The Astrophysical Journal Supplement Series, 70:731-812, 1989 August .731L © 1989. The American Astronomical Society. All rights reserved. Printed in U.S.A. .70. 9ApJS. A CO SURVEY OF REGIONS AROUND 34 OPEN CLUSTERS 198 David Leisawitz Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center; and Astronomy Program, University of Maryland Frank N. Bash Astronomy Department, The University of Texas at Austin AND Patrick Thaddeus HarvcU’d-Smithsonian Center for Astrophysics Received 1988 August 15; accepted 1988 December 12 ABSTRACT The Columbia 1.2 m millimeter-wave telescope was used to search systematically regions around 34 open star clusters for emission from the / = 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 deg2. 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 4 one molecular cloud more massive than 10 M0\ (b) clusters older than ~ 10 Myr do not have associated with 3 them molecular clouds more massive than a few times 10 Af0; (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. Subject headings: clusters: open — interstellar: molecules — nebulae: H n regions — stars: formation I. introduction Green, and Peters (1977; hereafter BGP) used CO spectral It is widely accepted that interstellar molecular clouds are line observations of regions around 63 young clusters to the birthplaces of stars, and, in recent years, a good deal of derive a characteristic molecular cloud lifetime. Spectroscopic discussion has centered on the question of whether and how and photometric observations of the stars in open clusters, molecular clouds are destroyed by star formation. Detailed interpreted with the aid of stellar evolution models, provide a studies of individual, isolated regions are inadequate if what chronometer with which the cluster ages can be gauged. one seeks is an understanding of the mechanism by which In principle, for example, if CO emission were to be de- stars interact with clouds in general. This is because the tected in regions around clusters younger than some age A, appearance of an individual region can be affected by initial but not in regions around clusters older than A, then one conditions, because the exact state of evolution of the system might conclude that the molecular cloud lifetime (after star under study is unknown, and because uncertainties exist in formation begins) is A. Indeed, the BGP survey indicated the positions and motions of the nascent stars relative to the that, on average, only clusters that contain stars of spectral clouds. type earlier than B0 show evidence for an associated molecu- Fortunately, there is a way to address the question of how lar cloud. Unfortunately, the BGP spatial resolution of ~ 2' stars interact with nearby interstellar matter (ISM) that cir- made severe undersampling of their regions necessary, and no cumvents or dilutes the impact of complications that arise in general conclusions regarding the nature of the cluster-cloud studies of individual regions. The method consists of examin- interaction could be drawn from their observations. ing, systematically, the interstellar contents of young open It was clear that several interesting questions could be cluster environments, considering clusters of a range of ages. addressed only with a CO survey more thorough than that of This approach was first employed by Gordon, Howard, and BGP. First, simply, what effect do stars have on the molecular Westerhout (1968; hereafter GHW) who used the 21 cm line clouds in which they form? Do the clouds survive their as a probe of the atomic gas in 28 cluster neighborhoods. interaction with the stars? What fraction of the molecular gas Schwartz (1971) sought radio continuum emission as an indi- is processed into stellar form? What physical mechanism cator of the presence of ionized gas around 41 clusters. Bash, dominates the interaction between a young cluster and the 731 © American Astronomical Society • Provided by the NASA Astrophysics Data System .731L 732 LEISAWITZ, BASH, AND THADDEUS Vol. 70 stellar matter to have been detected optically. A catalog of .70. molecular clouds in its vicinity, thus affecting the cloud life- . time and the arm-interarm contrast of molecular clouds in these clusters with an extensive bibliography has been pro- spiral galaxies? Also possibly influenced by the cluster-cloud vided by Alter, Ruprecht, and Vanysek (1970), supplemented interaction are the cloud-cloud velocity dispersion and the by Ruprecht, Balazs, and White (1981). A substantial fraction 9ApJS. stellar initial mass function (IMF). ( ~ 35%) of the cataloged clusters have been studied photo- 198 To learn more about the molecular gas content of young metrically, and for this fraction, distances, ages, and visual cluster environments, and spurred on by a major improve- extinctions are generally available. A smaller fraction, -10%, ment since BGP in millimeter receiver technology and by the have been observed spectroscopically and have measured ra- availability of a telescope designed specifically to survey large dial velocities (see, e.g., Hron 1987; Wramdemark 1982). The angular areas, we decided to map regions around a number of physical characteristics of 1180 open clusters are contained in young open clusters systematically for CO emission. the computer data base compiled by Lyngâ (1987), and some The present survey is one of several CO surveys of the statistical properties of the system of clusters are considered Milky Way conducted with the Columbia millimeter-wave by Lyngâ (1982) and Janes, Tilley, and Lyngâ (1988). telescope. Although CO surveys of the entire Galactic plane It was our aim to select from the set of known galactic have been made, with the Columbia telescope as well as with clusters, in a manner as free of bias as practicable, those others, these surveys were intended to show the large-scale which are the best suited for millimeter-wave observation. We molecular gas content of the Galaxy and, for a number of begin this section with a discussion of selection criteria and reasons, they do not provide the data that we seek. The inner then describe the characteristics of the sample and consider Galaxy either has been undersampled (Knapp, Stark, and the degree to which it is representative. Wilson 1985; Sanders et al 1986, and references therein), covered with a spatial resolving power too low to provide a) Cluster Sample Selection Criteria detailed information about distant star-forming complexes Four criteria were used to select a sample of clusters to (Dame et al. 1987, and references therein), or thoroughly survey in CO. A cluster must (a) be well-studied, (b) its mapped, but with limited Galactic latitude coverage (Cohen, distance must He in the range l<d (kpc) < 5, (c) its age must Dame, and Thaddeus 1986; Dame 1983). Confusion is an be <100 Myr, and (d) its declination must be > -20°. By additional major complication in the inner Galaxy. “well-studied,” we mean that agreement should exist among Because the molecular component of the outer Galaxy fills independent pubhshed estimates of the distance and age of a a relatively large part of the sky, coverage there has been cluster; additional weight is accorded to clusters with mea- either incomplete or obtained with low spatial resolution. In sured radial velocities. the low-angular-resolution survey of Dame et al. (1987), those Practical considerations led to the restriction on cluster of our clouds that are detected generally are not resolved. The distance. Since very few Galactic molecular clouds have a low-resolution survey, not surprisingly, turns out to be espe- dimension that exceeds 50 pc (see, e.g., Dame etal. 1986; cially sensitive to detections of local dark clouds that subtend Liszt, Xiang, and Burton 1981; Sanders, ScoviUe, and Solomon large solid angles and relatively massive clouds. 1985), it seemed reasonable to confine our observations gener- The Galactic plane surveys do show us the large picture of ally to within 25 pc of the cluster center in projected distance. the molecular Galaxy and thus provide a natural framework The sampling interval chosen for our CO observations was within which to evaluate the survey of open cluster neighbor- 7Í5, shghtly less than the beamwidth of the Columbia tele- hoods. Our survey is a link between the Galactic plane scope (see § III). A map of radius 25 pc at a distance of 1 kpc surveys and studies of individual star-forming complexes. The with this sampling would require 400 spectra or more, and open cluster survey coverage is compared to the coverage of one at a distance of 5 kpc would require about 20 spectra. To the Dame et al. (1987) survey in Figure 1. avoid biasing the sample by spending excessive time mapping Two major topics are treated in this paper: selection of a a few very nearby cluster regions, few clusters less than 1 kpc sample of clusters for observation and empirical results of a away were selected for observation.