3. Upgren 1
Upgren 1 was first noted by Upgren (1963). It is a group of seven F-type stars scattered over an area of 0.1 square degrees. It is located in Canes Venatici at (). Anderson (1966) and Osborn (1967) published proper motions for these stars and noted that not all of them belong to a physical group. Later Upgren et al. (1982) presented narrow-band photometry and radial velocities for the cluster stars. They concluded that five stars form a physical group, while the membership of the remaining two stars is less likely. Stefanik et al. (1997) confirmed these results, but noted that the velocity dispersion of the five stars is larger than one would expect if they form a bound system. Based on parallaxes and proper motions obtained with the multichannel astrometric photometer (MAP) Gatewood et al. (1988) also concluded that Upgren 1 consists of two dynamically different groups.
We have collected the parallaxes and proper motions of the 7 stars in Table 2. Star 2 could not be found in either Hipparcos or ACT, star 3 is not included in the Hipparcos Catalogue.
Fig. 2 shows parallaxes and proper motions. With the exception of stars 3 and 5 the proposed members share no common motion. Since the ACT and Hipparcos proper motions are in general agreement, short period binaries again cannot play a significant role. As in the case of Collinder 399 binaries with longer periods would require too massive companions to be a reasonable alternative. We therefore conclude that Upgren 1 is not a cluster.
Stars 3 and 5 are of special interest. The proper motions from the ACT suggest a common space motion. The radial velocities of the two stars differ by 1.8 times their standard errors and are not incompatible with such an assumption. In addition star 3 is known to be a long period binary which may complicate the determination of its radial velocity. The parallax of star 5 corresponds to a distance of about 110 pc. With such a distance the separation of stars 3 and 5 would be pc. Such systems are expected to survive for about 10 Gyrs before they are disrupted by encounters with passing disk stars (Bahcall et al. 1985). Hence, it is not unlikely to observe such systems.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998