An intermediate-mass stripped star with a dark companion in the Small Magellanic Cloud
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 Porb=19.95± 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 Mstripped = 3.84 ± 0.54 M⊙, temperature Teff = 68.3 ± 6.4 kK, surface gravity log g= 4.76 ± 0.12, luminosity log(L/L⊙)= 4.28 ± 0.13, radius R= 0.99 ± 0.09 R⊙, and an unusually high surface hydrogen abundance of XH, s=0.63 ± 0.04. The companion is undetected spectroscopically, and the orbital solution and stripped-star evolutionary mass imply a minimum companion mass of M2 ≳ 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.
Related
- Light curve spectral energy distribution fitting of 13 carbon-rich Mira variable stars in the Small Magellanic Cloud
- Discovery of the Slowest Radio Pulsar in the Small Magellanic Cloud
- Precise Physical and Kinematical Parameters of Massive Eclipsing Binaries in the Small Magellanic Cloud
- Precise physical and kinematical parameters of massive eclipsing binaries in the Small Magellanic Cloud
- Light curve spectral energy distribution fitting of 13 carbon-rich Mira variables stars in the Small Magellanic Cloud
BibTeX
@misc{blomberg2026intermediatemass,
title = {{An intermediate-mass stripped star with a dark companion in the Small Magellanic Cloud}},
author = {Lisa Blomberg and Kareem El-Badry and Peter Senchyna and Ylva Götberg and Bethany Ludwig and Maria R. Drout and Ana Bonaca and Warren R. Brown and Tim Cunningham and Brenna Mockler and Przemek Mroz and Andrzej Udalski},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.36050},
eprint = {2610.36050},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.36050},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - An intermediate-mass stripped star with a dark companion in the Small Magellanic Cloud
AU - Lisa Blomberg
AU - Kareem El-Badry
AU - Peter Senchyna
AU - Ylva Götberg
AU - Bethany Ludwig
AU - Maria R. Drout
AU - Ana Bonaca
AU - Warren R. Brown
AU - Tim Cunningham
AU - Brenna Mockler
AU - Przemek Mroz
AU - Andrzej Udalski
PY - 2026
DA - 2026/10/26
PB - arXiv
DO - 10.48550/arxiv.2610.36050
UR - https://arxiv.org/abs/2610.36050
AB - 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± 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 ± 0.54 M_⊙, temperature T_eff = 68.3 ± 6.4 kK, surface gravity log g= 4.76 ± 0.12, luminosity log(L/L_⊙)= 4.28 ± 0.13, radius R= 0.99 ± 0.09 R_⊙, and an unusually high surface hydrogen abundance of X_H, s=0.63 ± 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.
ER -