Systematische Bewegungen der Sterne im Orion-Nebel

Summary

I. A systematically homogeneous catalogue of proper motions (p.m.) in a field 80’ in diameter around θ¹ C Orionis had to be constructed. By intercomparison of the p. m. of all suitable stars in common to existing catalogues, both the relative weights and the parameters of reduction are evaluated. For good reasons our system of quasi-relative p.m. is based on carefully weighted means of p.m. taken from Parenago’s on the one hand, the “relative” p.m. derived by Meurers and Sandmann on the other: intermediate system Kl. Then the “absolute” p.m., determined by the latter authors, are reduced to and unified with the same system, thus forming the intermediate system K2. Finally, after appropriate reductions, Strand’s catalogue is equally incorporated.

II. Among 33 absolute p.m. from meridian circle observations within the field under discussion, at least eight seemed to be spurious when compared with the individually much more precise K2-values. The remaining ones showed much greater accidental deviations in RA than in δ, where, after a rediscussion of the final results, remained only two extremely great deviations. Hence we decided to ignore the rather great systematic reductions of scale and orientation of our p.m. that were suggested by the former (Δ μx) alone, without any confirmation by the latter ones (Δ μycedure has been justified by an analysis of the p.m. of 38 probably foreground stars with p.m. between 1”,5 and 7” per century. The systematic motions found in this sample are in fair agreement with the expected effect of parallactic motion. The resulting catalogue of p.m. of 319 stars is referred to as K4, since it is intended to fit systematically the FK4.

III. As p.m. of the cluster’s center of gravity we assumed the mean value of 77 stars within 380” from θ¹ C Ori, those with |μ| ≥ l”,5/100^a being excluded. Thus we hope to minimize the influence of undetected non-members, and to smooth out the divergent internal motions. Yet, the resultant, 0”25/100^a in position angle 50°, is of the same order of magnitude as the inevitable uncertainty of the reduction to absolute p.m. (c. II.). Fortunately, this does not affect the internal motions, which we transformed individually into polar coordinates with respect to the center of gravity before taking means over circular segments, tentatively formed in two different ways, quadrants and sextants of three concentric rings. The preponderance of a clock-wise angular velocity increasing towards the center is clearly seen from Table 4, whilst neither overall expansion nor contraction is indicated.