# Discovery of the missing intermediate-mass helium stars stripped in binaries

**Authors:** M. R. Drout, Y. Götberg, B. A. Ludwig, J. H. Groh, S. E. de Mink, A. J. G. O'Grady, N. Smith  
**arXiv:** [2307.00061](https://arxiv.org/abs/2307.00061)  
**Date:** 2023-06-30  
**Summary page:** https://www.summarizepaper.com/en/paper/2307.00061v1/

## Key points

- Theory of binary evolution predicts massive stars losing hydrogen-rich envelopes through interaction with a companion
- Formation of hot helium stars with masses ranging from 2 to 8 times the mass of the Sun
- Significant discrepancy between theoretical predictions and observational evidence
- New sample of stars identified through excess ultraviolet emission
- Luminosities, colors, and spectral morphologies consistent with predicted characteristics of missing population of helium stars
- Radial velocity variations indicate binary motion in these stars
- Newly discovered stars have high temperatures (60 to 100 thousand Kelvin), high surface gravities (logarithmic values around 5), and depleted surface hydrogen mass fractions (≤0.3)
- Estimated initial masses fall within the range of 8 to 25 times the mass of the Sun
- These systems bridge the mass range between subdwarfs and Wolf-Rayet stars
- Potential implications: ionizing sources for interstellar gas clouds, progenitors for stripped-envelope supernovae explosions, potential hosts for merging double neutron star systems producing detectable gravitational waves

## Summary

The theory of binary evolution predicts that many massive stars should lose their hydrogen-rich envelopes through interaction with a companion, resulting in the formation of hot helium stars with masses ranging from approximately 2 to 8 times the mass of the Sun. However, until recently, only one candidate system had been identified, leading to a significant discrepancy between theoretical predictions and observational evidence. In this study, the authors present a new sample of stars that have been identified through excess ultraviolet emission. These stars exhibit luminosities, colors, and spectral morphologies that are consistent with the predicted characteristics of the missing population of helium stars. The researchers also detect radial velocity variations in these stars, indicating binary motion. Further analysis reveals that these newly discovered stars have high temperatures (ranging from approximately 60 to 100 thousand Kelvin), high surface gravities (with logarithmic values around 5), and depleted surface hydrogen mass fractions (less than or equal to 0.3). These properties align with expectations for stars that have undergone binary interactions and have had their outer hydrogen layers stripped away. The estimated initial masses of these stripped helium stars fall within the range of 8 to 25 times the mass of the Sun. These systems fill an important gap in our understanding of stellar evolution as they bridge the mass range between subdwarfs and Wolf-Rayet stars. They are believed to be highly significant in astrophysics as potential ionizing sources for interstellar gas clouds, progenitors for stripped-envelope supernovae explosions which can enrich galaxies with heavy elements such as iron and nickel; and potential hosts for merging double neutron star systems which can produce gravitational waves detectable by Earth-based observatories such as LIGO/VIRGO. Overall, this study provides valuable insights into the population of intermediate-mass helium stars formed through binary interactions and sheds light on their astrophysical implications. The discovery of this new sample helps reconcile discrepancies between theoretical predictions and observations while expanding our knowledge of stellar evolution processes.