Sustaining Star Formation in the Galactic Star Cluster M 36?

Abstract

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 ( ${\mu }_{\alpha }\cos \delta =-0.15\pm 0.01$ <!-- --> 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 $27^{\prime} \pm 0\buildrel{\,\prime}\over{.} 4$ <!-- --> 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 (∼ $1\buildrel{\,\prime}\over{.} 9\times 1\buildrel{\,\prime}\over{.} 2$ <!-- --> ) 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) × 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.