The Open Cluster Database has been developed in order to provide easy reference to observational data and membership information for some of the nearer and better studied open clusters in the Galaxy. Clusters of particular interest are those for which candidate members in the solar-type and low-mass membership range have been identified through proper motion surveys, x-ray surveys, photometric/spectroscopic surveys or a combination of these methods. The data were compiled originally with the intent to provide an observational database to study the evolution of angular momentum among solar-type and low-mass stars as they evolve down to the zero-age main sequence, but undoubtedly may be applied towards other studies.
The format for the database on each cluster is one where the observational information for all the stars considered in the cluster has been segregated into separate files and listed in a columar format primarily for space and viewing considerations. These files are termed 'core datafiles' and currently contain the following information (where the prefix 'xxx' would represent an abbreviation for the cluster name) :
core files:
For these clusters, we have listed or included stars which may have been at one time considered as possible members, but for which later data may have found them to be nonmembers. A `reasonable' selection of stars considered as candidate members has therefore been included; we have not however blindly listed every possible star once cataloged in the region of the cluster. For example, in the Pleiades database we have not listed every HII star from Hertzsprung's survey, only those which had been considered as possible members based on proper motions. Since multiple star names may exist for the same star as a result of different membership surveys in a particular cluster, a `primary' star name has usually been chosen from an early survey of the cluster, with later alias star names listed in the xxx.mem core datafile. In the Hyades, the multiple star name issue is so complicated that a separate core datafile 'hya.alias' was created to list aliases. Of course, as new information becomes available to us, new stars and information may be added to these files.
The observational data listed in the core datafiles is usually flagged with a reference number corresponding to an entry in the xxx.ref file for the original source of the information. Using available sources, we have attempted to list what we consider to be the most accurate data on a particular star. In some instances, like for radial and rotational velocity determinations, we have listed more than a single value if there were estimates from more than one source. The reader is advised to refer to the original sources in the xxx.ref file for questions regarding the data listed.
In recent years the ROSAT satellite has provided x-ray flux measurements of members in several open clusters; in some clusters, new low-mass members themselves have been identified through their detection in x-rays and appear in the database files. While a core datafile 'xxx.xray' listing x-ray count rates and/or upper limits to cluster stars would be a desirable addition to the cluster database, we have not constructed them at this time. The reason for this is due to the variety of x-ray observations performed in the different clusters and the overlap occuring for some stars in some cases where they may have been detected in one observation, but have only an upper limit estimate from another observation. At some point, the addition of x-ray emission information will be considered, but the reader is referred to the original x-ray surveys at the present time for this information.
The xxx.mem file contains membership flags on the basis of different considerations such as photometry, radial velocity, and presence of lithium for example. These membership flags are qualitative in nature and range from members/candidate members (Y/Y?) to undecided (?) to nonmembers/likely nonmembers (N/N?); other researchers may assign slightly different membership flags than those listed in the table. As new observations become available the membership information is updated.
Currently there are two means of accessing the database. First, a researcher may wish to directly access one or more of the ascii core datafiles by their own plotting program or software routine to plot/input/view the contents. Second, a fortran program `cluster.f' has been developed by C. Prosser to prompt the user for the cluster of interest, read in data from the core files, and prompt the user for various selection parameters of interest to construct an output file. The cluster.f program provides a relatively easy means to segregate the cluster stars on the basis of membership and create files useful for plotting color-magnitude diagrams of members/candidate members of the cluster or for determining the completeness of radial or rotational velocity measurements, for example, among candidates. Questions on using the cluster.f program should currently be directed to C. Prosser (cprosser@noao.edu).