8. Collinder 135
Collinder 135 is located in Puppis at (). The discussion about the nature of Collinder 135 has been going on for 30 years. Williams (1967b) proposed that the cluster does not exist, an opinion which was shared by Claria & Kepler (1980), who only found evidence for a concentration of stars at a much larger distance. On the contrary, Eggen (1983) argued that 8 stars around Pup could form a cluster at a distance of approximately 300 pc. Amieux (1993) has measured radial velocities for 63 stars in a by field. She found a peak in the radial velocity diagram at +20 km/sec, but noted that most of the stars in this peak are field stars. Amieux & Röser (1995) have combined the radial velocities with proper motions from the PPM Catalogue. The proper motions favoured the hypothesis of a cluster, but were not accurate enough to definitely separate the cluster from the field stars.
Fig. 13 shows the proper motions of all Hipparcos stars in a by field centered on the position of the cluster. There is a clear concentration of stars around mas/yr, mas/yr, which becomes even more pronounced if one considers only the central by area. This is a first sign for the presence of a cluster.
All stars with proper motions between -14 mas/yr -6 mas/yr and 2 mas/yr 10 mas/yr were considered as members of a hypothetical cluster. Fig. 14 shows (clockwise from top left) their proper motions, positions, colour-magnitudes and parallaxes. The presence of a cluster becomes even more likely, since the suspected members may have a common parallax and proper motion, form a well defined main-sequence and show a clear spatial concentration. In addition, it seems possible that most stars share a common radial velocity (see Table 8). All aspects studied so far favour the hypothesis of a bound group of stars, and we therefore conclude that Collinder 135 is a cluster.
Table 8 lists the basic parameters of the candidate members. The red giant Puppis (HIP 35264) is found to be a member of Collinder 135. Stars 8 and 9 are members of double systems. This may explain their large proper motion deviations in Fig. 14a. Star 5 (HIP 35202) is an uncertain member, since it has the largest separation from the others and shows quite large proper motion and parallax differences too. Membership of HIP 35483 is also uncertain, since its radial velocity differs significantly from the cluster mean. The other stars seem to have similar radial velocities, which supports their membership. Omitting the two uncertain members, we obtain a mean parallax of mas for the cluster, which is in good agreement with the 300 pc (corresponding to a parallax of mas), that Eggen (1983) determined as the cluster distance.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998