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

TL;DR

This paper models the mass distribution of stars stripped in binary systems across different metallicities, revealing a metallicity-independent power law at lower masses and a 'helium-star desert' at higher masses at low metallicity.

Contribution

It introduces a population synthesis model predicting the mass distribution of stripped stars, highlighting the impact of metallicity on their prevalence and properties.

Findings

Mass distribution follows a power law with exponent -2 below 5 solar masses.

At low metallicity, a 'helium-star desert' appears around 15 solar masses.

Low metallicity stars are less likely to be stripped due to limited expansion.

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 $1 \,M_\odot$/yr and regardless of metallicity, a scalable model population contains ~30,000 stripped stars with mass $M_{\rm strip}>1M_\odot$ and ~4,000 stripped stars that are sufficiently massive to explode ($M_{\rm strip}>2.6M_\odot$). Below $M_{\rm strip}=5M_\odot$, the distribution is metallicity-independent and can be described by a power law with the exponent $\alpha \sim -2$. At higher masses and lower metallicity ($Z \lesssim 0.002$), 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 $M_{\rm strip} =15\, M_\odot$ 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.