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

Abstract

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.

Architecture of Hierarchical Stellar Systems and Their Formation

“…This explains the increase of the close-binary fraction at low metallicity found by Moe et al [35]. Interestingly, wide binaries exhibit an opposite trend: Hwang et al [36] discovered a reduced fraction of wide binaries in low-metallicity environments and explained it by the larger density and velocity dispersion in the metal-poor star-formation regions. Gravitational physics dictates that collapse at small scales proceeds much faster than at large scales [37], so when the smallest structures form, the larger structures still collapse.…”

“…Now we know that the metallicity of the gas affects its fragmentation scale and leads to a strong inverse dependence of the close-binary fraction on metallicity [35]. Conversely, the fraction of wide binaries positively correlates with metallicity [36]. At intermediate separations of 300-3000 au, the fraction of binaries in loose associations is twice lager than in the open clusters [20].…”

Binary fractions of G and K dwarf stars based on the Gaia EDR3 and LAMOST DR5: impacts of the chemical abundances

“…Consequently, it would lead to a slower slope of the metallicity dependency. Given that our sample also contains wide binaries in addition to the close binaries, this could be accounted for that the metallicity dependency of the binary fraction changes with the increasing period (e.g., El-Badry & Rix 2019;Hwang et al 2021;Bate 2019). As the period increases, the anti-correlation between the close binary fraction and metallicity weakens, even becomes positive.…”

Architecture of Hierarchical Stellar Systems and their Formation

“…Now we know that the metallicity of the gas affects its fragmentation scale and leads to a strong inverse dependence of the close-binary fraction on metallicity (Moe et al 2019). Conversely, the fraction of wide binaries positively correlates with metallicity (Hwang et al 2021). At intermediate separations of 300-3000 au, the fraction of binaries in loose associations is twice lager than in the open clusters (Deacon & Kraus 2020).…”