First discovery of a fast-rotating blue straggler in a compact binary with a sub-stellar 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, analysed 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−1 . 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 d, semi-amplitude K = 4 . 79 ±0 . 05 km s−1 , and systemic velocity γ = 64 . 97 ±0 . 03 km s−1 . 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 and 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 sub-stellar companions ever identified in a stellar system. Single-star SED fitting and a Gaia Renormalized Unit Weight Error (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.