To appear in the conference proceedings for the meeting "Stellar Clusters and Associations: Convection, Rotation and Dynamos", Astronomical Society of the Pacific Conference Series

Abstract

A new survey of open clusters is described. The Palomar-Las Campanas Observatory-NOAO (PLCON) Open Cluster Survey is intended to provide for CCD derived, VI color-magnitude diagrams of several hundred open clusters, most of which have been poorly studied in the past, or never studied at all. From these data, systematic estimates of cluster reddenings, distances and ages can be derived. When combined with previous CCD studies of clusters, fully one-half of all known open clusters will have available CCD data at the conclusion of the Survey.

Introduction

Star clusters are unique laboratories for investigating a wide range of astrophysical problems relating to star formation, stellar evolution, the formation and structure of our Galaxy, and the age and distance scale of the Universe. This unique role can be attributed to the reliability with which distances and ages can be determined for clusters. Consequently a large number of cluster studies were undertaken in the 1950s and 1960s, but the majority of the effort since then has been concentrated on the globular clusters. Despite the recognized importance of less massive, typically young open star clusters for investigating a wide range of astrophysical problems, only some 200-300 of the more than 1200 known clusters (Lynga 1987) have received more than superficial attention.

The advent of charge-coupled devices (CCDs) in the 1980s revolutionized astronomy, allowing for research to be undertaken on small telescopes that once was achievable only with the largest ones. The advantages of CCD detectors over those used previously are enormous; they are linear detectors and they can be used to measure reliably positions and brightnesses for thousands of stars, whereas previously only dozens or hundreds of stars were typically measured (and not always precisely). CCDs are also much more sensitive than earlier detectors, allowing small telescopes to reach faint stars. The fields of view currently available with CCDs match the angular sizes of most open clusters quite well, making them prime targets for smaller telescopes and CCDs.

Surprisingly, however, only about 100 open clusters have been observed with CCDs, and most of them were already the subjects of previous photographic and/or photoelectric studies. There remains, therefore, an untapped wealth of knowledge to be found within the nearly 900-1000 open clusters that have been the subject of few, if any, studies. The clusters that have been observed to-date are typically the largest, closest, more massive ones that are, not surprisingly, the easiest to observe. Without a more complete, homogeneous survey, the biases inherent in our current understanding of the open cluster population will seriously compromise our understanding of star formation history within the Galactic disk, the characteristic sizes, masses, and integrated luminosities and colors of Galactic open clusters (and hence our interpretations of integrated properties of extragalactic clusters), the dynamical evolution and subsequent disruption of open clusters (leading to many of the stars we see in the field), the age of the Galactic disk, the chemical enrichment history within the disk, and a host of other timely astrophysical problems.

For the first time, it is possible technologically to observe the majority of the known population of open clusters, thereby improving our understanding of a large number of astrophysical problems. As a step toward this goal, the "Palomar-Las Campanas Observatory-NOAO (PLCON) Open Cluster Survey", will provide an estimate of the reddening, distance and age for each of the 400-500 open clusters. The PLCON Survey will bring the total number of open clusters with available CCD data to ~600, or fully 50% of the known open cluster population. These data will significantly enhance our knowledge of the Galaxy's population of open clusters, and provide meaningful, self-consistent cluster parameters (e.g, ages and distances) that will form the basis for follow-up studies in a wide range of astrophysical disciplines.

Project Description

The PLCON survey is intended to provide a systematic, homogeneous database of cluster photometry. This goal is achievable by the use of a small number of telescopes (the Palomar Observatory 1.5 meter and the Las Campanas Observatory 1 meter telescopes), the use of consistent filters at each telescope, and a uniform observing strategy. Each cluster in the survey is observed in each of the V and I passbands, using short (typically 3 sec), medium (30 sec) and long (360 sec in V, 300 sec in I) exposures. The short exposures are used to obtain photometry of the brightest stars, which constrain the cluster age, while the longer exposure frames are used to reach farther down the cluster sequence in the color-magnitude diagram (CMD), where the shape better constrains the cluster distance. The V and I filters were selected for several reasons:

1. Although observations with more than two filters are highly desirable, the goal of observing several hundred clusters makes selection of only two filters necessary;

2. Much of the scheduled observing time is/was during bright time, necessitating the use of red filters to minimize the effects of the bluer, brighter sky background;

3. A primary goal of the Survey is the identification of "new" old open clusters. Their detection is enhanced by the use of redder filters, since the brightest stars in old clusters are red. Additionally, interstellar reddening effects for distant clusters, located near the Galactic plane, are lessened when using redder filters.

Data Quality

By utilizing the short exposure data for magnitudes brighter than ~15 mag, and the medium exposure photometry for magnitudes brighter than ~17 mag, internal errors in the photometry can be limited to s £ 0.02 mag to a limiting V magntude of ~19 mag. The internal photometric accuracy is better than 0.05 mag to a limiting V magnitude of ~20 mag.

The PLCON Survey is not intended to provide precise photometry for every star in each cluster. Additionally, the presence of field stars precludes precise membership information. The goal of the PLCON survey is an estimate, from the overall CMD, of the reddening, distance and age for each of the several hundred observed clusters. In order to achieve this goal, the telescope guider is set before the series of V and I exposures are taken - no attempt is made to use dithering to compensate for bad pixels/columns in the CCD, or for cosmic ray removal. This observing strategy leaves open the likelihood that the photometry for some small fraction of the stars will be compromised by bad pixels and/or cosmic rays, but the overall CMD derived for each cluster will be of sufficiently high quality to achieve the goals of the PLCON survey, that is a systematic, homogeneous determination of fundamental properties for the majority of known open clusters.

Uncertainties in the reddening, distances and ages vary depending upon the cluster, its reddening and the associated field star contamination. The uncertainties are typically larger than are found for more easily observed nearby clusters, but it is to be emphasized that this program will, for the first time, provide a self-consistent, homogeneous, large-scale determination of the reddenings, distances and ages, along with their uncertainties, for fully half of the known open clusters.

Data Distribution

For updated information on the status of the Survey, as well as information about data distribution, see the "PLCON" section under the "Research" heading at www.csus.edu/indiv/p/phelpsr (the Web page is under construction).

Acknowledgments

This research is supported, in part, by a grant (AST-9800126) from the U.S. National Science Foundation

References

Lynga, G. 1987, Catalog of Open Cluster Data, Lund Observatory