An intermediate-mass stripped star with a dark companion in the Small Magellanic Cloud

We present discovery of SMASH J004005 - 733351, an intermediate-mass stripped helium star in the Small Magellanic Cloud with a dark companion. The system was identified in a new optical color-selected survey for stripped stars and confirmed through spectroscopic follow-up. We measure an orbital period of P orb=19.95 plus or minus 0.002 hours and a radial-velocity semi-amplitude of K=91.7+3.2-3.1, km,s-1. Simultaneous modeling of the optical spectrum and ultraviolet-to-optical spectral energy distribution yields a stripped star evolutionary mass M stripped = 3.84 plus or minus 0.54,M, temperature T eff = 68.3 plus or minus 6.4 kK, surface gravity g= 4.76 plus or minus 0.12, luminosity (L/L)= 4.28 plus or minus 0.13, radius R= 0.99 plus or minus 0.09,R, and an unusually high surface hydrogen abundance of X H, s=0.63 plus or minus 0.04. The companion is undetected spectroscopically, and the orbital solution and stripped-star evolutionary mass imply a minimum companion mass of M 2 1.2,M. The absence of photometric variability from irradiation rules out a main-sequence companion. The companion is most likely a neutron star (NS), although a massive white dwarf or low-mass stripped star cannot be fully excluded. Binary evolution models predict that the stripped star will refill its Roche lobe after core helium exhaustion, likely producing a stripped or ultra-stripped supernova and ultimately a double NS binary if the system survives. SMASH J004005 - 733351 is among the first known stripped stars with a compact object companion. The system provides a new empirical benchmark for the late stages of binary interaction that lead to (ultra-)stripped supernovae and double NS binaries.