CCD () photometry of the open cluster NGC 6793 and its dynamical evolution

Abstract

We present new astrophysical parameters for the open cluster NGC~6793 based on new CCD $UBV(RI)_{KC}$ photometry. We derived a reddening of $E(B-V) = 0.24 \pm 0.02$~mag and a heavy element abundance of $Z = 0.024$ ($[Fe/H] = +0.20$~dex). Padova isochrone fitting to the $V \times (B-V)$ colour-magnitude diagram yields an intermediate age of $525 \pm 51$~Myr and a distance modulus of $μ= 8.80 \pm 0.05$~mag, corresponding to a distance of $d = 575 \pm 58$~pc from the Sun. The core radius of NGC~6793 appears to be shrinking due to advanced dynamical evolution ($\logτ_{2} = 1.13$), driven by mass segregation and the evaporation of low-mass stars from the central region. The ratios of core to half-mass radius ($R_{c}/R_{h}$) and half-mass to Jacobi radius ($R_{h}/R_{J}$) indicate that the cluster's evolution is governed by the combined effects of internal two-body relaxation, mass segregation, and external tidal perturbations. The ratio $R_{t}/R_{J} = 0.99$ suggests that the cluster is currently in a tidally filling state. The parameter pairs ($t_{diss}/t_{rlx_{1}} = 40$, $\log R_{J}/R_{c} = 0.72$) and ($R_{h}/R_{J} = 0.38$, $\logρ_{amb} = -0.88$) place NGC~6793 among the relatively compact clusters within $R_{GC} < 7.9$~kpc. This implies a compact internal structure that is stable against the combined effects of two-body encounters and tidal heating. Given its current state, NGC~6793 will likely dissolve and disperse before entering the final contraction phase ($R_{4}$ regime).