The non-monotonic, strong metallicity dependence of the wide-binary fraction

The metallicity dependence of the wide-binary fraction (WBF) IN stellar populations plays a critical role in resolving the open question of wide-binary formation. In this paper, we investigate the metallicity ([Fe/H]) and age dependence of the WBF (binary separations between 103 and 104 au) for field F and G dwarfs within 500 pc by combining their metallicity and radial velocity measurements from LAMOST Data Release 5 (DR5) with the astrometric information from Gaia DR2. We show that the WBF strongly depends on the metallicity: as metallicity increases, the WBF first increases, peaks at [Fe/H] ∝ 0, and then decreases at the high-metallicity end. The WBF at [Fe/H] = 0 is about two times larger than that at [Fe/H] = -1 and +0.5. This metallicity dependence is dominated by the thin-disc stars. Using stellar kinematics as a proxy of stellar age, we show that younger stars have a higher WBF at fixed metallicity close to solar. We propose that multiple formation channels are responsible for the metallicity and age dependence. In particular, the positive metallicity correlation at [Fe/H] < 0 and the age dependence may be due to the denser formation environments and higher mass clusters at earlier times. The negative metallicity correlation at [Fe/H] > 0 can be inherited from the similar metallicity dependence of close binaries, and radial migration may play a role in enhancing the WBF around the solar metallicity.