We identify member stars of more than 90 open clusters in the LAMOST survey. With the method of Fang et al. (2018), the chromospheric activity (CA) indices log R CaK for 1091 member stars in 82 open clusters and log R Hα for 1118 member stars in 83 open clusters are calculated. The relations between the average log R CaK , log R Hα in each open cluster and its age are investigated in different T eff and [Fe/H] ranges.We find that CA starts to decrease slowly from log t = 6.70 to log t = 8.50, and then decreases rapidly until log t = 9.53. The trend becomes clearer for cooler stars. The quadratic functions between log R and log t with 4000K < T eff < 5500K are constructed, which can be used to roughly estimate ages of field stars with accuracy about 40% for log R CaK and 60% for log R Hα .

We identify 1325 member candidates of the open cluster (OC) Stock 2 using data from Gaia DR2. We use the algorithms Clusterix 2.0 and HDBSCAN to select cluster candidates and further refine the final cluster membership by defining neighbors in 5D phase space (X cp, Y cp, /ϖ, κ · μ δ /ϖ). Among these candidates, less than half have G, G BP, and G RP extinctions from Gaia. When Gaia extinctions are unavailable, we compute extiction using empirical formulas and E(B − V) = 0.350. We analyze the spatial distribution and mass profile of Stock 2. Our results reveal Stock 2 is still a bound OC and we find evidence of mass segregation. By comparing initial mass functions, the present-day mass function indicates that Stock 2 is a massive stellar cluster with a mass of 4000 M ⊙. The core radius and tidal radius, calculated via the radial density profile and total mass, are 3.97 pc and 22.65 pc, respectively. Common stars between our selected member candidates and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope DR7 medium-resolution catalog give a metalliclity of [Fe/H] = −0.040 ± 0.147.

We report the discovery of emerged tidal tails around open cluster IC 4756 (∼1 Gyr) based on 644 members identified from Gaia EDR3. Three-dimensional spatial positions and two-dimensional tangential velocities x , y , z , κ · μ α * / ϖ , κ · μ δ / ϖ are utilized to determine the comoving member candidates of IC 4756. Using a Bayesian method, we correct the distance for each cluster member. Two tidal tails extend up to 180 pc and display an S shape in the X ′ Y ′ space (Cartesian coordinates focused on cluster center). A clean sequence of our members in the color–absolute–magnitude diagram indicates the coeval population and matches perfectly with the PARSEC isochrone with age from Bossini et al. Mass segregation is detected in this cluster as well. Finally, we derive the tidal radius and core radius of IC 4756 about 12.13 pc and 4.33 ± 0.75 pc, respectively.

We revisit the moving groups (MGs) in the solar neighborhood with a sample of 91,969 nearby stars constructed from LAMOST DR7. Using the wavelet technique and Monte Carlo simulations, five MGs together with a new candidate located at V≃−130 km s−1 are detected simultaneously in V − U 2 + 2 V 2 space. Taking into account the other known MGs, we conclude that MGs in the Galactic disk are spaced by approximately 15–25 km s−1 along V velocity. The origin of detected MGs is analyzed through the distributions of [Fe/H]−[Mg/Fe] and ages. Our results support attributing the origin to the continuous resonant mechanisms probably induced by the bar or spiral arms of the Milky Way.