Hidden Clusters in the Southern Milky Way

Extract

At visual inspection of objective-prism plates of Milky Way regions one frequently detects pairs or sometimes higher multiples of spectra which are so similar in appearance and so close together that they form a conspicuous configuration for the eye. A statistical analysis shows that the observed occurrence of this phenomenon, down to a certain limiting magnitude and up to a certain value of the angular separation of the components, considerably exceeds the expected one - if it should be considered as produced entirely by random coincidence. Therefore it is reasonable to assume a physical explanation behind this sort of agglomeration. According to the working hypothesis it is essentially a part of very loose or star-poor clusterings which do not show a sufficiently high degree of contrast against the surrounding galactic stellar background to be discovered “directly”. The phenomenon might thus be comparable to an optical multiple system, although it is produced by members of a physical cluster of some kind. In connection with a large survey of the Southern Milky Way, carried out at the Stockholm and Uppsala observatories, several thousands of such coincidences have been registered. About a hundred of them have been subjected to a special investigation, intended to give an answer to the question about their physical interpretation. Although the methods have been neither very rational, nor always point-to-point reliable, the scrutiny has shown pretty well that an overwhelming majority of the “candidate” objects do reveal stellar clusterings of various types. In fact, it has been possible to distinguish between at least four more or less well-defined types of objects in the shape of stellar clusterings. 1. Relatively ordinary clusters, a few of which are already known or even well-known. The “rediscovery” is then some sort of a confirmation of the working hypothesis. 2. Very loose clusterings. 3. Extremely small and star-poor clusters, possibly cluster remnants. 4. Multiple systems with extreme separation between the components, maybe the ultimate stage of a cluster in disintegration. The relative frequency of the four interpretations in order will approximately be: 1–20%, 2–35%, 3–20%, 4–10%. In addition, there is also a certain admixture of spurious objects without astrophysical interest.