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

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5. Melotte 227

Melotte 227 is a loose concentration of stars located around the 6.7 mag star HD 192074. Like NGC 1252 it is located at a high galactic latitude, its coordinates are [FORMULA] ([FORMULA]). It was discovered by Melotte (1915) on the basis of its appearance on Franklin-Adams chart plates. Melotte described the cluster as a few bright stars forming a coarse cluster scattered over an area of one square degree.

The cluster is only rarely mentioned in the literature. Collinder (1931) gave its diameter as somewhat over [FORMULA] and its membership as some 15 or 20 stars. Epstein (1968) published photoelectric photometry for 25 stars in the (uvby) system. He noted that the studied stars do not define a sharp main sequence and found a considerable dispersion in the derived distance moduli.

In order to reveal the nature of Melotte 227 we selected all stars within a [FORMULA] by [FORMULA] field centered on HD 192074 from the Hipparcos Catalogue. They can be found in Table 4. Column 3 gives the number used by Epstein for the stars in common. No radial velocities could be found for the stars of Table 4.


Table 4. Hipparcos stars in the field of Melotte 227
Column 4: Star numbers from Epstein (1968), Other columns: Same as Table 1

Fig. 5 shows the parallaxes and proper motions of the possible members. Although there seems to be a concentration of parallaxes around [FORMULA] mas, the proper motions scatter randomly and show no clustering. For a star cluster at a distance of 200 pc, the differences in the proper motions that are due to the internal motion of the stars are of the order of 1 mas/yr. Since the observed differences are of the order of 10 to 20 mas/yr, the stars cannot form a bound system. Only two stars share the same proper motion. These are stars 4 and 5 of Table 4, which are the components of the double star system CCDM 20041-784. Since in addition Epstein could not identify a proper main sequence among the stars we are forced to conclude that Melotte 227 is not a cluster.

[FIGURE] Fig. 5. Parallaxes (left) and proper motions (right) of Hipparcos stars in the field of Melotte 227

We find only a random concentration of stars of magnitude 7 to 10 in this field. Epstein found 25 stars brighter than 11th magnitude per square degree in the field of Melotte 227 and noted that this is about twice the normal star density for such a galactic latitude. This means that the density of bright stars is raised by 3[FORMULA] above average, still small enough to be explained by a statistical fluctuation.

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© European Southern Observatory (ESO) 1998

Online publication: November 9, 1998