Understanding the Star-forming Environment in Stellar Clusters By

The main goal of this thesis is to investigate the physical conditions of the star-forming environment in stellar clusters, especially · for the formation of low-mass cluster members. Embedded, young, and intermediate~mass stellar . clusters around Herbig . Ae/Be stars are sampled. Midand near-infrared observations identifying young stars and millimeter I interferometric observations probing dense molecular gas· and dust· continuum are presented. These observations are used to reveal the large-scale young steilar population around the vicinity where the sampled clusters form, probe the physical conditions of dense molecular clumps which are capable of forming individual low-mass cluster members, and examine the influence of the most massive star in the cluster on its siblings and natal cluster-forming cloud. This study shows that stars within the cluster tend to seem younger than those outside the cluster, suggesting a higher and continuous star-forming rate within the cluster than outside, or massive stars are initiated later than low-mass star~ within the same cloud. A thoroug~ investigation of young stars and dense gas toward the MWC·l080 cluster further suggests a domination of the.most massive star in the cluster on both the natal cloud dispersal and its low-mass cluster members. As active ' ,, ' outflows and winds from the Herbig Ae/Be stars increase the non-thermal motion in the cloud, low-mass cluster members are formed within denser and more. turbulent cores, than isolated lowmass star-forming cores. In addition, the strong gas dispersal from t~e Herbig Ae/Be stars also helps the removal of the circumstellar material around nearby fow-mass stars. This makes these low-mass cluster members appear older. In summary, this thesis provides the observational evidence showing how the most massive star in the cluster affects the formation and evolution of low-mass cluster members and the physical conditions of star formation in the cluster.