florida, 2001 Museum Rd, Gainesville, FL 32611, USA
¹⁰ Institute for Fundamental Theory, 2001 Museum Rd, Gainesville, FL 32611, USA
¹¹ Electrical and Computer Engineering, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 3 February 2026
Accepted: 18 April 2026

Abstract

Context. In isolated binary evolution, binary black hole (BBH) mergers are generally formed through stable mass transfer (SMT) or common envelope evolution. In recent years, the SMT channel has received significant attention due to detailed binary models showing increased mass transfer stability compared to previous studies.

Aims. In this work, we perform a full zero-age-main-sequence to compact object merger analysis using detailed binary models at eight metallicities between 10⁻⁴ Z_⊙ and 2 Z_⊙ to self-consistently model the population properties of BBH mergers in the SMT channel, determined their progenitor initial conditional, and investigate the binary physics governing their formation and metallicity dependence.

Methods. We used the population synthesis code POSYDON that incorporates detailed single-star and binary model grids to determine the population of BBH mergers from SMT. Using its extended grids of MESA binary models, we determined the essential physics in the formation of BBH mergers.

Results. The SMT channel produces BBH mergers predominantly from systems with P_ZAMS ≤ 10 days. In these systems, both the initial mass transfer between two stars and the subsequent interaction between the remaining star and the first-born BH take place while the respective donor star is on the main sequence (Case A). We find a limited contribution from wider Case B or C systems. Without a natal kick, the SMT channel does not produce BBH mergers above Z > 0.2 Z_⊙ due to orbital widening from stellar wind mass loss. The primary BH mass distribution shows a strong dependence on metallicity, while the mass ratio prefers unity independent of metallicity due to mass ratio reversal. Additionally, the χ_eff distributions contain peaks at χ_eff = 0 and ∼0.15, of which the former disappears at high metallicities. A mass-scaled natal kick leave this subpopulation unchanged but introduce a low-mass, unequal mass ratio subpopulation that merges within the Hubble time due to their eccentricity.