A CO survey of regions around 34 open clusters

Abstract

The Columbia 1.2 m millimeter-wave telescope was used to search systematically regions around 34 open star clusters for emission from the J = 1 - 0 rotational transition of 12 CO. The survey sensitivity is sufficient to enable 3 a detection of lines as weak as 1 K (7^*) over a range in velocity ± 83 km s -1 centered on the stellar radial velocity. Around each cluster, a region at least 25 pc or 10 cluster angular diameters was mapped with a uniform sampling interval of 7(5; a typical region was ~ 4 deg 2 . The criteria used to select clusters for observation are outlined. The clusters have well-determined distances ranging from -1 to 5 kpc and ages < 100 Myr. Sample selection effects are discussed. A description is given of features of the interstellar matter associated with each of the surveyed clusters. Whenever possible, a distinction is made between interstellar matter that is physically associated with a cluster and material that lies along the line of sight but is otherwise not related to the cluster. In general, we find that (a) all the surveyed clusters younger than ~ 5 Myr have associated with them at least one molecular cloud more massive than 10 4 M 0 \ (b) clusters older than ~ 10 Myr do not have associated with them molecular clouds more massive than a few times 10 3 Af 0 ; (c) molecular clouds are receding from young clusters at ~ 10 km s' 1 , and they also apparently are being destroyed by their interaction with the stars. Sites of ongoing star formation are found in a number of the clouds associated with young clusters.