2021ApJ...910...80P

We present comprehensive characterization of the Galactic open cluster M 36. Some 200 member candidates, with an estimated contamination rate of ∼8%, have been identified on the basis of proper motion and parallax measured by the Gaia DR2. The cluster has a proper motion grouping around (µ_αcosδ=-0.15±0.01 mas yr^–1, and µ_δ = - 3.35 ± 0.02 mas yr^–1), distinctly separated from the field population. Most member candidates have parallax values 0.7-0.9 mas, with a median value of 0.82 ± 0.07 mas (distance ∼1.20 ± 0.13 kpc). The angular diameter of 27^′ ±0′ / . 4 determined from the radial density profile then corresponds to a linear extent of 9.42 ± 0.14 pc. With an estimated age of ∼15 Myr, M 36 is free of nebulosity. To the southwest of the cluster, we discover a highly obscured (A_V up to ∼23 mag), compact (∼1′ / . 9×1′ / . 2) dense cloud, within which three young stellar objects in their infancy (ages <=0.2 Myr) are identified. The molecular gas, 3.6 pc in extent, contains a total mass of (2-3) x 10² M_☉, and has a uniform velocity continuity across the cloud, with a velocity range of -20 to -22 km s^–1, consistent with the radial velocities of known star members. In addition, the cloud has a derived kinematic distance marginally in agreement with that of the star cluster. If physical association between M 36 and the young stellar population can be unambiguously established, this manifests a convincing example of prolonged star formation activity spanning up to tens of Myr in molecular clouds.