Astron. Astrophys. 333, L27-L30 (1998)

3. Discussion

An immediate implication of the present results is that OCRs from poorly populated clusters are relatively easy to detect even using low resolution techniques (e.g. objective-prism plates) because of the presence of early-type, luminous stars. On the contrary, OCRs from rich clusters should be formed preferentially by low-mass, faint stars. In any case, an overabundance of binaries and hierarchical systems should be observed. Unfortunately, there have only been a few observational attempts to study OCRs. Using objective-prism plates, Lodén (1987, 1988, 1993, and references therein) has investigated the frequency of clusters in the Milky Way under the hypothesis that the stars should have similar luminosities and spectral types. He has found that about 30% of the objects in his sample could be catalogued as a possible type of cluster remnant. The membership for these objects is . The typical age of these systems is about 150 Myr with a range from 50 to 200 Myr. They show an increased density of binaries and a considerable probability for optical binaries. The stars of these OCRs have a tendency to be massive and hence early-type (A-F) stars although this observational method includes a noticeable selection effect (Lodén 1996) because bright early-type spectra are easier to detect than fainter and later ones. In fact, almost no stars with spectral type later than F appear among his objects. On the other hand, these results are not fully conclusive because there are known regions in the sky with many stars of the same spectral type but in which it is difficult to find two stars with the same velocity. A striking example of this fact is Upgren 1; initially, it was suggested that this small group of seven F stars was the remnant of an old cluster (Upgren & Rubin 1965) but later, Gatewood et al. (1988) concluded that Upgren 1 is only a chance alignement of F stars resulting from the close passage of members of two dynamically different sets of stars. Very recently, Stefanik et al. (1997) have shown that one of the sets is formed by 5 stars including a long-period binary and an unusual triple system. The properties of this unbound set match very well when those found in numerical models with an initial population of about 1,000 stars.

If the luminosity of the cluster stars is very low, the surface brightness of the object can be extremely small. OCRs from rich clusters would consist mainly of early K stars, which are extremely difficult to detect at a distance. As regards poor clusters, it is very probable that many of the loose clusters found by Lodén are poor OCRs (progenitor with 500 stars) in which the low-mass stars have not yet been detected because of the low resolution technique utilized. On the other hand, if most of stars were born in small open clusters the probability of finding these OCRs is higher; this point can help to explain Lodén's results. The properties of OCRs make them very suitable for the new generation of detectors, the STJ (Superconducting Tunnel Junction) devices (Peacock et al. 1996). The possible existence of a large number of OCRs in the Galactic disk may have some implications for dark matter. If stars form mainly in open clusters, the number of these systems had to be very large in the past in order to explain the stellar population of the disk. Considering a mean population of 300 stars for a typical open cluster and about 10¹¹ stars in the disk, the number of possible remnants could be about 3 . If the typical mass of such an object is about 30 , it is possible that the mass in OCRs could be as large as 10 , roughly a sixth of the Galactic disk mass. This hypothesis could be tested by using the capabilities of the proposed global astrometry mission GAIA.

© European Southern Observatory (ESO) 1998

Online publication: April 20, 1998
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