Magnetic braking saturates: evidence from the orbital period distribution of low-mass detached eclipsing binaries from ZTF

ABSTRACT We constrain the orbital period (Porb) distribution of low-mass detached main-sequence eclipsing binaries (EBs) with light-curves from Zwicky Transient Facility (ZTF), which provides a well-understood selection function and sensitivity to faint stars. At short periods (Porb ? 2 d), are predicted evolve significantly due magnetic braking (MB), shrinks orbits ultimately brings into contact. The is thus sensitive probe MB. find that intrinsic (0.1 M1/M? < 0.9) basically flat (${\rm d}N/{\rm d}P_{\rm orb} \propto P_{\rm orb}^0$) Porb = 10 d down contact limit. This strongly inconsistent predictions classical MB models based on Skumanich relation, widely used in binary evolution calculations predict ${\rm orb}^{7/3}$ at periods. observed distributions best reproduced by field saturates torque scales roughly as $\dot{J}\propto orb}^{-1}$, opposed $\dot{J} orb}^{-3}$ standard law. also no significant difference between containing fully partially convective Our results confirm saturated law, was previously found describe spin-down rapidly rotating isolated M dwarfs, operates tidally locked binaries. advocate using calculations. work supports previous suggestions cataclysmic variables (CVs) much weaker than assumed evolutionary model, unless mass transfer leads additional angular momentum loss CVs.