Exploring the Near Galactic Centre: A Comprehensive Study of Bulge OCs HSC 25, HSC 37, HSC 2878 Utilising Gaia DR3 Data

Abstract

We present a comprehensive photometric and kinematic study of the open clusters HSC 25, HSC 37, and HSC 2878, located in the innermost regions of the Galactic disc. Utilizing data from Gaia DR3 and the UPMASK membership algorithm, we identify 44, 55, and 112 most probable members for HSC 25, HSC 37, and HSC 2878, respectively. The mean proper-motion components are obtained as (-5.901 +/- 0.41, -6.213 +/- 0.40), (-3.231 +/- 0.56, -4.564 +/- 0.47), and (-3.830 +/- 0.51, -5.198 +/- 0.44) mas yr^-1 for HSC 25, HSC 37, and HSC 2878, respectively. The open clusters span a broad range of evolutionary stages, with estimated ages of log(t/yr) = 8.38 +/- 0.08, 7.04 +/- 0.09, and 9.04 +/- 0.09, and corresponding heliocentric distances of 7.36 +/- 0.37, 6.79 +/- 0.18, and 6.17 +/- 0.22 kpc. The obtained metallicities are 0.0388 +/- 0.0039, 0.0259 +/- 0.0028, and 0.0209 +/- 0.0023, respectively. Total mass estimates are 135, 755, and 204 solar masses, respectively, highlighting notable differences in stellar content across the clusters. An analysis of dynamical relaxation times suggests that HSC 25 and HSC 2878 are dynamically evolved, whereas the much younger HSC 37 is still in an early phase of dynamical evolution. The high space velocities and orbital parameters of these clusters reveal significant deviations from typical disc kinematics. HSC 25 and HSC 37 exhibit eccentric orbits and small perigalactic distances, consistent with dynamically heated or accreted origins within the Galactic bulge. In contrast, HSC 2878's relaxed, planar orbit suggests in situ bulge membership despite its age. These findings point toward a heterogeneous dynamical origin for the clusters, with implications for star formation and evolution in the inner Milky Way.

Figures (11)

• Figure 1: Spatial distribution of probable members for HSC 25, HSC 37, and HSC 2878. The underlying grey contours depict the stellar density levels derived via Kernel Density Estimation (KDE) based on $Gaia$ DR3 data. Blue-filled circles represent the members identified in this study, while red open circles indicate the members adopted from Hunt2024. The cluster centres are marked by the intersection of vertical and horizontal lines: blue dashed lines correspond to the centres determined in this work, and red dotted lines represent the coordinates given by Hunt2024.
• Figure 2: The King profile fitting to the surface density extracted RDP parameters. The solid curve represents the best-fit empirical King profile, calculated by weighting the data points with their errors. The red-shaded band corresponds to the 1$\sigma$ confidence interval of the fit.
• Figure 3: Distributions of the mean proper-motion components ($\mu_\alpha\cos{\delta}$ and $\mu_\delta$) for the candidate OCs members.
• Figure 4: Distribution of member stars for HSC 25, HSC 37, and HSC 2878 in the trigonometric parallax ($\varpi$) versus $Gaia$$G$-band magnitude diagram. The solid red line shows the median parallax value, and the surrounding black dashed lines show the range of values that fall within one standard deviation of the median.
• Figure 5: Distance distributions of member stars in the clusters HSC 25, HSC 37, and HSC 2878 based on their individual Bailer-Jones distances ($d_{\mathrm{BJ}}$). The histograms represent the estimated distance values, while the red curve indicates the Gaussian fit applied to the distributions. The shaded region shows the $\pm1\sigma$ range around the mean, corresponding to the internal uncertainty.