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: $T_{\rm eff} = 8800 \pm 700$ K, $Log(g) = 4.49 \pm 0.58$, $[{\rm M/H}] = -0.31 \pm 0.15$, and $v\sin (i) = 95.63 \pm 9.78$ km s−1. A Keplerian fit to multi-epoch radial velocity data reveals a nearly circular orbit ($e = 0.03 \pm 0.01$) with period $P = 0.234 \pm 0.007$ d, semi-amplitude $K = 4.79 \pm 0.05$ km s−1, and systemic velocity $\gamma = 64.97 \pm 0.03$ km s−1. The primary has a mass of $1.72 \pm 0.12$ M$_\odot$, radius $1.23 \pm 0.22$ R$_\odot$, and age of $0.51 \pm 0.07$ Gyr, while the orbital separation is $1.94 \pm 0.05$ R$_\odot$. The companion mass can range between 0.0199 and 0.099 M$_\odot$, depending on inclination; thus, the lightest BSS companion detected so far. The system is likely tidally synchronized, implying an inclination of $i = 21.08^{\circ } \pm 4.49^{\circ }$ and a companion mass of $0.056 \pm 0.011$ M$_\odot$, along with $T_{\rm eff} \sim$1000–2500 K and radius of $\sim 0.08 \pm 0.13$ R$_\odot$, 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.