Photometry of NGC 1907.

A preliminary estimate of the separation in space of the two galactic clusters M 38 and NGC 1907 was made on the basis of revised and improved photo-red color indices. An attempt was then made to evaluate the mass of M 38 from the conditions for stability of the companion cluster, NGC 1907, under the tidal action of M 38. The limiting diameter beyond which NGC 1907 can no longer be considered a stable cluster was estimated by star counts on long exposure photographs. The critical density for stability for NGC 1907 then led to an estimate of the mass of M 38 of the order of 6 X i~~ solar masses. Such a value was felt by the author to be much too high, for M 38 is not a very rich cluster. In fact, from star counts, and from the luminosity curve of M 38, one would be apt to regard 5 X I& solar masses as an upper limit to its mass. A possible cause for the discrepancy may be found in the fact that the region of the clusters in Auriga is loaded with field stars belonging to the Auriga star cloud. The clusters, therefore, should not be regarded as isolated objects in space, but instead as parts of the Auriga star cloud. Under this condition, the stability criterion, derived by Hill, later by Bo~, and by others, on the basis of the inverse square law of force does not apply to this particular problem. Loading of the cluster region with field stars may give an approach to uniform density in the region, and therefore the integrated law of force outward from M 38 may approach the inverse first power, and the method of mass estimation from stability considerations becomes completely insensitive. The presence of the field stars, however, does offer an explanation for the stability, under the shearing effects of galactic rotation, of the more widespread group of clusters, such as M 38, M 36, M 37, NGC 1907, NGC 1893, and others, in Auriga. Kirdwood and Goethe L~nk Observator~es, Ind~na University, Bloomington, Ind.