The proper motion and the distance of the Praesepe cluster

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Table 6. Twenty stars, belonging to the Cluster, fainter than the fourteenth magnitude.

Nr. Phot. magn. Nr. Phot. magn.

576 14*07 569 16-15 568 14-33 570 16-37

574 1443 558 16-65

560 14-70 571 16-71 559 14*99 573 1699 572 15-10 564 17-12

575 1526 562 17-16

561 15-41 565 1725

577 i5*7i 567 1768 566 15-97 563 1807

7b Cluster-stars The proper motions of the stars fainter than the 14* magnitude have been 18>m>14. derived with a larger probable error. Moreover the measured area is one fifth of the maximum area of the other plates (table 1, plate XVII).

The proper motions are again plotted in a diagram for each magnitude separately. It is a little more difficult to distinguish the cluster-stars from the background-stars than in the case of section 7a but the presence of a group motion cannot well be doubted (figure 3). The 20 stars, lying within the dotted line of figure 3, are contained in table 6. The six cluster-stars in common with the other plates {m — 13*0 to 14*0) have been found also on this plate.

The absolute proper motion of these faint Praesepe-stars, derived in section 9, is practically identical with that of the stars brighter than the magnitude 14.

8. The Magnitude The accuracy of the proper motions, derived in this paper is high as far as Error. the accidental errors are concerned. The probable error of the mean of the plates

is o"*ooi5 or less, up to the magnitude 12 or 13. It is therefore disappointing to find that a systematic error, of several times the value of this probable error, has vitiated the results. This error depends on the magnitude and will therefore be called magnitude error; this implies however no indication of its cause.

The value of this magnitude error can be determined by three different methods: First method: the relative proper motions are compared with known meridian proper motions.

Second method: the dinerences are computed between the theoretical parallactic motion of the stars of determined magnitude and the mean relative proper motion of stars of the same magnitude.