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

The theory of binary evolution predicts that many massive stars should lose their hydrogen-rich envelopes via interaction with a companion – revealing hot helium stars with masses of $\sim$2–8M${}_{\odot }$. However, only one candidate system had been identified, leaving a large discrepancy between theory and observation. Here, we present a new sample of stars – identified via excess ultraviolet emission – whose luminosities, colors, and spectral morphologies are consistent with predictions for the missing population. We detect radial velocity variations indicative of binary motion and measure high temperatures ($T_{\mathrm{e}\mathrm{f}\mathrm{f}}\sim 60-100$kK), high surface gravities ($\log (g)\sim 5$) and depleted surface hydrogen mass fractions ($X_{\mathrm{H},\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}}\lesssim 0.3$), which match expectations for stars with initial masses between 8–25 M${}_{\odot }$ that have been stripped via binary interaction. These systems fill the helium star mass gap between subdwarfs and Wolf-Rayet stars, and are thought to be of large astrophysical significance as ionizing sources, progenitors of stripped-envelope supernovae and merging double neutron stars.