WEBDA Presentation

Contents

This site

The need for a database

Open clusters

I hope WEBDA will become a place where anybody could get the data he or she needs and contributes his or her own data.

This database has been conceived to bring at one place most data that could be useful to decide upon the membership of the stars and their physical characteristics, and store them with a coherent numbering scheme. In spite of the limitations due to the lower precision of some older data, the database is the best starting point for many astrophysical studies involving open clusters. Nowhere else are complete data collections to be found and one merit of the database is to give a clear report of the present observation status.

The present database offers astrometric data in the form of coordinates, rectangular positions, and some proper motions, photometric data in the major system in which star clusters have been observed (UBV, uvby, Geneva, Vilnius, DDO and others), spectroscopic data, like spectral classification, radial velocities, rotational velocities. It contains also miscellaneous types of data like membership probabilities, orbital elements of spectroscopic binaries, periods of variability for different kinds of variable stars. List of interesting and peculiar stars have also been compiled. Finally a whole set of bibliographic references allows every one to locate the interesting publications on his or her favorite open clusters easily.

The channels called Information and Interrogation offer two different ways to find the clusters that are potentially interesting for a given study. Once some clusters have been identified, the existing data can be reached with the help of the tools proposed in each cluster page. These tools include search by star numbers, selection on astrophysical parameters, searching the cross-reference tables, looking at the detailed content of the data files and the corresponding references, plot of colour-magnitude diagrams, display of cluster photographs and maps.

A facility has been developed to give the possibility of transfering any part of the data for a cluster on the user computer. Another part of the site is devoted to the access of the numerous archive files containing the published original data installed in the database.

More details are given in the pages describing the site design and philosophy.

The need for a database

The venue in a near future of instruments capable of obtaining spectra of several tens of stars at the same time will favour the acquisition of better and more numerous observations in the area of star clusters. Indeed, although much information has already been obtained, there are still large gaps in several domains that render the understanding of cluster formation and evolution still difficult and incomplete. In this context, the database is able to provide enough information as to make the best selections of stars to be observed, by providing most of the present knowledge accumulated over many years of a collective efforts by passionated observers.

The realisation of a new atlas of colour-magnitude diagrams based on improved and homogeneous cluster parameters was the first scientific use of the database foreseen and the primary motivation of its development. The way to achieve this goal is however quite long. If the production of a colour-magnitude diagram seems rather easy, the real situation is unfortunately not simple and the data are not always of sufficient quality and the question of the membership of each star to its parent cluster do not usually receive a straightforward answer. Therefore a large amount of work is necessary to plot any reliable colour-magnitude diagram that could be used, for example, to test stellar evolutionary models.

Open star clusters

Modern observations of open clusters developed very rapidly after the definition of the UBV photoelectric photometric system by Johnson & Morgan in 1953. These observations produced a number of colour-magnitude (Hertzsprung-Russell) diagrams which made fundamental contributions to the understanding of stellar evolution. At the same time photographic photometry allowed to observe larger areas and reach fainter stars.

Additional information, mostly spectroscopic, was gradually obtained, first for stars in the nearby clusters and later in more distant clusters, thanks to the existence of larger telescopes and more efficient detectors. Two-dimensional detectors are best adapted for the observations of star clusters and CCD observing is today becoming the preferred technique. It replaces both the photoelectric and photographic photometry.

The interest for star clusters results from four properties usually considered:

The stars are at the same distance

This is true for most objects, because the effect of the cluster volume is smaller than the usual errors on magnitude determination and negligible in comparison with other effects like binarity and rotation. There is one cluster, namely the Hyades, for which a precise determination of the distances of the individual stars has been possible, thanks to the Hipparcos satellite.

The stars have the same age

This asumption is true for intermediate-age and old clusters, but is questionnable for very young and extremely young open clusters. The problem arises from our lack of knowledge on how a molecular cloud contracts and which is the sequence of stellar formation. Which one of the low mass stars or the massive stars do form first? Obviously, in young clusters, the massive stars are already on the main sequence or even started their evolution away from the main sequence, while the low mass stars are still in a phase of contraction. This is a domain in which the contribution of star clusters to the understanding of the laws of star formation has been and will be fundamental.

The stars have the same chemical composition

So far, it has not been possible to prove the opposite and this is a good assumption. It implies that the material from which the stars formed was rather homogeneous. But the precise determination of the chemical composition is a difficult task and the uncertainties on the results are still rather large.

The stars differ in their mass

Open clusters usually contain stars over a large range of mass from more than 80 solar masses in the extremely young clusters, to stars less masssive than 0.08 solar masses, i.e. the limit for brown dwarfs.