6. Collinder 132
Collinder 132 is a loose concentration of stars located in Canis Major at (). Collinder (1931) gave its membership as 18 stars spread out over a region of about 85 arcmin. He estimated a distance of 270 pc to the cluster. During the last 20 years a debate arose about the nature of this cluster. Claria (1977) performed photoelectric UBV measurements for 35 stars, as well as H measurements for 18 stars in the cluster area. He interpreted the data as demonstrating the existence of two clusters, which he called Collinder 132A and 132B. For the two clusters, he determined distances of 560 and 330 pc respectively. Eggen (1983) performed intermediate-band and H photometry for 14 stars in the cluster region. He found evidence for the existence of two unbound groups of stars in the cluster area. He interpreted the first group as being connected to the nearby cluster Collinder 140 and supposed that the stars in the second group were members of the CMa OB2 association. The distances and assigned members of both groups differ significantly from Claria's groups.
This controversial situation calls for a re-examination of Collinder 132. In a first step we selected all stars within a by field around the cluster centre from the Hipparcos Catalogue. Fig. 6 shows a proper motion diagram of the stars found. There is a concentration of proper motions around mas/yr, mas/yr, which indicates the presence of a star cluster or association in this field.
In order to reveal the nature of this concentration, a larger field of by was searched for stars having proper motions close to mas/yr, mas/yr. We proceeded in the following way: For every star, we first calculated a -value according to
where z is the (two dimensional) difference vector between observed and expected proper motion and is the covariance matrix from the Hipparcos Catalogue. Stars with -values higher than a certain threshold were rejected as members. We chose , which corresponds to the confidence level for a system with two degrees of freedom. We next removed stars with incompatible parallaxes. Finally, we removed stars that were clearly above the main-sequences of Fig. 7 but were too faint to be giant members, so that they are non-members without doubt.
Fig. 7 shows a colour-magnitude diagram of the remaining stars. There seem to be two groups of stars present in this field.
Stars which belong to the lower of the two main sequences are shown as filled dots in Fig. 7. They show no clear concentration on the sky, instead they are distributed throughout the upper half of the right diagram in a more or less random way. The natural explanation is that they are members of an unbound association, not of a cluster. From the Hipparcos parallaxes of the suspected members, we derive a mean parallax of . Eggen derived a distance modulus of for his second group. This corresponds to a distance of pc, which is in rough agreement with our value. In addition, most stars of Eggen's second group belong to the lower main-sequence. The stars of the lower sequence in Fig. 7 may therefore be members of the CMa OB2 association. We have marked them by 'AS' in Table 5. We note that the stars of Collinder 121, which is about 8 degrees north-east of Collinder 132, have magnitudes and colours comparable to the stars of this association. In addition, Collinder 121 has a similar proper motion (Baumgardt et al. 1998). Hence, there may be a connection between this association and Collinder 121.
Table 5. Stars in the field of Collinder 132
The stars of the upper sequence (open circles) show a noticeable concentration around , , which may indicate the presence of a cluster. Claria measured distance moduli for the four central stars in Fig. 7 and obtained = 7.44 (HIP 34898), 7.44 (35342), 8.97 (34954) and 9.16 (34937). Eggen has determined distances for 3 of the 6 stars. His distance moduli differ considerably from Claria, he found = 7.9 (HIP 35342), 9.1 (34954) and 8.6 (34937). He noticed that HIP 34898 is an eclipsing binary and derived distance moduli between 8.1 and 8.8, depending on the nature of the variable. From these stars only HIP 35342 seems to be a foreground star, since both Claria and Eggen derived significantly shorter distances to this star than to the others. The other stars may form a cluster with a distance modulus of , corresponding to a distance of pc. With such a distance, their angular separation corresponds to a displacement of about 5 pc. Hence they may be members of a star cluster. In addition, the Hipparcos parallaxes of all three stars are compatible with this distance.
Based on their colours and proper motions two additional stars from the ACT may belong to this group: HD 56374 and HD 56638. Claria derived distance moduli of 8.47 and 7.78 for them, Eggen derived 8.5 and 8.9. At least the distances of Eggen are compatible with a cluster membership of the two stars. We have marked the possible members of this cluster by a CL in Table 5. Precise radial velocities would help to establish the reality of this group.
Eggen speculated that HIP 34937 and HD 56374 together with HIP 35174 and HIP 35348 form a group of stars which is connected to the star cluster Collinder 140. This group is likely not to exist, since the proper motion of HIP 35348 differs clearly from the other stars. Furthermore the proper motions of HIP 34937 and HIP 35174 differ by about 4 mas/yr from each other.
Claria suspected the presence of two clusters, Collinder 132 A and B. Some stars of Collinder 132 A belong to our hypothetical cluster. Furthermore the distances of both agree with each other. Fig. 8 shows the proper motions of the suspected members of Collinder 132 B. The reality of this cluster is doubtful, since most stars do not share a common motion. In addition one of the stars (HIP 34898) with a proper motion close to mas/yr, mas/yr seems to be located at a much larger distance than the 330 pc that Claria determined for Collinder 132 B. Moreover the photometric distances of Eggen argue against a cluster.
© European Southern Observatory (ESO) 1998
Online publication: November 9, 1998