An Estimate of the Binary Star Fraction Among Young Stars at the Galactic Center: Possible Evidence of a Radial Dependence

We present the first estimate of the intrinsic binary fraction of young stars across the central $\approx$ 0.4 pc surrounding the supermassive black hole (SMBH) at the Milky Way Galactic center (GC). This experiment searched for photometric variability in 102 young stars, using 119 nights of 10"-wide adaptive optics imaging observations taken at Keck Observatory over 16 years in the K'- and H-bands. We photometrically detected three binary stars, all of which are situated more than 1" (0.04 pc) from the SMBH and one of which, S2-36, is newly reported here with spectroscopic confirmation. To convert the observed binary fraction into an estimate of the underlying binary fraction, we determined experiment sensitivity through detailed light curve simulations, incorporating photometric effects of eclipses, irradiation, and tidal distortion in binaries. The simulations assumed a population of young binaries, with stellar ages (4 Myr) and masses matched to the most probable values measured for the GC young star population and underlying binary system parameters similar to those of local massive stars. The detections and simulations imply young, massive stars in the GC have a stellar binary fraction $\geq$ 71% (68% confidence), or $\geq$ 42% (95% confidence). This inferred GC young star binary fraction is consistent with that typically seen in young stellar populations in the solar neighborhood. Furthermore, our measured binary fraction is significantly higher than that recently reported by Chu et al. (2023) based on RV measurements of young stars <~1" of the SMBH. Constrained with these two studies, the probability that the same underlying young binary fraction extends across the entire region is <1.4%. This tension provides support for a radial dependence of the binary star fraction and, therefore, for the dynamical predictions of binary merger and evaporation events close to the SMBH.