Old open clusters NGC 188 and M 67 in Gaia DR3

Abstract

We performed a statistical study of two old open clusters NGC 188 and M 67 using Gaia DR3 data. No tidal tails of the clusters were detected, which most likely had been destroyed when the cluster passed through the Galactic plane. The size estimates of the clusters depend on the range of astrometric parameters and stellar magnitudes of the stars used for star counts. The mass spectra of two clusters differ significantly. NGC 188 shows a deficit of low-mass stars compared to M 67. In the halo region of NGC 188 (compared to the core region of the cluster), there is a relative excess of the low-mass stars (just as in the case of M 67) and a deficit of stars in the mass range from 0.66 to 0.9 solar masses. Comparison with the Hunt & Reffert samples showed that almost all the stars from these samples are contained among the stars we selected for counting. Moreover, the group probability of these stars belonging to clusters, estimated by the uniform background method, is higher than 60%. It is shown that the velocity dispersion of single stars (selected according to the `stellar magnitude -- color index' diagrams) is significantly smaller than the velocity dispersion of unresolved binary stars.

Figures (12)

• Figure 1: Diagram 'proper motion in right ascension $\mu_\alpha$ vs. G magnitude' for stars within 30 arcmin from the center of the M 67 cluster. Red lines show the 'standard' selection range, blue lines show the 'strict' selection range.
• Figure 2: Diagram 'parallax $\varpi$ vs. G magnitude' for stars within 30 arcmin from the center of the M 67 cluster.
• Figure 3: Surface density maps for the M 67 cluster. Left: without preliminary star selection; right: after the 'strict' selection (\ref{['interval_2_67']}).
• Figure 4: Maps of stars from Hunt & Reffert HR2023 samples and cluster sizes determined from surface density profiles using the method of Seleznev2016. Left: NGC 188. Red line --- cluster size for $G<18$ mag and constraints on the astrometric parameters (\ref{['interval_1_188']}), green line --- for $G<21$ mag and constraints (\ref{['interval_2_188']}). Right: M 67. Red line --- $G<18$ mag and constraints (\ref{['interval_1_67']}), blue line --- $G<21$ mag and constraints (\ref{['interval_1_67']}), green line --- $G<21$ mag and constraints (\ref{['interval_2_67']}).
• Figure 5: Brightness functions. Black lines --- cluster region (cluster and field stars), blue lines --- comparison region (field stars), red lines --- difference between them (cluster stars). Solid lines — brightness function, dashed lines --- 2$\sigma$ confidence interval. Left: NGC 188, right: M 67.