Detailed Analysis of the NGC 2168 Cluster, Leveraging Gaia DR3

Abstract

NGC 2168 (M35) serves as a fundamental benchmark for studying stellar evolution and dynamical environments at the transition between young and intermediate-age populations. We present a comprehensive analysis of the cluster's kinematic, structural, and astrophysical properties utilizing high-precision astrometry and photometry data from Gaia Data Release 3 (DR3), complemented by 2MASS data. A statistical membership assessment yields a clean sample of probable members (N ~ 1397), with mean proper motion components of mu_alpha cos(delta) = 2.278 +/- 0.006 mas/yr and mu_delta = -2.893 +/- 0.006 mas/yr, along with a mean trigonometric parallax of varpi = 1.154 +/- 0.052 mas. We derived the cluster's fundamental parameters via isochrone fitting, determining an age of 190 +/- 12 Myr, a metallicity of [M/H] = -0.048 dex, and a probabilistic distance of 840 +/- 54 pc. The radial density profile is well described by a generalized King model with beta = 1 (rc = 7.97', rcl = 36.69'), revealing the presence of a loosely bound, extended stellar halo. Furthermore, we detect a spatial elongation oriented perpendicular to the Galactic plane, likely a signature of vertical tidal heating or disk shocking. The mass function analysis exhibits a multimodal Gaussian structure, suggesting a complex dynamical formation history beyond a simple power-law distribution. Finally, orbital integration confirms NGC 2168 as a thin disk object with a maximum vertical excursion of ~171 pc, consistent with the observed vertical morphological deformation.

Figures (21)

• Figure 1: Uncertainties in the astrometric parameters (trigonometric parallax and proper-motion components) and in the photometric $G$ magnitude as a function of $G$ magnitude for stars in the field of NGC 2168.
• Figure 2: Surface number density maps in the field of NGC 2168 under different astrometric selection criteria. In the first panel, two stellar overdensities are visible, corresponding to NGC 2168 (larger circle) and the more distant background cluster NGC 2158 (smaller circle). Applying a stricter parallax cut suppresses the contribution from NGC 2158, while the combined parallax and proper-motion selection effectively removes the background cluster, yielding a clean density distribution dominated by NGC 2168.
• Figure 3: Radial density profile of NGC 2168. The solid black curve represents the best-fit generalized King profile with $\beta = 1$, while the red dashed curve illustrates the $\beta = 2$ solution. The horizontal dashed line marks the background density level. Notably, the statistical preference for the $\beta=1$ model indicates a shallower density fall-off in the outer regions, revealing the presence of an extended stellar halo that reaches beyond the classical tidal radius estimates.
• Figure 4: Histogram of the membership probabilities for stars in the NGC 2168 region, derived from the pyUPMASK algorithm.
• Figure 5: Our methodology involves analyzing the Probability $P_{i}$ at every ring as a function of the radius $r_i$.