Year: 2026
Type: preprint
Abstract: 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 $P_{\rm orb}=19.95\pm 0.002$ hours and a radial-velocity semi-amplitude of $K=91.7 ^{+3.2}_{-3.1}\,{\rm km\,s^{-1}}$. Simultaneous modeling of the optical spectrum and ultraviolet-to-optical spectral energy distribution yields a stripped star evolutionary mass $M_{\rm stripped} = 3.84 \pm 0.54\,M_\odot$, temperature $T_{\rm eff} = 68.3 \pm 6.4$ kK, surface gravity $\log g= 4.76 \pm 0.12$, luminosity $\log(L/L_\odot)= 4.28 \pm 0.13$, radius $R= 0.99 \pm 0.09\,R_\odot$, and an unusually high surface hydrogen abundance of $X_{\rm H, s}=0.63 \pm 0.04$. The companion is undetected spectroscopically, and the orbital solution and stripped-star evolutionary mass imply a minimum companion mass of $M_2 \gtrsim 1.2\,M_\odot$. 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.
Source: arXiv (Cornell University)
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Science Foundation, National Aeronautics and Space Administration, Research Corporation for Science Advancement, Carnegie Institution of Washington, Massachusetts Institute of Technology, Space Telescope Science Institute, Indian Space Research Organisation, Canada Research Chairs, University of Toronto, Korea Astronomy and Space Science Institute, Ministério da Ciência, Tecnologia, Inovações e Comunicações, Agencia Nacional de Investigación y Desarrollo, Natural Sciences and Engineering Research Council of Canada, Agencia Estatal de Investigación
OpenAlex ID: W7224090767