Abstract

New observational data are presented for 464 stars of Henry Draper type B5 and earlier, m <= 6.5 and > - 20 , which are likely candidates for membership in a local system defined by the Gould belt. These data include new spectral types on the MK system, classified by the author on Yerkes 40-inch plates; previously unpublished UB V photometry, kindly furnished by Dr. D. L. Crawford of the Kitt Peak National Observatory; and new proper motions on the system of the FK4, calculated under the author's direction at the Kapteyn Astronomical Laboratory, Groningen, The Netherlands. The observations have been combined to calculate space motions for the 294 program stars having well-determined distances within 600 pc. It is shown that if these stars originated within a small volume of space at some unique time in the past, the present values of the velocity gradients au/ax, aU/aY, aV/aX, and aV/aY can be used to determine the age of the group. This determination is independent of the location of the Sun, with respect to the center of the group, and of any incompleteness in observation of the group members. However, it is affected by the possible inclusion of non-members in the solution. Least-squares solutions for the velocity gradients have been made for various subsets of the stars with calculated space motions. These solutions provide conclusive evidence for the presence of an expanding element among the nearby B stars but do not yield a unique result for its age. Two models are proposed, one in which the entire sample consists of a mized population with an over-all expansion age of 90 X 106 years and the other in which the associations constitute an expanding subset aged 45 X 106 years and account for about 30 per cent of the total number of stars. It is shown that either of these models is compatible with the observed gradient perpendicular to the galactic plane and also with a time scale of expansion derived on purely kinematic grounds.