2. Collinder 399
Collinder 399 is a concentration of bright stars, located in a one square degree big field around (). It was first mentioned by Meyer (1903, 1905) and then briefly discussed by Doig (1926) and Collinder (1931). The most extensive membership study so far was done by Hall and van Landingham (1970). They concluded on the basis of UBV magnitudes, spectral classifications, radial velocities and proper motions, that Collinder 399 is a cluster, but that it consists of only 5 early-type stars (HD 182422, 182620, 182761, 182919 and HD 182972) and one red giant (HD 182955).
All suspected members could be found in the Hipparcos and ACT catalogues and we have collected the relevant data in Table 1. Fig. 1 shows the Hipparcos parallaxes and proper motions of the suspected members. On the basis of their parallaxes two of the six stars cannot belong to the cluster: Star 1 (HIP 95340) seems to be a background star, while star 4 (HIP 95560) is clearly foreground. Only four stars may share a common parallax of mas, corresponding to pc, consistent with the distance modulus of m-M = 5.5 derived by Hall & Van Landingham (1970).
Table 1. Suspected members of Collinder 399
We have also plotted the proper motions of the suspected members in Fig. 1. The proper motions differ by about 20 mas/yr from each other, with a typical 1 mas/yr uncertainty of a single proper motion. The differences are therefore real and not due to errors. They correspond to roughly 15 km/sec with the adopted distance of 145 pc, which is about a factor of 100 higher than the value one would expect if the stars form a bound system. Since the Hipparcos and ACT proper motions are in general agreement (see Table 1), it is impossible that the Hipparcos proper motions are perturbed by short period binaries ( years). We cannot rule out the possibility that the stars are members of long period binary systems ( years), since then both the Hipparcos and ACT proper motions would deviate in the same way from the true motion of the star. However, to explain deviations of 20 mas/yr by binary effects would require much too high masses for the companions to be a reasonable alternative. The proper motion differences must therefore reflect a real difference in the space motion of the studied stars. They cannot form a bound system. In addition, the density of fainter stars is not raised in the cluster area (Hall & Van Landingham 1970). We therefore conclude that Collinder 399 is not a cluster, but only a chance alignment of some stars of magnitudes 5 to 7.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998