First Discovery of a Fast-Rotating Blue Straggler in a Compact Binary with a Substellar Companion

Abstract We report the first discovery of a brown-dwarf (BD) companion using a radial velocity-based study of a rapidly rotating blue straggler star (BSS) in a short-period close binary system in NGC 2243. Multi-epoch spectra from VLT/FLAMES-GIRAFFE, analyzed using iSpec, yield stellar parameters for the primary: Teff = 8800 ± 700 K, Log(g) = 4.49 ± 0.58, M/H = −0.31 ± 0.15, and vsin (i) = 95.63 ± 9.78 km/s. A Keplerian fit to multi-epoch radial velocity data reveals a nearly circular orbit (e = 0.03 ± 0.01) with period P = 0.234 ± 0.007 days, semi-amplitude K = 4.79 ± 0.05 km/s, and systemic velocity γ = 64.97 ± 0.03 km/s. The primary has a mass of 1.72 ± 0.12 M⊙, radius 1.23 ± 0.22 R⊙, and age of 0.51 ± 0.07 Gyr, while the orbital separation is 1.94 ± 0.05 R⊙. The companion mass can range between 0.0199 - 0.099 M⊙, depending on inclination; thus, the lightest BSS companion detected so far. The system is likely tidally synchronized, implying an inclination of i = 21.08○ ± 4.49○ and a companion mass of 0.056 ± 0.011 M⊙, along with Teff ~ 1000 − 2500 K and radius of ~0.08 ± 0.13 R⊙, it is likely to be a BD. This is the shortest-period binary known inside the BD desert for main-sequence stars, and one of the most compact substellar companions ever identified in a stellar system. Single-star SED fitting and a Gaia RUWE of 1.01 show no excess or astrometric anomalies, supporting a faint companion. This rare non-eclipsing BSS-BD system offers a valuable insight into the binary interaction during BSS formation.