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Astron. Astrophys. 340, 402-414 (1998)

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4. NGC 1252

NGC 1252 is located in Horologium at [FORMULA] ([FORMULA]). The New General Catalogue describes it as a group of 18 or 20 stars, but it is not included in the Catalogue of Star Clusters and Associations by Ruprecht et al. (1981). If it exists, it would be one of the few clusters located at a high galactic latitude. The cluster is of general interest because the carbon star TW Horologii could be a member of it. Eggen (1972) found a distance modulus of [FORMULA] to this star, corresponding to a distance of [FORMULA] pc. Bouchet & Thé (1983) performed UBVRI photometry for 38 stars in a region of radius [FORMULA] around TW Hor. They found 16 probable cluster members, and a cluster distance of approximately 470 pc. TW Hor was found to be a likely cluster member. In a later investigation Eggen (1984) searched the Cape Zone Catalogue for proper motions of the suspected members. He found six stars of which only three could share a common motion, casting doubt on the reality of NGC 1252.

Due to the relatively big errors of the Cape Zone Catalogue and the limited number of stars no definite conclusion can be drawn from Eggen's investigation. We therefore searched the Hipparcos and ACT catalogues for the probable cluster members of Bouchet & Thé. 12 stars could be found, including all stars brighter than 11th magnitude. They are listed in Table 3. Fig. 3 shows their proper motions. It is obvious that they do not form a cluster, since at most only two stars can share a common motion. In particular the proper motion of TW Hor, [FORMULA] mas/yr, [FORMULA] mas/yr, is not shared by any other star.


Table 3. Suspected members of NGC 1252
1: TW Hor, Column 1 gives the star numbers from Bouchet and Thé. The proper motions in columns 8 to 11 are taken from the Hipparcos (1) or ACT catalogues (2).

[FIGURE] Fig. 3. Proper motions of the stars, that Bouchet & Thé (1983) considered as members of NGC 1252.

There might be a slight chance that Bouchet & Thé have missed the real cluster members. We therefore searched the Hipparcos and ACT catalogues for stars which lie in a one square-degree field centered on the position of NGC 1252. Fig. 4 shows their proper motions and magnitudes (a few high velocity stars have already been omitted). It seems possible that some stars have the same motion, but their magnitudes and colours are incompatible with the assumption of membership to a cluster. On the other hand, those stars which may form a main sequence have very discrepant proper motions. Combining this with the result of the previous paragraph, we therefore conclude that NGC 1252 is no cluster.

[FIGURE] Fig. 4. Proper motions (left) and colour-magnitudes (right) of all Hipparcos stars in the direction of NGC 1252

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998