The mass distribution of stars stripped in binaries: The effect of metallicity

¹ Department of Physics and Astronomy, Northwestern University, 2145 Sheridan Rd, Evanston, IL 60208, USA
² Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, 1800 Sherman Ave, Evanston, IL 60201, USA
³ Division of Physics, Mathematics, and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
⁴ The Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
⁵ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
⁶ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁷ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85738 Garching bei München, Germany
⁹ David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S3H4, Canada

^⋆ Corresponding authors; beryl@u.northwestern.edu, ylva.gotberg@ista.ac.at

Received: 26 November 2024
Accepted: 14 March 2025

Abstract

Stars stripped of their hydrogen-rich envelopes through binary interaction are thought to be responsible for both hydrogen-poor supernovae and the hard ionizing radiation observed in low-Z galaxies. A population of these stars was recently observed for the first time, but their prevalence remains unknown. In preparation for such measurements, we estimate the mass distribution of hot, stripped stars using a population synthesis code that interpolates over detailed single and binary stellar evolution tracks. We predict that for a constant star formation rate of 1 M_⊙/yr and regardless of metallicity, a scalable model population contains ∼30 000 stripped stars with mass M_strip > 1 M_⊙ and ∼4000 stripped stars that are sufficiently massive to explode (M_strip > 2.6 M_⊙). Below M_strip = 5 M_⊙, the distribution is metallicity-independent and can be described by a power law with the exponent α ∼ −2. At higher masses and lower metallicity (Z ≲ 0.002), the mass distribution exhibits a drop. This originates from the prediction, frequently seen in evolutionary models, that massive low-metallicity stars do not expand substantially until central helium burning or later and therefore cannot form long-lived stripped stars. With weaker line-driven winds at low metallicity, this suggests that neither binary interaction nor wind mass loss can efficiently strip massive stars at low metallicity. As a result, a “helium-star desert” emerges around M_strip = 15 M_⊙ at Z = 0.002, covering an increasingly large mass range with decreasing metallicity. We note that these high-mass stars are those that potentially boost a galaxy’s He⁺-ionizing radiation and that participate in the formation of merging black holes. This “helium-star desert” therefore merits further study.