Tracing Radial Migration in the Outer Disk: A Comprehensive Analysis of the Old Open Cluster Berkeley 36

We present a chemo-kinematical, structural, and photometric analysis of the old open cluster Berkeley 36 using Gaia DR3 astrometry and photometry together with high-resolution spectroscopy from the Gaia-ESO Survey DR5.1. Applying a Gaussian mixture model to Gaia astrometry, we identify 946 high-probability cluster members. We derive a core radius of 2 . ′ 63 ± 0 . ′ 21 and a tidal radius of 14 . ′ 84 ± 0 . ′ 47, indicating a moderately concentrated and dynamically relaxed system. By fixing the cluster metallicity to the spectroscopic value of [Fe/H] = −0.19 ± 0.02 dex and adopting an independently determined geometric distance of 4377 ± 510 pc, we minimize the classical age–reddening–metallicity degeneracy and derive a robust isochrone age of 6.8 ± 0.5 Gyr. Independent Ba-based chemical clocks yield a mean age of 7.75 ± 1.94 Gyr, supporting the isochrone solution. The cluster exhibits a high main-sequence binary fraction of 48.0% ± 1.7% and hosts 83 blue straggler star candidates with an extended spatial distribution, contrary to classical mass segregation. Orbit integration shows that Berkeley 36 follows a nearly circular orbit in the outer Galactic disk ( R GC = 11.42 ± 0.44 kpc). Accounting for the Galactic warp and disk flare increases the cluster’s maximum vertical excursion by 86% and the local disk scale height by 17%, respectively. Comparison with its chemically inferred birth radius ( R b = 6.71 ± 0.66 kpc) indicates an outward radial migration of approximately 5 kpc, predominantly driven by churning. These results establish Berkeley 36 as a benchmark for investigating radial migration, secular evolution, and the dynamical history of old Galactic open clusters.